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This Journal of Environmental Horticulture article is reproduced with the consent of the Horticultural Research Institute (HRI – www.hriresearch.org), which was established in 1962 as the research and development affiliate of the American Nursery & Landscape Association (ANLA – http://www.anla.org).

HRI’s Mission:

To direct, fund, promote and communicate horticultural research, which increases the quality and value of ornamental plants, improves the productivity and profitability of the nursery and landscape industry, and protects and enhances the environment.

The use of any trade name in this article does not imply an endorsement of the equipment, product or process named, nor any criticism of any similar products that are not mentioned.

HRI’s Mission:

The use of any trade name in this article does not imply an endorsement of the equipment, product or process named, nor any criticism of any similar products that are not mentioned.

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Impact of Mulches on Landscape Plants and the Environment — A Review1

Linda Chalker-Scott2 Washington State University, Puyallup Research and Extension Center

7612 Pioneer Way E., Puyallup, WA 98371

Abstract Mulches provide aesthetic, economic and environmental benefits to urban landscapes. Mulching is especially useful in the establishment of trees in landscapes that receive minimal care, such as restoration sites. In general, mulches improve soil health, creating healthy populations of plants and associated animals. These biodiverse, stable landscapes are more resistant to stress, are more aesthetically pleasing, require fewer applications of pesticides and fertilizers, and are ultimately more sustainable than those without mulch cover. All mulches are not created equally, however, and this review compares the costs and benefits of landscape mulches as reported in the scientific literature. It also presents real and perceived problems associated with various landscape mulches.

Key words: aesthetics, economics, inorganic mulch, living mulch, mulch management, organic mulch, pesticide reduction, plant establishment, soil protection, urban landscapes.

Significance to the Nursery Industry There is a vast array of mulch materials available for land-

scape use, benefiting plants and soils through weed suppres- sion, evaporation reduction, and other environmental modi- fications. Given the available choices, it can be difficult to determine which mulch materials are best suited for a par- ticular landscape. The purpose of this review is to provide a comprehensive analysis of the scientific research on the ben- efits and drawbacks of mulches used in ornamental and ur- ban landscapes. The article will be of particular value to in- dustry professionals who sell, apply, or manage mulch mate- rials, allowing them to make informed recommendations spe- cific to their customers’ landscaping needs.

Introduction The term ‘mulch’ is derived from the Germanic word

‘molsh’, which means soft. Though not all mulches are soft, for many the word connotes the soft, spongy layer found in forest ecosystems. Mulches are defined as materials that are applied to, or grow upon, the soil surface, as opposed to ma- terials that are incorporated into the soil profile (amendments). Therefore, any material laid or grown over the soil surface can be considered a mulch, though some materials are more beneficial than others.

Mulching received serious study as an environmental modi- fication in forest, agriculture, and landscape applications be- ginning in the late 1930s, perhaps spurred by earlier studies examining the negative impacts of grasses on tree growth (9). ‘Deep, permanent mulches’ were recommended for shrubs and trees as early as 1941 (101) as a way of prevent- ing drought stress (also 62, 131); likewise, prevention of freeze damage and frost heave were documented benefits (27, 72). In a comparative study, leaf material used as a mulch was found to be more effective in terms of water conserva-

tion than the same leaf material incorporated into the soil (116).

The increase in mulch research in the mid-1900s was closely tied to interest in reusing agricultural and forestry byproducts including wood pulp and shredded paper (22). More recently, other recyclables such as arborist trimmings, yard waste, and agricultural crop byproducts have been in- corporated as mulching materials. Though landscape mulches were first reviewed in 1957 (2), there have been no analyses summarizing the scientific research on landscape mulches. Given the substantial use of mulch materials in the manage- ment of urban or ornamental landscapes, such a summary is long overdue.

Comparative Benefits of Mulches Improved soil moisture. Exposed to heat, wind, and com-

pacting forces, bare soil loses water through evaporation and is less able to absorb rainfall or irrigation as it becomes in- creasingly compressed. Weeds can increase evapotranspira- tion of soil moisture by 25% in a summer day (54). In con- trast, mulches will increase soil water by increasing percola- tion and retention, reducing evaporation, and reducing weeds. An early study (105) demonstrated that a layer of straw only 3.8 cm (1.5 in) thick reduced evaporation by about 35% com- pared to bare soil. Later, Kacinski (62) demonstrated that most mulched soil has greater water retention than bare soil, with the exception of competitive living mulches such as turf.

What is less consistent is how different mulch types influ- ence water movement. For instance, black plastic generally inhibits water movement (7, 12) between the soil and the above-ground environment, thus limiting recharge. Soil wa- ter recharge is dependent upon infiltration, which in turn is influenced by surface permeability. Activities and products that compact soils and/or create hydrophobic conditions will limit recharge while increasing runoff and erosion. Plastics, geotextiles, fine-textured organic mulches, sheet mulches, and mulches with waxy components are poor choices in this regard. Therefore, though these mulches may initially increase soil water retention since evaporation is reduced (68), over the long term they will create soils that are unnaturally dry.

Viewpoint

1Received for publication May 12, 2007; in revised form July 24, 2007. Acknowledgements: I am most grateful for the thoughtful, constructive com- ments provided by two anonymous reviewers and for Dr. Fretz’s assistance in preparing this Viewpoint article. 2Associate Professor. Email: <[email protected]>.

