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Mulch is material that you put over soil around plants, shrubs, and trees. Fallen leaves form a natural mulch on forest floors. Like leaf litter, mulch conserves moisture and keeps soil from eroding.
More reasons to mulch your garden:
Mulch helps stop weeds by blocking weed seeds from sunlight. Although it won't completely prevent weeds, it reduces their number and makes yanking them easier.
It insulates soil. "Hot or cold surface soils can kill new transplants that have not had time to generate a large root mass," horticulture professor Dr. Linda Chalker-Scott reports in the Journal of Environmental Horticulture. A Michigan State University study showed some mulches can keep the soil as much as 18 degrees cooler.
It protects plants over winter, when soil freezes and thaws on repeat, according to the Chicago Botanic Garden.
Keeps soil from compacting. Heavy rains and people walking on wet ground can compact soil, which makes it harder for air and water to reach roots. Mulch acts as a cushion, says Chalker-Scott.
Shields trees from lawn mowers. Mulch acts like a bodyguard around landscape trees. (Learn more about mulching trees.)
Helps prevent plant diseases by keeping bacterial spores in soil from splashing onto plants, notes Chalker-Scott.
Reuses waste. Plant waste, tree debris, and wood pallets can be turned into mulch instead of being sent to a landfill.
Beautifies your landscape. Even a basic row of bushes looks better when mulch is added.
With all the types of mulch available, how does a gardener choose? Here’s a look at some commonly available mulches. We found that some mulches are more beneficial than others.
Types of Beneficial Mulches
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Wood mulch comes from several sources, including tree cutting, lumber mills, construction sites, and wood pallet recycling.
The best wood mulch comes from tree trimming. This mix of leaves, wood, and bark breaks down at different rates, adding a variety of nutrients to the soil.
Mulch made from construction debris and paper mill waste could contain chemicals. Chalker-Scott reported a study that found tree seedlings were less likely to survive when mulched with mill wastes containing "formaldehyde and other wood processing residues."
Tips for using wood mulch (sources: Michigan State University Extension, Texas A&M Extension):
- Don't use around vegetables and annual flowers
- Do use around shrubs, trees, and natural areas
- Don't use dyed wood mulches in organic gardens
- Do use in perennial beds
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Rocky mulches such as gravel, pea stone, and cobbles are good when you want a permanent, nice-looking mulch. But a couple warnings. First, you won't be able to easily enrich the soil underneath stones. Second, the Chicago Botanic Garden points out that stones can harm tender plants in hot dry weather, because "rocks absorb and reflect heat" and don't hold moisture.
For these reasons, stone mulches are better suited for drought-tolerant, hardy trees, shrubs, and plants or for areas that receive some shade.
Pine needles are attractive and aromatic—and free if you have pine trees that annually drop needles. They give off "naturally occurring compounds that suppress weed seedlings. Contrary to popular belief, pine straw mulch will not change your soil’s pH," according to the MSU Extension.
Cocoa Bean Hulls
Pretty, sweet-smelling cocoa mulch is made of reused shells from processed cocoa beans. It's been found to increase soil nutrients and suppress weeds better than bark mulch and is more fire resistant than other organic mulches.
But cocoa shell mulch could be toxic to dogs, so be cautious if you have fur babies.
Straw is excellent "for crops that prefer cooler soil temperatures such as cabbage or broccoli, but may slow the growth of warm season crops, such as tomatoes or melons," says Gary Heilig, MSU Extension. Strangely, it can cause more damage to squash “after a light frost compared to squash on bare soil." Sometimes straw contains seeds.
Free, if you make your own. Leaves, herbicide-free grass clippings, kitchen scraps, and disease-free plant material can all be composted. Compost is ideal for vegetable gardens and annual flower beds because it enriches the soil. It’s also great for perennial beds, but you might want to top it with a more decorative wood mulch.
Researchers at University College Cork in Ireland found butterhead lettuce grew better when mulched with grass than hay.
The drawbacks? If you’ve applied herbicide on your grass, the clippings can hurt—even kill—your plants. Besides decaying unattractively, dried cut grass can mat, blocking water. The Missouri Botanical Garden recommends using only a very thin layer around plants. Better yet, add grass clippings to your compost pile for later use.
You can use black-and-white newspaper as mulch. Ink in today's newsprint no longer contains lead, according to the Virginia Cooperative Extension.
You should cover newspaper with an organic mulch, because it isn't pretty and can blow away.
Rubber landscape mulch usually comes from recycled tires, which might be good for landfills, but not so much for your garden.
Here are some issues with rubber mulch (from Washington State University and Purdue University Extension):
- More flammable than several organic mulches
- Not as good at controlling weeds compared to organic mulch
- Can leach heavy metals and chemicals into soil and water
- Could get "hot enough to burn tender plants," according to Rosie Lerner, Purdue University Extension
Plastic mulch has more negatives than positives, according to gardening experts and researchers. Here are a few pros and several cons (from Linda Chalker-Scott, MSU Extension, Missouri Botanical Garden).
- Can use plastic short-term with vegetables or small fruit trees
- Red plastic mulch benefits tomatoes and strawberries
- Not recommended for landscape trees and shrubs
- Blocks water and air movement in and out of soil
- Not good for long-term weed control
- Not good at moderating soil temperatures
- Degrades under sunlight and hard to reuse
- Difficult to recycle because it gets dirty
- Can potentially pollute ecosystems with microplastics, phthalates, and agrochemicals
- Can compact soil and in the long term will cause unnaturally dry soil
Properly applied, mulch will help your garden thrive. To learn about other organic mulch materials you can use, check out this organic mulch chart from the University of Illinois at Urbana-Champaign Extension.