Organic mulches serve several important functions in gardens and landscape plantings.
Mulches help:
- control annual weeds
- conserve soil moisture
- reduce soil erosion by reducing the impact of raindrops and water runoff
- reduce the severity of some diseases, such as blights on tomatoes
- keep fruits, vegetables, and flowers free of rain-spattered soil
- reduce fruit and vegetable spoilage
- moderate soil temperatures
- provide an attractive background for plantings.
These many benefits make the use of mulch very beneficial in a wide range of garden settings. When choosing which organic mulch to use, consider availability, cost, appearance, function, and durability.
Types of Organic Mulches | Mulch Depth | When to Apply | Inorganic Mulches | Potential Problems | More Information
Types of Organic Mulches
Bark
Bark mulches may be sold in bags or bulk. They can be purchased in various particle sizes varying from fine to large chunks. Some are colored/dyed for decorative purposes. Bark mulches are attractive, weed free, and decompose slowly, with cedar and cypress being slowest to decompose.
Use bark mulches around trees, shrubs, and roses, and in perennial beds.
Wood Chips
Wood chips are an excellent mulching material that may be available from local arborists or from municipal or private yard waste sites. The material is obtained by passing tree and shrub trimmings through a mechanical chipper. Sometimes it is processed into chips, and other times it is shredded. Often, shredded wood mulch is less likely to blow or wash away than chips.
Some commercially available wood chips are made by shredding or chipping lumber or pallets. This product is inferior to those processed directly from tree material, as it often contains low quality wood that will break down quickly and may contain chemicals or other compounds. Some wood chip mulches are processed from this inferior wood source and colored or dyed to mask it.
Wood chips are best used in landscape plantings, such as around trees, shrubs, and roses, and in perennial beds.
Leaves
Leaves should be shredded or composted before being applied as a mulch. Shredded or composted leaves do not mat down as readily as whole leaves, are less likely to blow away in the wind, and decompose more quickly.
Shredded or composted leaves are an excellent mulch for vegetable gardens, annual flower beds, raspberry plantings, and around perennials, trees, and shrubs. Leaves are a poor winter mulch for strawberries and herbaceous perennials because they tend to mat down and smother plants.
Grass Clippings
Allow lawn clippings to dry before applying to gardens. Fresh, green material may settle and form a dense mat or produce an unpleasant odor. If the lawn has been treated with a broadleaf herbicide, don’t use the clippings until the lawn has been mowed two or three times after the application. Grass clippings from a weed-infested lawn will undoubtedly contain a large amount of weed seed.
Grass clippings decompose quickly making them great to use in vegetable gardens or annual flower beds.
Straw
Wheat, oat, or soybean straw that is free from crop and weed seed can make an excellent short-term mulch. Straw is also used as an overwintering mulch to insulate cold-tender plants during the winter months because it is less likely to mat or smother the crowns of plants. Straw may provide a winter habitat for mice and other rodents, so avoid using straw around trees and shrubs.
Straw is an excellent mulch for the vegetable garden and strawberry bed. It is an ideal mulch for overwinter protection of perennials, roses, and other tender plants.
Newspapers and Cardboard
Shredded newspapers or whole sheets may be used as mulch. Most newspapers use organic inks so gardeners need not worry about lead contamination. Avoid the use of waxed cardboard and glossy paper, such as magazines. Remove packing tape and staples from boxes. When using newspaper sheets, place a layer of 2 or 3 sheets (up to 10 layers thick) between plant rows in the garden. Layer cardboard 1 or 2 layers thick, being sure to overlap the edges. Water the sheets so they stick to one another and to the soil surface, then weigh them down with soil to prevent them from blowing away in the wind. Newspaper and cardboard can also be successfully used with other mulches. Cover newspaper and cardboard with a layer of wood mulch or compost to help hold it in place and prevent it from blowing away. Landscape staples can also be used to hold cardboard in place.
Use newspaper and cardboard as a mulch in vegetable gardens, garden pathways, and around trees and shrubs.
Cocoa Bean Shells
Cocoa bean shells, also called coco hulls, are a by-product of chocolate production. They are light, easy to handle, and have an attractive brown color. They also have a delightful aroma. (Unfortunately for chocolate lovers, the aroma lasts for only a few days.) They are somewhat expensive, but a mulch depth of 1 to 2 inches is sufficient. Dog owners should be aware that dogs may develop signs of chocolate poisoning, such as vomiting, if they eat large amounts of cocoa bean shell mulch. Owners of dogs with indiscriminate eating habits should use alternate mulches.
Cocoa bean shells are excellent mulches for annuals, large containers, perennials, vegetable gardens, and roses. They generally last only one growing season.
Pine Needles
Pine needle mulch, or pine straw, is light, airy, decomposes slowly, and makes an attractive mulch. It may last several years and may be easily removed if necessary. Pine needles are acid in reaction and are excellent mulches for acid-loving plants, although their effect on soil pH is relatively minor. The best source of pine needles is a large, established windbreak.