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In contrast, there is a wide variety of mulching materials that do not limit soil water infiltration and retention: their one similarity is that they are all permeable materials. Most comparative studies among mulch types indicate that organic mulches conserve water more effectively than inorganic (6, 60); organic and inorganic are better conservers than syn- thetic (5, 73) and all are better than bare soil (6, 60, 65, 68, 76, 107, 112, 134). Mulches with demonstrated ability to re- tain water include gravel and stone (6, 60, 65, 112, 133), livestock manure (15), and a vast array of plant materials. These consist of rapid decomposers such as grass clippings (31, 107), leaves (116, 134) and local crop residues (68, 76, 92, 108, 127); moderate decomposers including hay and straw (73, 93, 103, 108, 133), coir pith (6, 53, 76), and jute (8); and slowly decomposing timber residues (46) including sawdust (5, 66), and barks and chips from both hard- and softwoods (31, 43, 60, 65, 68, 73, 96, 99, 131, 134). Cover crops are generally less effective than either organic or inorganic mulches (31, 68, 96, 107, 133) as they must compete with other landscape plant materials for water.

From a practical viewpoint, an appropriate mulch will sig- nificantly reduce the amount of irrigation needed for all land- scapes, and in some cases can eliminate it altogether (99). In addition to protecting soil reserves, coarse organic mulches will hold water much like a sponge, thereby capturing rain- fall and irrigation water for later release and preventing run- off. An early study demonstrated that 1.5 cm (0.6 inch) of straw mulch reduced water runoff by 43% (10); mowed sod and bark were likewise found to reduce runoff (87). Less runoff and improved retention will translate to reduced needs for supplemental irrigation. In addition, mulch protection from drought stress can also protect trees and shrubs from subsequent environmental stresses such as cold injury (118).

Reduced soil erosion and compaction. Mulch will protect soils from wind, water, and traffic-induced erosion and com- paction, all of which contribute directly to root stress and poor plant health. Though living mulches are often the most effective in this regard, holding the soil matrix together even on the steepest slopes, they may not be the best practical or economic choice. Grass sowing, for instance, can reduce ero- sion but often increases runoff compared to other mulch choices (104, 126) as did barley (Hordeum vulgare) (109).

Adding even a thin organic mulch will protect soils: Borst and Woodburn (10) found that a 1.5 cm (0.6 in) layer of straw mulch reduced soil erosion by 86%. Straw from rice (Oryza sativa) and other grains continues to be commonly used (107) and in some cases can outperform living mulches such as legumes and grasses (107, 126). Straw mulch in combina- tion with an erosion net was found to decrease erosion by 95% over bare soil treatment in a forest plantation (84). Fallen pine (Pinus spp.) needles resulting from beetle attack helped prevent soil erosion (80), and logging debris was used to in- tercept water and reduce overland flow (104). These studies all underscore the importance of leaving fallen vegetation on forest sites. [It is important to note that mulches cannot be use to ‘stabilize’ slopes but only reduce soil loss. Slope sta- bilization requires an engineering solution, not a horticul- tural one.]

Compaction is a common aliment of urban soils; while the impacts of foot and vehicular traffic are self-evident, it’s less obvious that rainfall will compact unprotected soils. Adding organic mulch such as bark (96) or jute (8) disperses

the direct impact of water droplets, feet, and tires, thus re- storing soil aggregation and porosity. It is better to apply mulch before compaction occurs rather than after the fact. Research has demonstrated that proactive mulching will pro- tect soil integrity, while the same mulch applied after com- paction could not reverse bulk density changes even after two years (30).

Maintenance of optimal soil temperatures. Mulches pro- tect soils from extreme temperatures in that soils can be kept cooler in hot conditions (37, 43, 74) and warmer in cold con- ditions (66). Temperature extremes will kill fine roots and while rarely killing established plantings, they can induce a chronic stress as the plant expends energy to generate new fine roots. Temperature modification is especially important near the soil surface, where fine roots can be killed by freez- ing and frost heaving (49). Hot or cold surface soils can kill new transplants that have not had time to generate a large root mass and establish into deeper, more moderate, surround- ing soils.

This moderating effect is especially important during the winter and/or in alpine and arctic regions, where warmer soil can enhance root growth and thus establishment of desirable plants. Cold hardiness of the plants, however, is not lessened by this treatment (140). Protection of sensitive root systems from freezing has the added benefit of preventing opportu- nistic root rots from attacking stressed seedlings (128). In summer months or in hotter geographical regions, organic mulches have been shown to lower soil temperature nearly 10C (50F) compared to unprotected soil (76).

Coarse mulches are more temperature moderating than finely textured mulches of the same general category; for example, the soil under cobbles is cooler than that under gravel, and the soil under leaf mulch is cooler than that un- der compost (127). Likewise, thicker applications of organic mulches are more temperature moderating than thin applica- tions (131). Once again, coarse mulches are better in this regard (57) as thick layers of finely textured mulches can inhibit both water and gas transfer.

