How to apply mulches
What is mulch?
Mulch is a protective covering that's spread or left on the ground to reduce evaporation, maintain even soil temperature, prevent erosion, control weeds, and enrich the soil. Mulch can also keep ripening fruits and vegetables from contact with the soil, keeping them clean and preventing them from rotting.
Types of mulch
- Bark chips are the most common landscape mulch: they're attractive and resist insects and artillery fungus.
- Wood chips are an excellent mulch, especially good for garden paths since they don't compact under foot traffic. Disadvantages: they can blow away, and like other wood products, can tie up nitrogen.
- Shredded leaves are cheap, contain lots of nutrients, and they're everywhere! Run over them with the lawnmower to chop them up before using as a mulch: this keeps them from matting, allowing air and water penetration.
- Pine needles are good for acid-loving plants. They're light, clean, weed-free, and easy to work with.
- Straw should be chopped and free of seeds and is often used for newly-seeded lawns. It's also good for vegetable gardens. Disadvantages: it can be a fire hazard. Also, small animals such as mice like it for bedding and may take up residence.
- Salt marsh hay is a great mulch available in coastal areas. It's good for vegetable gardens as it contains no weeds (unlike regular hay) and is a great winter mulch.
Below are organic mulches that are good, but that may be best used for other purposes.
- Grass clippings are best left on your lawn. While they're a good cheap mulch, if spread too thickly, they get slimy and have a foul odor. Also, if the lawn has weeds, so will your mulched area.
- Compost is a super mulch, but unless you have an endless supply, it's probably best used to feed plants rather than mulch them.
Also consider inorganic mulches, such as those below.
- Stone mulches—including gravel and crushed marble—are decorative and used mostly around trees, shrubs, and other ornamentals. Stone is water permeable and retains the sun's heat, warming soil underneath. On the downside, stone is expensive and heavy, and weeds can often grow through.
- Geotextiles are also called "landscape fabrics." Most are made from polypropylene—a petroleum byproduct—or polyester and are not recommended for a "green" landscape. Look for newer geotextiles made from coir and other natural fibers.
- Plastic mulches are usually made from polyethylene and are not recommended. Look for newer "plastic" mulches made from materials such as corn starch.
Mulches that are NOT recommended
The following mulches are not recommended due to health and environmental risks. See Related health issues below:
- Rubber "playground" mulch
- "Bark mulches" containing old asphalt roofing
- Cocoa bean shells
How much mulch?
- The recommended mulching depth, depending on the type of mulch, is 2 to 2.5 inches. At this depth, most mulches will control weeds and conserve moisture in the soil.
- To cover a 100-square-foot area, it takes about one-third of a cubic yard of mulch.
- Mulch shouldn't be more than 3 to 4 inches deep. Too much mulch repels water, invites unwanted insects, and may cause decay on tree trunks.
When to mulch
- Apply mulch in the spring after the soil has warmed and begun to dry. Depending on soil texture, temperature, and other weather conditions, this is usually mid to late spring.
- Mulching too early can result in soil taking too long to dry out. This delays root growth, which depends on sufficient aeration (oxygen content in the soil) and a reasonably warm soil temperature. In moist or cool climates, mulching should be delayed until late May or June.
- Mulch after it rains, when the ground is moist but not waterlogged.
- Cultivate around plants first. Loosen soil and remove weeds.
- Apply mulch after seeds are established otherwise you risk covering the seeds. Keep an area six inches around seedlings free of mulch.
- Use a shovel or garden fork (depending on the type of mulch) to put the mulch where you want it.
- Use a garden rake to spread the mulch evenly.
Before you buy
Before purchasing mulch, you might want to look for sources of free mulch, including public composting sites, tree trimming services, and local power companies. Buying in bulk can also save money, if you have a pickup truck to haul your own. Some landscape and garden centers will deliver a truckload within a certain distance. If you don't need that much mulch, consider splitting a load with a neighbor. Buying in bulk rather than in bags also eliminates unnecessary packaging. To find out more about how to cut your mulching costs, check out LessLawn - Mulch for Less Moola.
Also beware of buying "sour mulch" which is toxic to plants. Mulch becomes toxic from improper composting, when conditions become anaerobic, or low in oxygen with a water content of over 40 percent. The resulting byproducts—methane, alcohol, ammonia gas, or hydrogen sulfide gas—build up to levels that can damage or kill plants. Hardwood bark is susceptible to this problem; pine bark is generally not. Healthy mulch smells like newly-sawn wood or garden soil. Sour mulch smells like vinegar, ammonia, or sulfur. If it doesn't smell fresh, don't buy it.
