Mulch is made from a wide range of organic and inorganic materials, from shredded bark and wood chips to rubber, gravel, and plastic sheeting. The most common types sold at garden centers are organic: chipped or shredded wood, bark, leaves, straw, and composted yard waste. But stone, rubber, and synthetic fabrics all serve as mulch too, each with different trade-offs for your soil, plants, and wallet.
Wood and Bark Mulch
The bulk of bagged and bulk mulch sold in North America is wood-based. It comes from chipped or shredded hardwoods (oak, maple, hickory), softwoods (pine, cedar, cypress), or bark stripped during lumber processing. Cedar and cypress are popular because they contain natural oils that resist decay and repel some insects, so they last longer in a landscape bed. Pine bark nuggets hold their shape well but can float away in heavy rain. Hardwood blends break down faster, which means they feed the soil sooner but need replacing more often.
One thing to understand about wood mulch is its carbon-to-nitrogen ratio. Fresh wood chips have a very high ratio, sometimes 100:1 or more, meaning the microbes that decompose them pull nitrogen from the surrounding soil to fuel the process. This can temporarily starve shallow-rooted plants of nitrogen. Composted or aged wood mulch has a lower ratio and causes far less of this effect. If you’re using fresh chips, keeping them on the surface rather than mixing them into the soil limits nitrogen tie-up.
Dyed Wood Mulch
Red, black, and brown mulches get their color from two types of dye. The most common is iron oxide, a simple compound of iron and oxygen. As it breaks down, it releases small amounts of iron into the soil but is not considered toxic. The second type uses carbon-based colorants similar to those found in ink and cosmetics. According to UMass Extension, there is currently no evidence that either dye type is toxic to plants, pets, or people.
The bigger concern with dyed mulch is what the wood itself came from. Some colored mulch is made from recycled pallets, construction debris, or demolition wood. The EPA states that wood treated with chromated copper arsenate (a preservative containing arsenic) should not be reused as mulch. Manufacturers voluntarily stopped producing CCA-treated wood for residential use in 2003, but older treated lumber still circulates. If you’re buying dyed mulch, purchasing from a reputable supplier who sources clean wood reduces the risk of contaminants.
Straw, Leaves, and Agricultural Byproducts
Plenty of effective mulch comes from farm and garden waste rather than a bag at the hardware store. Straw (the dry stalks left after grain harvest) is a classic vegetable garden mulch. Rice straw, widely available in late summer and fall from Central Valley harvests, is naturally weed-free and can be applied right away. It does break down quickly under heavy winter rain and typically needs annual replacement.
Leaf mulch, or leaf mold, is simply fallen leaves that have been shredded or allowed to decompose. Its nutrient content is modest: nitrogen and potassium each run about 0.5 to 1.0 percent on a dry basis, and phosphorus sits around 0.1 percent. The real value isn’t fertility. Composted leaves dramatically improve soil structure and water-holding capacity by adding organic matter, which is why they’re a favorite among gardeners building up poor or sandy soils.
Other agricultural byproducts used as mulch include cocoa bean hulls, rice hulls, buckwheat hulls, and pine straw (fallen pine needles). Cocoa hulls are attractive for their rich reddish-brown color and initial chocolate scent, but they carry a real risk for pet owners. Cocoa shells contain roughly 10 to 13 milligrams of theobromine per gram of dried material. Theobromine is the same compound that makes chocolate dangerous for dogs and cats. A curious dog eating a handful of cocoa mulch can develop vomiting, tremors, or worse. If you have pets with garden access, a different mulch is a safer choice.
Compost and Grass Clippings
Composted yard waste, a mix of decomposed leaves, small branches, and grass, works as both a mulch and a slow-release soil amendment. It sits lower on the carbon-to-nitrogen scale than raw wood, so it won’t rob your plants of nitrogen. A two- to three-inch layer suppresses weeds, retains moisture, and feeds soil organisms as it continues to break down.
Fresh grass clippings can also be used as a thin mulch layer (one to two inches) around vegetable beds. They decompose rapidly and return nitrogen to the soil. Thicker layers tend to mat down and become slimy, restricting airflow, so it’s better to apply them thinly and let them dry first. Avoid clippings from lawns recently treated with herbicides, as the chemicals can damage garden plants.
Rubber Mulch
Rubber mulch is made from shredded recycled automotive tires, ground into particles and sometimes mixed with sand. It’s marketed for playgrounds and landscape beds because it doesn’t decompose and doesn’t need annual replacement. However, research from the National Toxicology Program has confirmed that tire crumb rubber contains a cocktail of chemicals: polycyclic aromatic hydrocarbons (PAHs), metals including zinc with trace amounts of lead and cadmium, plasticizers like phthalates, and bisphenol A. Other manufacturing chemicals found in tires include sulphenamides, thiazoles, and phenylenediamines.
Whether these substances leach into soil at levels that cause harm is still debated, but rubber mulch does nothing to improve soil health. It adds no organic matter, no nutrients, and no microbial food. It also absorbs and radiates heat, which can stress plants in hot climates. For purely decorative beds or high-traffic play areas where cushioning matters, some people accept those trade-offs. For garden beds where soil health is the goal, organic options are a better fit.
Stone and Gravel Mulch
Mineral mulches include pea gravel, river rock, lava rock, crushed granite, and slate chips. They’re permanent, don’t decompose, and work well in xeriscape designs or around foundations where you want to keep organic material away from the structure.
The main drawback is heat. Stone and gravel absorb heat during the day and release it at night, drying out soil much faster than organic mulch would. Lava rock is especially known for this, making it a better match for dry-climate plantings that tolerate heat. River rock, with its smooth rounded shapes and mixed colors, is popular for decorative borders and dry creek beds where drainage matters more than soil moisture. None of these materials add nutrients or organic matter to the soil, so they’re a functional choice rather than a soil-building one.
Landscape Fabric and Plastic Film
Synthetic mulches include woven and non-woven landscape fabrics, typically made from polypropylene or polyethylene. Woven polypropylene fabric allows water and air to pass through while blocking sunlight to suppress weeds. Non-woven versions use bonded fibers for the same purpose but tend to clog with soil particles over time, reducing drainage.
Black plastic sheeting (polyethylene film) is used in commercial agriculture to warm soil, retain moisture, and suppress weeds around crops like strawberries and tomatoes. It doesn’t let water through, so drip irrigation underneath is usually necessary. In home gardens, landscape fabric is more common and is often topped with a layer of wood mulch or gravel for appearance. Over several years, fabric can degrade, tear, and become difficult to remove as roots grow through it, so it’s most practical in permanent installations like pathways or under gravel beds rather than in planting areas you’ll rework regularly.
Choosing by Purpose
Your best mulch depends on what you’re trying to accomplish. For vegetable gardens and annual flower beds where you want to build soil over time, organic options like shredded leaves, straw, or composted yard waste break down and feed the soil each season. For perennial beds and foundation plantings where low maintenance matters, aged hardwood or bark mulch lasts a full growing season before needing a top-up. For pathways, dry creek beds, and areas where you never plan to plant, stone or gravel provides a permanent solution with no upkeep beyond occasional raking.
Regardless of material, two to four inches is the standard depth for most mulches. Thinner than that and weeds push through. Thicker and you risk trapping too much moisture against plant stems, which invites rot. Keep all mulch pulled back a few inches from tree trunks and plant crowns to allow air circulation.