Among mulch categories, living (133) and organic (6, 60, 76, 89, 133) mulches are more temperature moderating than inorganic mulches. Chunky inorganic mulches such as gravel and lava rock are more effective temperature moderators (6, 60, 89) than solid inorganic surfaces such as concrete. Syn- thetic mulches including asphalt, fabrics and plastics are poor- est in this regard (73, 89, 133), routinely raising the underly- ing soil temperature as deep as 30 cm (12 in) below the sur- face (33). For some special applications (such as soil solar- ization to kill pests [82]), this might be desirable, but not for general landscape or garden maintenance. Black plastic mulches can either raise (73) or lower soil temperatures (130), probably depending on how much light is absorbed by the plastic and whether heat is retained or reflected. Clear plas- tic mulches routinely raise soil temperatures since radiation (including infrared wavelengths) is transmitted through the plastic and heat is retained.

While the impacts of mulches on soil temperature have been well documented, there is also an effect of mulch type on surface temperature. There are far fewer studies on this phenomenon, but it is clear that some mulches heat the soil as a function of solar radiation absorption more than bare soils and living mulches (89). The increased surface tem- perature due to pine bark mulch caused nearby leaves to lose

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more water (143), though at greater distances [e.g. 1 m (3.3 ft)] there is no mulch effect upon air temperature (144).

Living mulches such as turf release water vapor through evapotranspiration and reduce surface temperatures by evapo- rative cooling (89), though they use more soil water than non-living mulches. Interestingly, the soil temperature be- neath turf was shown to be higher than that below mulch (23), perhaps because soil beneath turf was drier and thus less protected against high temperatures. Heat-reflecting mulches are sometimes useful, especially in improving fruit maturation (48); for most urban or managed landscapes, how- ever, this is probably not a priority.

Increased soil nutrition. Living and organic mulches can increase, decrease, or have no effect upon nutrient levels de- pending upon mulch type, soil chemistry, and particular nu- trients of interest. As living and organic mulches decompose under appropriate water and temperature levels, nutrients are released into the soil and become available for root uptake or microbial use. Generally, green and animal manures used as mulch supply nutrients at higher rates than other mulch choices (such as straw, bark and wood chips) and often per- form better than inorganic fertilizers (3, 31, 100, 116). While immediately available nutrients are sometimes desirable for a landscape, it is important to note that overapplication of these materials can lead to excess mineral availability, caus- ing damage to plants, soil organisms, and nearby watersheds. Therefore, nutrient-rich mulches should be applied sparingly and may be most effective as part of a mulch layer.

While living mulches often compete for nutrients as well as water, this characteristic can be valuable on landscapes where fertility is too high. Fast-growing plant materials will reduce soil nutrient levels (94) as can microbial activity in low-fertility organic mulches. This has been helpful in resto- ration of ecosystems with naturally low fertility, allowing native plants to compete more effectively with invasive spe- cies (145). Low nutrient mulches such as uncomposted bark or straw were found to decrease nitrogen levels of soil water while not impacting plant nutrition (95), thus reducing wa- tershed pollution.

While mulches with relatively high nitrogen content often result in higher yields (127), low nitrogen mulches can also increase soil fertility and plant nutrition. For example, straw (125), sawdust (5) and bark (99) mulches have been shown to increase nutrient levels in soil and/or foliage. Likewise, mulches of husks were most effective in increasing available soil nutrients compared to grasses and leaf-litter (114), which presumably would have higher nitrogen levels.

Reduction of salt and pesticide contamination. Many land- scapes experience salinity stress besides those found near marine coastlines. Arid landscapes in particular are often highly saline as evaporating water leaves behind salt crusts. Irrigation water in arid environments and improperly treated greywater (domestic, non-sewage waste water) can also con- tain high levels of salts from fertilizers, detergents and other chemical sources. Container plants that are over-fertilized will likewise experience increasing levels of salts.

Because mulches reduce evaporation, more water is left in the soil and salts are diluted. Furthermore, organic mulches can reduce the effect of salt toxicity on plant growth (3, 69, 142) or actively accelerate soil desalinization (29). Plastic mulches are not effective in this regard (123), probably be-

cause they are not able to bind ions as organic materials can. Organic mulches can also help degrade pesticides and other contaminants (45, 117), presumably by providing increasing microbial populations that degrade pesticides.

Increased binding of heavy metals. Organic as well as liv- ing mulches can be effective in removing heavy metals from landscape and garden soils. Common urban contaminants such as lead and cadmium can be removed from the soil so- lution by mulched leaves of eucalyptus (Eucalyptus spp.), pine, poplar (Populus spp.), and arborvitae (Thuja spp.) (106). Likewise, a mixture of compost and woodchips was found to decontaminate forest soils by complexing copper into a less toxic form (63).

Improved plant establishment and growth. Mulches are used globally to enhance establishment of many woody and herbaceous species. There are hundreds of controlled studies demonstrating that mulches improve seed germination and seedling survival, enhance root establishment and transplant survival, and increase overall plant performance when com- pared to unmulched controls. Practically, this translates to healthier trees and shrubs requiring less maintenance and chemical application.

A. Improved seed germination and seedling survival. Kacinski (62) demonstrated that mulching planting pits with manure or sawdust improved oak (Quercus spp.) seedling survival compared to unmulched pits and surface plantings. Since then, numerous studies have found mulch to enhance seed germination and seedling survival.