Find it! Mulches and Mulching Books
Bio-Film Mulch is made of cornstarch, biodegradeable and compostable polyester, and vegetable oil. A company trial showed that 90 percent of the mulch—which looks like black plastic—biodegrades in 90 days, blocking weeds until plants are established.
Applying mulches helps you go green because…
- They prevent water loss and conserve precious water.
- They suppress weeds and eliminate the need for toxic herbicides.
- They cut down on plant disease and eliminate the need for toxic fungicides.
- They improve the soil and lessen the need for fertilizer.
Using mulches is a great way to conserve water. A study by Weyerhaeuser indicated that 2 inches of bark mulch reduced moisture loss in summer by 21 percent. The future of water, and consequently food (due to the need for irrigation) is highly unpredictable. Over 1 billion people around the world don't have enough potable water to meet minimum health and income requirements.
The United Nations projects that the human need for water could double over the next 50 years. While factors such as weather are uncontrollable, other critical factors can be affected by the collective choices of the world's people, including allocation of water for personal use. Americans use tens of thousands of gallons of water on their lawns, gardens, and landscape plantings every year. Outdoor landscape watering accounts for 20–50 percent of all residential water use in the US (more in some areas of the country).
Using mulches also prevents the use of harmful herbicides (weed killers) such as atrazine. The Environmental Working Group found agricultural weed killers—including atrazine, cyanazine, metolachlor, and acetochlor—in the tap water of 28 of 29 cities tested. In 13 cities, the average level of weed killers in the tap water exceeded federal standards. The commonly used herbicide atrazine has been shown to cause severe hormonal damage to wildlife, including amphibians, reptiles, and fish. The European Union (EU) banned the herbicide because of safety concerns in October of 2003, while the US Environmental Protection Agency (EPA) chose not to restrict the herbicide, despite hundreds of pages of research findings by the EPA's own scientists detailing the environmental and health risks of the chemical.
Related health issues
When mulch is shoveled or turned, it may emit a cloud of
grayish, smoke-like material. This cloud is made up primarily of spores of actinomycetes (microscopic bacteria) and fungi, including Penicillia and Aspergilli. In 5 to 10 percent of people, inhaling these microbes produces an allergic reaction, the result of which may be an asthma-like fluid buildup in the lungs commonly called “farmer’s lung.” The allergic reaction can be mild to severe, depending on the severity of a person’s allergy and the extent of exposure.
The US Centers for Disease Control (CDC) documented cases in which the inhalation of organic dust contaminated with microbes created an occupational hazard for persons who work with mulch. On June 21, 1983, five employees at a municipal golf course became ill with a flu-like syndrome after unloading a trailer truck full of wood chips from an enclosed trailer. Along with the order of fresh wood chips, the supplier included old chips that had been stored in the front of the truck for about a year. These old chips were extremely moldy, and cultures revealed they contained a wide variety of mesophilic and thermophilic bacteria and fungi. All five workers who unloaded the moldy wood chips suffered an acute toxic reaction from inhaling large amounts of dust heavily contaminated with microbial toxins contained in the decomposing wood.
"Playground" rubber mulch was shown by The Connecticut Agricultural Experiment Station to outgas and leach high levels of benzothiazole; butylated hydroxyanisole; n-hexadecane; and 4-(t-octyl) phenol. About two dozen other chemicals were found at lower levels. Benzothiazole causes skin and eye irritation. Butylated hydroxyanisole is a known carcinogen, gastrointestinal toxicant, immunotoxicant, neurotoxicant, skin and sense-organ toxicant, and suspected endocrine toxicant. The chemical 4-(t-octyl) phenol is corrosive and destructive to mucous membranes. The toxins released from recycled crumb rubber used in rubber mulch can cause severe irritation of the respiratory system; severe irritation of the eyes, skin, and mucous membranes; systemic effects on the liver and kidneys; neurotoxic responses; allergic reactions; cancers; and developmental effects. A 2006 Rutgers University study of "tire crumbs" from synthetic turf fields in New York City found six polycyclic aromatic hydrocarbons (PAHs)—which are suspected carcinogens—at levels exceeding those set by the state of New York. Skin, eye, and respiratory irritation are the most common reactions to rubber mulch.
In 2004, Oregon's Department of Environmental Quality (DEQ) tested samples of "bark mulches" composed of old asphalt roofing wastes sold in the state under the brand names “Budget Bark” and “No Spark Bark.” The results showed levels of contaminants in quantities of concern for human health and aquatic life, specifically unsafe levels of PAHs and arsenic. Arsenic and several of the PAHs are classed as probable human carcinogens by the EPA. The DEQ advises homeowners and landscapers to avoid the use of these two products or any other mulches containing asphalt roofing wastes.
