Organic mulches serve several important functions in gardens and landscape plantings.
Mulches help:
- control annual weeds
- conserve soil moisture
- moderate soil temperatures
- reduce soil erosion by reducing the impact of raindrops and water runoff
- reduce the severity of some diseases, such as blight on tomatoes
- keep fruits, vegetables, and flowers free of rain-spattered soil
- reduce fruit and vegetable spoilage
- provide an attractive background for plantings.
These many benefits make 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, available in bags or bulk, come in various particle sizes, varying from fine to large chunks. Some are colored/dyed for decorative purposes. They are attractive, weed-free, and decompose slowly, with cedar and cypress being the slowest to decompose.
Use bark mulches around trees, shrubs, and roses, and in perennial beds.
Wood Chips
Wood chips, available from local arborists, municipal facilities, or waste sites, are produced by chipping or shredding tree and shrub trimmings. Shredded 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. They are an inferior product, breaking down quickly and potentially containing chemicals. These products are sometimes dyed to mask their origin.
Wood chips are best used in landscape plantings, such as around trees, shrubs, and roses, and in perennial beds.
Leaves
Shred or compost leaves before using them as mulch to prevent matting, make them less likely to blow away, and speed up decomposition.
They're great for vegetable gardens, annual flower beds, raspberry plantings, and around perennials, trees, and shrubs. However, they're poor winter mulch for strawberries and herbaceous perennials because they tend to mat down and smother plants.
Grass Clippings
Let lawn clippings dry before applying to gardens, as fresh clippings may form a dense mat or produce an odor. Avoid using clippings treated with broadleaf herbicide until after two or three mowings. Clippings from weed-infested lawns will contain weed seeds.
Grass clippings decompose quickly, making them ideal for vegetable gardens and annual flower beds.
Straw
Wheat, oat, or soybean straw free of seeds makes excellent short-term mulch and overwintering insulation for cold-tender plants. It’s less likely to mat and smother crowns but avoid using it around trees and shrubs as it may provide a winter habitat for mice and other rodents
Straw is an excellent mulch for the vegetable garden and strawberry bed. It is an ideal mulch for overwintering protection of perennials, roses, and other tender plants.
Newspapers and Cardboard
Shredded newspapers or whole sheets can be used as mulch. Most newspapers use organic inks, so there is no worry about lead contamination. Avoid waxed cardboard and glossy paper like magazines. Remove tape and staples from boxes.
Place 2-3 layers of newspaper (up to 10) or 1-2 layers of cardboard, making sure to overlap the edges. Avoid placing too many layers down as it can potentially inhibit water and air movement into the soil, negatively affecting the soil health. Water them so they stick to one another and the soil surface, then weigh them down with soil to prevent them from blowing away. Cover with wood mulch or compost to help further keep them in place and improve appearance. Landscape staples can also be used to hold cardboard down.
Use newspaper and cardboard as 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, light, easy to handle, and have an attractive brown color with a short-lasting aroma. They're somewhat expensive, but a depth of 1 to 2 inches is sufficient. Dog owners should monitor closely as dogs might develop signs of chocolate poisoning if ingested.
Cocoa bean shells are excellent mulches for annuals, large containers, perennials, vegetable gardens, and roses. They typically last about one growing season.
Pine Needles
Pine needle mulch, or pine straw, is light, airy, slow to decompose, and attractive. It lasts several years and can be easily removed if necessary. Pine needles are slightly acidic and great 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 in the landscape, especially around trees and shrubs, perennials.
Sawdust
Sawdust is easy to apply, weed-free, and decomposes slowly. Compost or age sawdust for a year before use, or apply a thin layer (1 inch or less) if fresh, ensuring it doesn't crust over, reducing water infiltration. Avoid using sawdust from treated lumber.
Use sawdust in vegetable gardens, perennial plantings, or around trees and shrubs.
Compost and Leaf Mold
Compost, used alone or with other mulches like shredded leaves or newspaper, helps improve soil structure, drainage, and fertility but doesn't suppress weeds as well as other mulches.
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; faster if shredded.
Compost and leaf mold can be used in nearly all garden settings, including perennial and annual beds, vegetable gardens, and around trees and shrubs.
Mulch Depth
The optimal mulch depth depends on factors like soil texture, climate, mulch type, plant age, and management objectives. Generally, 2 to 4 inches is effective, with coarser mulches (like wood chips) on the higher end and finer mulches (like grass clippings) on the lower end. Under mature trees, mulch can be deeper than in beds of annuals and perennials.
A thin mulch layer is less effective at weed suppression and moisture conservation and needs frequent replenishment increasing maintenance time and costs. Conversely, an excessively deep mulch layer can lead to waterlogging, reduced soil oxygen levels, shallow rooting, and keeping soils overly warm in winter.