Seedling emergence and survival presents a management conundrum: we want to encourage desirable plants yet pre- vent weeds from establishing. Unfortunately, mulches do not distinguish between weeds and desirable plants. For this rea- son, many mulches are not appropriate for annual flower beds and vegetable gardens. On the other hand, these same mulches are excellent choices for repelling weed colonization.

Success in this respect may be determined by mulch depth and/or seedling maturity. Deeper mulches are associated with improved weed control and are not the best choices for areas that are to be seeded rather than planted, especially if the species of interest have small seeds. Broadcast seeding on a restoration site was successful when a thin mulch layer was applied post-seeding, but significantly reduced when the mulch depth was doubled or when mulch was applied before seeding (103). Organic mulch may be a better choice for seed germination than gravel (90), which in deeper layers can pre- vent seedling emergence (137). Once seedlings have emerged, mulches are associated with improved seedling performance under both nursery (88, 128) and field (92) conditions.

B. Enhanced root establishment and transplant survival. Numerous studies have demonstrated that improved water retention and reduced weed growth are correlated with in- creased root growth and exploration by desirable plants (39, 138). Mulches allow roots of trees and shrubs to extend and establish far beyond the trunk compared to bare soil (17, 134) and thus become increasingly stabilized.

Mulch choice is important in determining how well roots will explore the underlying soil. Root development and den- sity was greatest under organic mulches compared to that under plastic (39), bare soil (134) or living mulches (50). In contrast, sheet and film mulches that act as barriers to water and air movement will encourage root growth on top of the

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mulch (4), which can eventually injure desirable plants when and if the sheet mulch is removed. In another study (58), plastic mulches used with a fertilizer treatment led to in- creased mortality of transplanted materials.

Roots tend to grow into organic mulch layers (134–135), but by and large these are fine roots whose presence is tran- sient. Generally, these roots exploit water and nutrient re- sources in mulch until conditions become unfavorable (e.g. when much begins to dry in the summer). These roots die back and new feeder roots appear where resources are more available. In any case, it does not appear to injure the plant to have roots exploring the mulch layer. However, roots will also colonize landscape fabrics and if these materials are even- tually removed they could cause extensive damage to fine root systems. This is one reason not to use landscape fabrics around woody plants.

If roots can establish successfully, then plant survival is more likely. Thus, use of landscape mulches have been shown to decrease mortality of new transplants even in harsh envi- ronments such as mine tailings (91, 139), saline soils (3, 123) and subarctic systems (58). Enhanced survival through mulch- ing has been seen in nursery and field production (76, 79, 92), silvipasture systems (108), forest plantations (31, 46, 59), and restoration sites (18, 145). Competitive cover crops such as turf will increase mortality of transplants (31).

C. Increased overall plant growth performance. As early as 1942, researchers found that mulched trees grew 67% bet- ter than those grown on bare soil (56). Many others since then have shown similar improvements in growth of trees, shrubs, and other plant materials in field and nursery condi- tions (6, 15, 19, 42–43, 50, 99, 112, 114, 116). Specifically, increases in plant height (6, 18, 73, 76, 91, 102, 108, 115), stem or trunk diameter (6, 31, 50, 91, 107, 108, 115), leaf size and/or number (26, 31, 76, 91, 99), and flower, fruit and/or seed production (99, 127, 131) have all been reported as a result of mulching with appropriate materials.

The best mulches for overall plant performance are or- ganic materials, consistently rated as the best or second best in comparative field trials. Tested mulches include rapid de- composers such as grass clippings, leaves, and compost (50, 99, 107, 115, 127), moderate decomposers including paper (102), hay and straw (26, 73, 108, 131), and other crop resi- dues (6, 76, 115) and slow decomposers, especially bark and woody chips (31, 50, 99, 102). The exceptions to this trend are almost exclusively found in annual crop production re- search, where slow decomposers such as bark can create nu- tritional problems for fast-growing species with limited root mass (26).

Gravel and stone are generally not as effective as organic mulches (6, 108, 112, 115) in optimizing plant performance. Sheet mulches can also produce disappointing results (73, 112). Not surprisingly, competitive ground covers such as turf grasses result in reduced growth (107, 131) even com- pared to bare soil conditions (31, 65).

Reduction of disease. Physically, mulches will reduce splashing of rain or irrigation water, which can carry spores of disease organisms up to the stems or leaves of susceptible species. Additionally, the populations of beneficial microbes that colonize many mulch materials can reduce soil patho- gens either through direct competition for resources or through chemical inhibition. Regardless of the mechanism involved, disease reduction is an important benefit of many mulches.

Researchers have long suspected that mulches play com- plex roles in disease prevention and recovery. Spaulding and Hansbrough (119) found the ‘retention of a normal layer of fallen dead needles under the trees … will help the trees to resume normal growth’ after suffering needle blight. Part of the complexity is that mulches can act indirectly and/or di- rectly to prevent disease establishment. This relationship was explored by Downer et al. (32) who identified both short- and long-term effects of mulching on the incidence of Phytophthora root rot. Indirect effects are both short- and long-term and include increased soil moisture, soil tempera- ture moderation, improved soil nutrition, and improved soil aggregation and drainage. Thus, mulches maintain an opti- mal soil environment, which in turn supports healthy plants that are less susceptible to opportunistic pathogens (130).