Cocoa bean shells, sold as mulch, are a byproduct of chocolate production. Dogs are sensitive to theobromine and caffeine, chemicals called methylxanthines. Dogs who eat cocoa bean shell mulch could develop methylxanthine toxicosis, the signs of which are similar to those in chocolate poisonings. Vomiting and muscle tremors are the most common symptoms. Other symptoms include rapid heart beat, hyperactivity, and diarrhea. Dr. Steve Hansen, director of the American Society for the Prevention of Cruelty to Animals' (ASPCA) Poison Control Center, said that there have been no reports of lethal poisonings from ingesting cocoa mulch, but that the center has received reports of [dogs] ingesting the mulch. The Hershey Company, which produces 100 percent organic HERSHEY'S Cocoa Mulch, using byproducts of its chocolate making operations, warns on the company website that dogs are "sensitive to … theobromine, which can lead to toxicity and even death in some animals." Cocoa shell mulch packaged by the Hershey Company contains a warning on each bag. The company notes that horses are also sensitive to theobromine, but cattle are less so. Cats are somewhat sensitive, but are "more discriminating in their diets" [than dogs].
After Hurricanes Katrina and Rita hit New Orleans, there were many downed trees. Rumors spread that these trees, which contained Formosan termites, were being turned into mulch to be sold by major do-it-yourself chain stores such as Home Depot and Lowe's. This story is an urban legend, however two facts of the story are true: Formosan termites are prevalent in the parishes affected by the hurricanes, and a large amount of cellulose debris resulted from the two hurricanes. The state of Louisiana, however, took measures to prevent the spread of Formosan termites in mulch and other cellulose-based products.
Also, large chain stores such as Home Depot and Lowe's have strict quarantine regulations, and don't sell mulch from suppliers in southern Louisiana. While finding Formosan termites in your mulch is highly unlikely, Louisiana State University entomologist Dennis Ring recommends thoroughly inspecting wood, paper products, and potted plants to prevent an infestation.
- VideoJug - How To Apply Mulch To Your Garden Bed Watch a video on how to apply landscape mulch.
- Iowa State University Extension - Using Mulches in Managed Landscapes Read this 12 page illustrated bulletin on landscape mulching, which also discusses mulching problems and solutions.
- Ohio State University Extension - Mulching Landscape Plants Learn how to mulch landscape plants from Cooperative Extension experts.
- The Master Gardeners - Mulching—More is not Always Better Learn how—and how not—to mulch from a Master Gardener from Adams County, Pennsylvania.
- North Carolina Cooperative Extension Service - Mulching Trees and Shrubs
- Campbell, Stu (2001) Mulch It!. North Adams, Massachusetts: Storey Publishing: 23
- Campbell, Stu (2001) Mulch It!. North Adams, Massachusetts: Storey Publishing: 27-47
- Ohio State University Extension - Mulching Landscape Plants
- Reich, Lee, "Digging into Mulches" Old House Journal 35, no. 3 (May/June 2007): page
- Landscape-America.com - Top 10 landscaping mistakes
- Campbell, Stu (2001) Mulch It!. North Adams, Massachusetts: Storey Publishing: 79
- Campbell, Stu (2001) Mulch It!. North Adams, Massachusetts: Storey Publishing: 78
- LessLawn - Mulch for Less Moola
- Virginia Cooperative Extension - Beware of Sour Mulch
- Highbeam Encyclopedia - Will the world run dry? Global water and food security
- The New York Academy of Sciences - A Course in Good Water
- The National Audubon Society - Conserve Water
- Environmental Working Group - Weed Killers by the Glass
- Union of Concerned Scientists – Chemical Industry Pressures EPA to Protect Herbicide, not Wildlife
- Iowa State University Extension - Using Mulches in Managed Landscapes
- US Centers for Disease Control - Acute Respiratory Illness Following Occupational Exposure to Wood Chips—Ohio
- Environmental and Human Health, Inc. - Exposures to Recycled Tire Rubber Crumbs Used on Synthetic Turf Fields, Playgrounds and as Gardening Mulch
- State of Oregon Department of Environmental Quality - Consumer Alert: Bark Mulches Made from Roofing Wastes May Be Unsafe to Use
- American Veterinary Medical Association - Danger to dogs from cocoa bean mulch put in perspective
- HERSHEY'S Cocoa Shell Mulch - Frequently Asked Questions
- Louisiana State University Agricultural Center - Efforts Under Way To Prevent Spread Of Formosan Subterranean Termites In Mulch From Louisiana Following Hurricanes Katrina And Rita
- This Old House - The Termites Are Coming