Use pine needles around trees and shrubs, perennials, and in areas like the vegetable garden, although this mulch will be present for several growing seasons so you may have to remove it from a vegetable garden at the end of the season.
Sawdust
Sawdust is easy to apply, weed free, and decomposes slowly. Generally, sawdust should be allowed to age or compost for a year before being applied. If fresh sawdust is used, apply only a 1-inch layer and make sure the sawdust doesn’t crust over, reducing water infiltration. Do not use sawdust from treated lumber in the yard and garden.
Use sawdust in vegetable gardens, perennials plantings, or around trees and shrubs.
Compost and Leaf Mold
Compost can be layered on top of the soil as a mulch material or used in conjunction with other mulches, such as shredded leaves or newspaper. While this mulch material will not have the same level of weed suppression as other organic mulches, it can help to improve soil structure and drainage, as well as increase soil fertility.
Leaf mold is partially decomposed leaf matter. A pile of leaves around 3 feet tall and wide will transform into leaf mold over a period of one to two years. Shredding the leaves allows it form faster.
Compost and leaf mold can be used in nearly all garden settings, including perennial and annual beds, vegetables gardens, and around trees and shrubs.
Mulch Depth
The optimal depth of mulch will vary depending on soil texture, climate, type of mulch, age of plants, and management objectives. In most cases, a layer 2 to 4 inches thick works well.
Mulch can be applied under the drip line of mature trees to a greater depth than in a bed containing annual and perennial herbaceous plants.
A thin layer of mulch does not suppress weeds or conserve moisture as effectively as a deeper layer. A thin layer also needs to be replenished more often, which increases maintenance costs.
On the other hand, an excessively deep mulch layer can promote waterlogging of heavy soils, decrease soil oxygen levels, result in shallow rooting, and keep soils too warm during winter.
When to Apply Mulches
Mulches, including compost, can be applied at any point during the growing season. Although it is most convenient for most gardeners to do it in early spring before planting or before plants emerge, or in the fall as plants start to go dormant but before the ground freezes.
For early spring applications, it is typically best to wait until the ground starts to warm and the perennials are just emerging as putting down a layer of mulch too early can further insulate the ground and slow plant emergence. Plus, the mulch pile may still be frozen anyway!
For late-season applications, the mulch can help protect newly planted perennials or other plants from harsh cold temperatures. Ideally, the mulch would go down after plants go dormant in the fall. Do not place the mulch on too early as it can slow plants from going dormant and make them more susceptible to damage from cold temperatures.
Inorganic Mulches
Inorganic mulches, like rock and plastic sheeting, can be used but have limitations.
Common inorganic mulches include:
- river rock
- lava rock
- pea gravel
- crushed brick
- limestone gravel
- caramel rock
- rubber chips
- woven landscape fabric
- plastic sheeting
Inorganic mulches do not contribute to building soil fertility or composition. Most will help warm soils quickly in the spring and keep them warmer in the fall but may keep soils too warm in the summer months. These mulches can be effective at weed suppression, but organic material and soil will build up on top of them over time, leading to weed issues. They are typically difficult to garden in, making planting and transplanting challenging. Some inorganic mulches like plastic sheeting will prevent water infiltration.
Landscape fabric used in conjunction with organic mulches can create problems as well. The organic mulch, such as wood chips, does not mix with the soil and therefore many of its benefits, such as increasing soil fertility or improving soil structure, are not realized. Over time, weeds will establish on top of the fabric in the mulch. Additionally, organic mulches placed on top of landscape fabric are more likely to be washed or blown away because they are not in contact with the soil.
Potential Problems with Mulch
Overall the most significant problems with mulch come with inproper use, such as piling it too deeply. Other issues with organic mulches are rare and often relatively minor. The benefits of mulch far outweigh the potential problems.
Mulch “Volcanoes”
Mulch piled high (more than 2-4”) against tree trunks is a common sight in some landscapes, especially those around commercial or municipal properties. This deep pile at the base of the tree is referred to as a “mulch volcano”. The practice is problematic because trunks encircled by mulch stay constantly moist, which interferes with respiration of cambium, phloem, and other living cells in the trunk by limiting their exchange of oxygen and carbon dioxide with the atmosphere. Furthermore, this practice also creates conditions that favor infection by plant pathogens causing cankers and root rots and it favors moisture-loving insects, such as carpenter ants, which could colonize and expand decayed areas of the trunk.
To prevent these problems, it is advisable to keep mulch no more than 4” deep and to feather it away from the trunks of woody plants.
Mushrooms, Fungus, Slime Molds
Mulches can support decay fungi that can become a nuisance in certain situations. Stinkhorn fungi (Mutinus caninus and M. elegans), artillery or shotgun fungus (Sphaerobolus stellatus), bird’s nest fungus (Cyathus striatus), and slime molds may colonize organic mulch. These organisms can be unsightly or, in the case of shotgun fungus, can stain nearby buildings and surfaces.