Mulches can combat disease organisms directly as well. Researchers have found that Western red cedar (Thuja plicata) heartwood contains thujaplicin, a water-soluble tropolone not only inhibitory to various bacteria and fungi, but with anti- tumor activity as well. This antimicrobial activity is prob- ably responsible for the rot-resistant nature of cedar wood. In addition to plant-derived antibiotics, healthy organic mulches may also contain a variety of soil microbes that can exert biological control over pathogens, either through resource competition or enzymatic degradation (24). Many microbes produce cellulase enzymes that attack the cell wall of patho- gens such as cinnamon fungus (Phytophthora cinnamomi) (32). Mulching soils to encourage populations of indigenous, beneficial soil microbes will increase the effectiveness of biological control in managing disease (38). This may ex- plain why organic mulches such as straw (11) and wood chips (26) are more effective in suppressing disease than landscape fabric and black polyethylene, respectively.

Some mulches, however, can increase the incidence of dis- ease by exacerbating already poor soil conditions (e.g. using a plastic mulch). Sawdust was implicated as a likely medium for shoestring root rot (Armillaria mellea) when used as a garden mulch in 1948 (77). Bacterial soft rot (Erwinia carotovora) was significantly greater in plants grown with a black polyethylene mulch than with bark or wood chips (26). Therefore, selecting an appropriate mulch is crucial as part of an IPM program.

Reduction of weeds. Mulching as a means for landscape weed control is highly effective, though the mechanism(s) responsible for control are not completely understood for all mulch types. Mulches can effectively be used in nursery pro- duction as well as in the field; Wilen et al. (136) found a 92% reduction in weeds of container plants that were mulched rather than left bare. Nearly all mulches reduce light, which will stress existing weeds and prevent the germination of many weed species, especially those with small seeds. A com- parison of 15 mulch types showed that all significantly re- duced weed growth as compared to bare soil, but there were no differences between types tested (122). The physical bar- rier created by other mulches can prevent weeds from emerg- ing, though this effect is temporary and disappears as mulches decompose. Certain organic mulches, especially wood chips, may control weeds chemically through the leaching of al- lelopathic chemicals naturally occurring in the wood. Addi- tionally, the protected soil habitat created by the use of mulches can increase beneficial organisms that prey upon weeds or eat their seeds.

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Living mulches can reduce weed problems through both competition for resources and allelopathy. Ideally, cover crops and ground covers suppress weed seed germination and es- tablishment while having little effect on desirable plants. This ideal is realized in situations where ground covers occupy a different niche than the desirable plants (e.g. trees and large shrubs whose roots are typically deeper than ground covers) (52). If ground covers are too much like other plants in the landscape, such as low-growing herbaceous perennials, then they may compete more directly for limited resources like water, nutrients and sunlight.

Although they can be highly effective in immediately elimi- nating weeds, plastic films and landscape fabrics should not be used as a long-term approach of weed control in land- scapes. White and green plastics do not eliminate photosyn- thetic radiation, thus allowing weeds to continue to grow un- derneath; darker mulch colors will eliminate these wave- lengths and prevent weed growth (57). Regardless of mulch color, eventually, weeds will colonize soil above these mulches and some weeds can pierce and grow through plas- tic films (57). Replacement of plastics and fabrics is not only time-consuming and expensive but also damages the roots of desirable plants that invariably will grow through and over these mulches (4).

In general, inorganic and organic mulches are most effec- tive in weed control when applied at sufficient depth (51) and are least susceptible to compaction (37). Inorganic mulches such as gravel will prevent weed growth if the lay- ers are at least 4 cm (1.5 in) deep (137). Because inorganic mulches do create otherwise optimal conditions (i.e. adequate soil moisture and moderated temperature), the absence of light in these deeper mulches is probably responsible for the lack of germination of weed seeds that require light for ger- mination.

Organic mulches are variable in their weed-controlling abilities. Nutrient-rich, finely textured materials like com- post are not satisfactory mulches for weed control (75, 79, 95). Instead, they act as a fertile base and potential seed bank for establishment of new weeds or enhancement of peren- nial weeds. Weed seeds that settle on top of organic mulches are more likely to germinate, especially if the mulch layers are thin (meaning seedling roots can more quickly reach the underlying soil. Applying two, rather than one organic mulch layer results in significantly less seed germination (103).

Organic mulches that are coarse, applied in thicker lay- ers, and/or less nutrient-rich are more effective in control- ling weeds — sometimes even better than herbicides (18, 44). Locally-derived residues from crops (26, 76, 93, 95) and forest products (19, 26, 37, 46, 57, 95) have all proved effective in reducing weed success in a variety of agricul- tural and landscape situations, especially in uncomposted form (95). While some studies recommend the use of saw- dust for weed control on forest lands (5, 131) or in container production (90), others have noted that thick layers of saw- dust can be impermeable to gas and water movement (120). This material might best be used in situations where soils are less frequently compacted by vehicular or foot traffic: not urban landscapes.