Once you know your desired mulch depth, use this article to determine how much mulch you need to purchase: How to Determine the Amount of Mulch Needed for a Garden Bed.
When to Apply Mulches
Mulches, including compost, can be applied at any point during the growing season. For many gardeners, it is most convenient to apply it in early spring before planting and before plants emerge, or in the fall as plants start to go dormant (or just after).
In early spring, wait until the ground warms and perennials emerge. 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!
In the fall, mulch can help protect newly planted perennials or other plants from harsh cold temperatures. For late-season applications, apply mulch after plants go dormant in the fall, but not too early, as it can delay dormancy and increase cold damage risk.
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 warm in the fall, but can keep soils too warm in the summer. They suppress weeds for several years, but organic material and soil will build up on top of them over time, leading to weed issues. They can be difficult to garden in, making planting, transplanting, and weeding challenging. Some inorganic mulches, like plastic sheeting, will prevent water infiltration.
Landscape fabric used in conjunction with organic mulches can be problematic. Organic mulch, like wood chips, does not mix with the soil. so they don't improve soil fertility or structure. Weeds will eventually grow 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
The most significant problems with mulch come with improper use. 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 called a "mulch volcano". It is seen in some landscapes, especially those around commercial or municipal properties.
This practice keeps trunks constantly moist, interfering with the respiration of cambium, phloem, and other living cells in the trunk by limiting their exchange of oxygen and carbon dioxide with the atmosphere. 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.
To prevent these problems, keep mulch no more than 4” deep and feather it away from the trunks of woody plants.
Mushrooms, Fungus, Slime Molds
Mulches can support decay fungi, like stinkhorn fungi (Mutinus caninus and M. elegans), artillery or shotgun fungus (Sphaerobolus stellatus), bird’s nest fungus (Cyathus striatus), and slime molds. These nuisance fungi can be unsightly or, in the case of shotgun fungus, can stain nearby buildings and surfaces.
Fungi can be raked or scooped up when they appear. Management strategies are preventive in nature. Dry mulches with high wood content (less than 34% moisture) cause most problems since fungi prefer these conditions. Moisture levels greater than 40% foster bacteria growth, which competes with nuisance fungi. 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, and food for many insects, most of which are beneficial or innocuous. These insects suggest a natural ecosystem and include predators of insect pests, like rove beetles, ground beetles, firefly larvae, and centipedes. Recyclers, like decay fungi, ants, sowbugs, millipedes, springtails, and mites, feed on fungi and decaying plant debris improving soil quality.
However, mulch can also increase pests like slugs, snails, sowbugs, and earwigs. Treatments are available, so eliminating mulch is unnecessary. Ants in mulch are generally beneficial but may occasionally wander indoors. 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 large, black ants 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 not already present in the area or increases the incidence of termite damage. If termites are present in your area, mulches (organic or inorganic - both are foraging grounds for termites) 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, stimulate microbial growth, which depletes available nitrogen in the soil potentially causing nitrogen deficiency in plants . Organic mulches with a high C:N ratio that resists decay, like cypress mulch, have a lower effect on nutrient availability because they decompose slowly.
The nitrogen-depleting effect decreases as mulch decomposes. This impact is greater on herbaceous plants and newly transplanted woody plants than on established trees and shrubs. Alternatively, these products can be blended with composted materials that have a low C:N ratio, such as yard waste, animal manure, or sewage sludge. 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 fine-textured, it may block air penetration and deplete soil oxygen. Coarse textured mulches piled deep are much less likely to prevent air penetration. 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. A mulch layer of 2 to 4 inches thick (closer to 2 inches for fine-textured; up to 4 inches for coarse-textured) suppresses weed growth and promotes shoot growth in woody plants better than deeper or unmulched treatments.
Sour Mulch
When mulch is stored in excessively large piles or overly saturated conditions, it creates low-oxygen conditions. This can turn the mulch "sour" due to fermentation caused by anaerobic microorganisms producing methanol and acetic acid, which are toxic to plants. Vulnerable plants include annuals, perennials, and low shrubs. Symptoms include leaf chlorosis, scorch, abscission, and even plant death, appearing within hours to days.
Sour mulch has a strong odor of vinegar or silage, while properly stored mulch smells like freshly cut bark or fertile garden compost. Prevent sour mulch by storing it in smaller piles (no more than 10' high), ensuring good drainage, and turning the pile regularly.
More Information
- Organic Mulches (pdf)
- Using Mulches in Managed Landscapes (pdf)
- How to Determine the Amount of Mulch Needed for a Garden Bed
- How to Overwinter Plants
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