When fungus appears, in most cases it can be raked or scooped up and disposed of. Management strategies are preventive in nature. Dry mulches (moisture content less than 34 percent) that are high in wood content cause most problems since fungi are the primary colonizers of dry wood. Moisture levels greater than 40 percent foster the growth of bacteria, which compete with nuisance fungi, reducing their potential to cause problems. Most problems with nuisance fungi can be avoided by composting woody mulch before use, thoroughly soaking mulches after they have been applied, and avoiding sour mulches.
Insects
Mulches provide shelter, moisture, or food for many different insects and related organisms. However, few insects found in mulch are destructive or harmful. Most mulch inhabitants are beneficial or innocuous. Presence of insects in mulch generally is not a cause for alarm or treatment but rather suggests the existence of an environment similar to natural ecosystems. Mulch provides a habitat for beneficial insects that are important predators of insect pests, including rove beetles, ground beetles, firefly larvae, and centipedes. Other beneficial organisms are “recyclers” that feed on fungi and decaying plant debris. The decomposition of organic mulch by recyclers, such as decay fungi, ants, sowbugs, millipedes, springtails, and mites, converts the mulch into valuable organic matter that improves soil tilth.
Mulch may increase the number of certain plant pests by providing ideal living conditions. Examples include slugs, snails, sowbugs, and earwigs. Treatments are available for combating these plant feeders, so eliminating mulch is not necessary. Ants that live in the moist, loose soil under mulch are not harmful to people or structures and are generally beneficial to the soil and plants. Unfortunately, some species of ant occasionally wander indoors while foraging for food. These nuisance insects are present around homes whether mulch is present or not.
Carpenter ants live in galleries chewed into decayed wood, such as stumps, logs, firewood, hollow trees, and dead limbs. These familiar, large, black ants do not nest in wood chips. They may forage for food, such as dead insects, in mulch but they do not live there. Carpenter ants are best controlled by locating and treating their nest.
Termites routinely feed on woody mulch and other wood products on or in the soil (lumber scraps, boards, firewood, pallets, etc.). However, there is no evidence that mulching with wood chips attracts termites that were not already present in the area or increases the incidence of termite damage. Any form of mulch, organic or inorganic, can provide the moist environment termites use for foraging, exploration, tunneling, and feeding. If termites are present in your area, mulches should be inspected regularly for signs of activity. To minimize the risk of infestation, mulch should be kept several inches away from the house foundation. Mulch that covers windowsills or contacts siding may provide termites direct and undetected access into homes.
Soil Nitrogen and Oxygen Depletion
Organic mulches can influence soil microbial activity and nutrient availability. Mulches with a high carbon-to-nitrogen (C:N) ratio, such as hardwood bark, ground wood pallets, straw, and sawdust, may induce nitrogen deficiency in plants by stimulating microbial growth, which depletes underlying soils of available nitrogen. Some mulches, such as cypress, are resistant to decomposition and thus have a much lower effect on nutrient availability, despite having a higher C:N ratio.
The potential nitrogen-depleting effect of mulch diminishes over time as mulch decomposes. Nitrogen immobilization by microbes will have a greater impact on herbaceous plants and newly transplanted woody plants than on well-established trees and shrubs. Avoid the extensive use of high C:N ratio mulches around these plants. Alternatively, these products can be blended with composted materials with a low C:N ratio, such as yard waste, animal manure, or sewage sludge. If possible, composting wood chips before use will further decrease their C:N ratio, making them safter to use around sensitive plants.
If mulch is piled too deep or if its texture is fine (for example, sawdust), air may not be able to penetrate the mulch layer, and the underlying soil can become depleted of oxygen. Coarse textured mulches piled deep are much less likely to prevent air penetration, but finer textured mulches piled deeply (greater than 10 inches) are likely to block oxygen movement to the soil because the mulch itself uses all the available oxygen. Mulch 2 to 4 inches thick suppresses almost all weed growth and results in more shoot growth from woody plants than either an unmulched treatment or mulch applied to depths of 6 to 10 inches.
Sour Mulch
If mulch is produced under low-oxygen conditions, it can turn “sour.” Anaerobic microorganisms are favored when oxygen availability is limited because of excessively large mulch piles or saturated conditions. When oxygen levels are low, fermentation occurs, producing compounds such as methanol and acetic acid, which are toxic to plants. Annual bedding plants, herbaceous perennials, and low-growing woody shrubs are the most vulnerable to poisoning by sour mulch. Symptoms include marginal leaf chlorosis or scorch, leaf abscission, and in severe cases even plant death. The symptoms may appear within hours to a few days after sour mulch is applied. Sour mulch can be recognized by its strong odor of vinegar or silage. Properly stored mulch will smell like freshly cut bark or fertile garden compost. To keep mulch from souring, store it in smaller piles (no more than 10’ high) on a crowned surface so that rainwater drains away. Turning the pile regularly is also helpful.
More Information
- Organic Mulches (pdf)
- Using Mulches in Managed Landscapes (pdf)