Reduced pesticide use. Mulches reduce plant stress and susceptibility to pests. This important function means that plants will be more resistant to weed invasion and opportu- nistic pests and pathogens, which leads to reduced use of

herbicides, insecticides, and fungicides. Reduction in unnec- essary chemicals not only saves money but also preserves the health of beneficial insects, bacteria, fungi, and other soil organisms that might otherwise be negatively affected.

Aesthetic improvement. Mulches can be beautiful as well as functional; though this is not a scientifically measurable aspect, the fact remains that aesthetics will influence mulch choices and usage. Many mulches, such as ground covers or tumbled glass, can add to a landscape’s design elements while protecting soil. Visually distinctive mulches can be used to control foot traffic by directing pedestrians through a land- scape, which both protects sensitive root zones and adds a design element. Some mulches add other sensory elements in addition to visual interest: smooth rock and soft ground covers invite touching, while fragrant ground covers and fresh organic mulch add enjoyable scents to the landscape. The aesthetic appeal of mulches is critical to their acceptance by consumers, who may otherwise perceive mulches as ‘messy’ and prefer the appearance of bare soil.

Economic value. For any landscape management practice to become widely adopted its economic viability must be established. Many decades of research has demonstrated that mulching improves crop production. Far fewer studies have addressed the economic impacts in urban landscapes and so it is difficult to make detailed economic arguments based solely on tangible costs and benefits.

Over 40 years ago Hunt (59) found that ‘the increase in survival of mulched plants more than compensated for the extra cost.’ Likewise, Brantseg (14) found that retaining log- ging slash increased increment size of new tree seedlings and thus the economic value of replanted pine forests. Not all mulches result in defensible cost:benefit ratios. Paper, plas- tic discs and black polyethylene mulches all failed to im- prove survival and growth for several tree species, causing the researcher to recommend against their use for economic reasons (70). Furthermore, the synthetic mats and films tend to be the most expensive choice (111, 141).

Cost savings have been more specifically identified by oth- ers and include reduced use of pesticides (21, 46) or other weed control methods (46). Utilizing locally-produced woody debris as a production mulch was described as ‘a useful and affordable tool’ with low external input for restoring dam- aged land, improving crop tree growth, and increasing farmer income (91). In a more urban application, brush mulch was found to be ‘applied easily and economically’ during reveg- etation of roadsides (104).

Locally available materials continue to be good economic choices. An early study (131) recommended timber harvest residue as a mulch over peat materials based on both cost and performance. Another study which compared several dif- ferent mulches found unprocessed bark from regional saw- mills to have the greatest cost:benefit ratio as long as the cost did not exceed $2.75 per m3 ($2.10 per yd3) (102). In tropical regions, paddy straw mulch is routinely used and one study found it to have the highest benefit:cost ratio (93).

Mulch Problems — Real and Perceived Acidification. Organic mulches such as wood chips and

bark are thought by some to be soil acidifiers. No scientific research supports this, and in fact studies refute this percep- tion. One study found neither pine bark nor pine needles had

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any affect on soil pH (51). A second report (60) found bare soil to be more acidic than soil covered by inorganic mulch, and that shredded bark and wood chips were least acidifying of all treatments. Similarly, a year-long study found that the soils under organic mulches were either more alkaline or not affected by mulch treatment (100).

It’s likely that in artificial conditions, such as nursery pro- duction, that woody materials do have an acidifying effect when they are used as part of a potting medium. Release of phenolic acids is one stage of the decomposition of woody material, and if this material comprises the bulk of medium then acidification is likely to occur. In a field situation, how- ever, where the woody material is used as a mulch (and not worked into the soil), any acidification will be localized within the mulch layer and have little effect on the vast underlying soil environment below. Thus, soil acidification due to mulch- ing with woody plant material is unlikely to occur under real world conditions.

Allelopathy. Allelopathy is the inhibition of seed germina- tion and growth of plants through the release of chemicals and apparently plays a large part in the weed-controlling be- havior of many organic and living mulches. A few growth- inhibiting substances have been isolated and identified, in- cluding the classic example of juglone (and juglonic acid) which is produced in all parts of black walnut (Juglans ni- gra). Juglone can injure or kill seedlings and shallowly rooted plants, though it apparently has little effect on established plants (54). In laboratory tests, allelopathic activity of a com- pound is usually confirmed by inhibition of seed germina- tion (34) rather than how it affects mature plant materials. Other well-studied, plant-derived natural pesticides, such as those within the Thuja species described earlier, have no dem- onstrated negative effect upon plant materials.

Seeds and seedlings, whether weeds or desirable species, are most sensitive to mulch suppression as they do not have the extensive root systems of established plants. Mulches made of pine, eucalyptus, and acacia (Acacia spp.) were able to suppress germination of several common weed species as were water extracts of these materials, supporting an allelo- pathic function (110). Grasses may be less affected than di- cot weed species (110), and this may help explain the appar- ently contradictory evidence demonstrating that eucalyptus leaf mulch has no effect on rice seed germination (Oryza is a monocot genus) (71).

It is unlikely that any properly applied landscape mulch will have allelopathic effects on established landscape plants, but is most likely to injure newly planted or shallowly rooted plants in the landscape. For such plantings, a short period of composting and correct application of woody mulch will pre- vent damage.

Competition. As mentioned earlier, living mulches can be competitive with landscape plants for water, nutrients, and space. Bedford and Pickering (1919) were perhaps the first to document both the interference of grasses with tree growth, as well as its subsequent recovery upon grass removal (9). Turf and other grasses are very competitive (9, 47), espe- cially during plant establishment (65, 139). Thus, turf grass must be kept away from newly installed shrubs and trees and can easily be replaced with an organic mulch. These ‘tree skirts’ allow rapid root establishment without competition from turf roots (Fig. 1).

Chemical contamination. As with composts, woody mulch quality is influenced by the source of materials. Mulches cre- ated from branches and tree trimmings often contain a diver- sity of leaves, wood, and bark, which contributes to a highly functional mulch. In contrast, woody mulch made from wood recovered from construction and demolition debris can con- tain pressure-treated lumber. In one Florida study, 18 of 22 samples collected from debris processing facilities contained arsenic (from chromated copper arsenate-treated wood) at concentrations greater than the state’s allowable levels (129). Similarly, mill wastes that contained formaldehyde and other wood processing residues reduced survival of tree seedlings when used as a mulch (85).

Disease. Mulches made from diseased plant materials can contain those pathogens. For this reason, many mulches are composted (55) or otherwise treated at temperatures that kill the pathogens along with other harmless or beneficial organ- isms. Therefore, many commercially available organic mulches are relatively sterile.

While mulches made from diseased wood can contain vi- able populations of pathogens such as honey locust canker (Thyronectria austro-americana) (64), few examples of dis- ease transference exist in the literature. Wood chips made from infected maple trees and used as a mulch failed to spread Verticillium spp. to healthy trees (28). Likewise, a wood chip mulch containing the shoestring root rot pathogen did not exhibit disease transmission, perhaps because the organism

Fig. 1. Tree skirt of coarse arborist wood chips creates a competitor- free zone to allow root establishment in turf.

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dried out rapidly (98). Finally, a six-year study of diseased woody mulches found no transmission of either butt rot (Armillaria gallica) or canker (Botryosphaeria ribis) patho- gens to common landscape tree species (61). The only evi- dence of disease transmission occurred when foliage from Austrian pines (Pinus nigra) infected with tip blight (Sphaeropsis sapinea) was used as a mulch around healthy saplings of the same species (61). No trees from other spe- cies were affected, and the author acknowledges that Aus- trian pines are particularly susceptible to this pathogen. It is not surprising that so few examples of mulched-mediated disease transmission have been documented. The pathogen of interest must be present in such a way as to fit the epide- miology of the disease cycle; simply existing in a mulch source is not enough.

While disease transmission from mulch to tree is unlikely, there is greater probability of infection if backfill soil is amended with wood chips. A researcher working with rhodo- dendron (Rhododendron spp.) lost plant material to Phytophthora root rot after amending the soil with 33% composted wood chips (97). A similar transmittal of verticil- lium wilt (Verticillium dahliae) was seen when infected wood chips were used as part of a potting mix (41). Not only is this a poor practice for installing woody plants, it also casts doubt on the efficacy of composting to eliminate pathogens.

Many landscape pathogens are both opportunistic and per- vasive in the soil environment. Armillaria spp., for instance, are widespread in many soils as a decomposer but can be- come pathogenic under unhealthy soil conditions. Healthy soil communities, on the other hand, have diverse fungal and bacterial species, many of which are symbiotic partners of plant root systems. These beneficial species can outcompete pathogens as long as soil conditions remain optimal for root growth. When soils become compacted and anaerobic, plants decline and become susceptible to opportunistic pathogenic microbes — always present but inactive in healthy soils.

Given the distance between wood chip mulch and plant roots, it’s doubtful that pathogens would travel far under healthy soil conditions. Fresh wood chips have been used as a mulch in other long-term studies without any report of dis- ease transmission (43, 95). It does, however, point out the importance of keeping wood chip mulches away from the trunks of trees and shrubs as moist trunk conditions are at risk of pathogen infection. In addition, only unprocessed wood should be used in making wood chips. Mulches de- rived from shipping pallets and other wood packing materi- als, especially if uncomposted, could introduce exotic plant pathogens (64).

Though they do not qualify as disease organisms, other fungal species should be mentioned as possible nuisances in woody mulches. The artillery fungus (Sphaerobolus stellatus) can be found on landscape mulches where it can propel sticky spore masses onto the sides of nearby light-colored cars and houses (13, 55). Spent mushroom compost has recently been identified as an antagonist to this fungus (25) and might be a wise choice as a mulch component in affected landscapes. The colorfully named ‘dog vomit fungus’ (Fuligo septica) — actually a slime mold — creates a bright yellow mass on woody mulches. This is not a pathogenic species but may be of questionable aesthetic value.

Flammability. Though there are documented incidences of spontaneous combustion of yard wastes (16), in general wood-

based mulches are not flammable. A recent comparison of 13 landscape mulches (121) found rubber mulch to be the most flammable, followed by fine textured organic mulches (dried pine needles, straw, shredded bark), coarse textured organic mulches (chipped wood, bark nuggets, cocoa shells), mulches with higher water content (composted yard waste and sod), and finally brick chips (which never ignited). These com- parisons should be carefully considered when mulching in regions where there is significant fire danger.

Nitrogen deficiency. A common misconception about woody mulches is that they impose a nutrient deficiency upon plant materials. This is based on the fact that woody mulches have a high C:N ratio and nitrogen will be ‘tied up’ by mi- crobes during the decomposition process. Furthermore, woody materials that are used as amendments incorporated into soil or potting mixes will create zones of nitrogen defi- ciency, which is visualized by spindly, chlorotic growth of plants in these zones.

Experimental research reveals that neither nitrogen im- mobilization nor growth suppression occurs as a result of using woody materials for mulch (51, 100). To the contrary, many studies have demonstrated that woody mulch materi- als actually increase nutrient levels in soils and/or associated plant foliage (5, 99, 114, 125). A zone of nitrogen deficiency exists at the mulch/soil interface (Chalker-Scott, unpublished data), possibly inhibiting weed seed germination while hav- ing no influence upon established plant roots below the soil surface. For this reason, it is inadvisable to use high C:N mulches in annual beds or vegetable gardens where the plants of interest do not have deep root systems.

Pests. Many organic mulches, especially those based on wood products, have an undeserved reputation as ‘pest mag- nets.’ In fact, many of these wood-based mulches are not attractive to pest insects but are actually insect repellent. Salvia spp., pine needles, and cedar shavings were found to repel fire ants (Solenopsis invicta) under laboratory condi- tions (1). Thuja species have developed a number of chemi- cal weapons against pests including thujone, one of several essential oils found in arborvitae foliage and that of other non-Thuja species. Best known for its ability to repel clothes moths, thujone and other foliar terpenes also repel, inhibit, or kill cockroaches (Blattodea), termites (Isoptera), carpet beetles (Dermestidae), Argentine ants (Iridomyrmex humilis), and odorous house ants (Tapinoma sessile).

A common concern is whether wood-based mulches are attractive to termites. There have been specific studies tar- geting this question with sometimes surprising results. One recent study (74) compared subterranean termite (Reticulitermes virginicus) activity underneath both organic (bark and wood) and inorganic (gravel) mulches. The great- est termite activity was found beneath the gravel mulch. Not only were the wood and bark mulches unappealing to ter- mites, but when fed a diet of these materials in the lab they suffered increased mortality. These results are partially ex- plained by an earlier study (35), which found that termites preferred mulches with higher nitrogen and phosphorus con- tent. Martin and Poultney (76) confirm this in a study dem- onstrating termite partiality for banana mulch, a relatively nutrient-rich material. Therefore, in regions where subterra- nean termites are potential pests, organic mulches should be selected that are low in nutrients.

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Research upon the ability of mulch to exacerbate or con- trol other pest insects and nematodes is sparser and more variable, so it is not easy to draw generalizations. Dry-sur- faced mulches, such as gravel or wood chips, are recom- mended as deterrents to ticks (Ixodes scapularis) that carry Lyme disease (78); other insects (beneficial or otherwise) may likewise be discouraged. Black plastic and landscape fabric may reduce certain pest species in the short term (33, 83), but their long-term negative impacts on soil and plant health are so significant that their perpetual use in a land- scape cannot be recommended. In general, any mulch which increases the general health of the soil environment will un- doubtedly enhance the diversity of beneficial microbes and insects in the landscape (124).

Mulches are variable in their ability to attract or repel mam- malian pests as well. Some materials may naturally repel her- bivorous mammals by virtue of their thorns or odors (20) or texture (125), while others may attract undesirable pests, es- pecially rodents, who can use dense ground covers or sheet- type mulches for shelter (40, 86, 113).

Weed contamination. Mulches lacking pedigrees can be carriers of weed seeds and other undesirable plant parts. While controlled research on this problem is lacking, anecdotal evi- dence suggests that improperly treated crop residues and com- posts as well as bark mulches are often carriers of weed seed. Woody mulches may contain invasive species associated with tree materials that are chipped. Many of these species, such as English ivy (Hedera helix), can easily grow from seed or regenerate vegetatively, thus colonizing landscapes. If it is not possible to document the mulch source, it might be pru- dent to use it on a small area of the landscape and monitor it for problems before using it more widely.

A successful mulch must be deep enough to suppress weeds and promote healthy soils and plants: research has demon- strated that weed control is directly linked to mulch depth (81), as is enhanced plant performance (42). As mentioned earlier, coarse materials are more effective in this regard as their depth will not have the negative impacts found with fine-textured materials.

A review of the research on coarse organic mulches and weed control reveals that shallow mulch layers will promote weed growth (67) and/or require additional weed control mea- sures (144). Coarse organic mulches applied at depths of 7.6 cm (3 in) or less are most likely to fall into this category. On the other hand, mulches applied at 10 cm (4 in) (36) or 15 cm (6 in) (57) were effective in weed control.

Other considerations. In addition to the objective science that should be used to guide landscape management deci- sions, there are other factors to consider. Perhaps most im- portant are those that affect the sustainability of not only the local landscape but global ecosystems as well. While this literature review has focused directly on plant and soil crite- ria associated with landscape mulches, other important en- vironmental issues may well play a role in the ultimate choice of landscape mulch materials. Such issues as environmental degradation associated with acquisition of some mulching materials, use of invasive species as ground covers, nutrient overload from overuse of organic mulches, and toxic leachates from some synthetic mulches, must also be con- sidered.

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