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Mulch: What it Is and What it Does

By Teo Spengler ; Updated March 14, 2019

Longing for help in the garden? One way to cut down on garden chores is mulch, material you apply to the soil surface. Mulch acts as a protective covering, keeping down weeds and holding moisture in the soil. If you never heard of mulch, aren't using mulch or aren't sure how to use it, it's time to read up on the basics.

What Is Mulch?

Mulch is a lumpy word for something that offers valuable assistance to a gardener. Applying a layer of mulch on top of garden soil is a very beneficial practice.

What is mulch made of? You can use different types of materials for mulching as long as the material lets water and air pass through to the soil below. Mulch materials can be divided into two groups, organic mulch and inorganic mulch.

Organic Versus Inorganic Mulch

Organic mulches are materials that are or were once living, such as organic compost, grass clippings, chopped leaves or chipped wood bark. This type of material disintegrates into the soil over time, enriching it with nutrients. That makes it the top choice for gardeners.

Inorganic mulch is made from material that has never been living, often petrochemical-based products like permeable plastic sheeting or rubber chips. These materials do not disintegrate and nurture the soil. However, they do last much longer than organic mulch and work well for garden pathways.

Benefits of Mulch

If you ask half a dozen people why they use mulch, you may get six different responses. By providing a water-permeable layer of protection over the soil, mulch does many useful things for your garden.

Mulch can:

  • Block out sunlight and keep weeds from germinating;
  • Stop or reduce soil erosion by making the ground more porous;
  • Protect plants from cold winter weather by providing a warming "blanket" of material;
  • Cool the soil in summer, keeping plant roots comfortable;
  • Hold moisture from rain or irrigation, keeping it available longer for plants;
  • Break the winter cycle of ground freezing and thawing that heaves plants from the soil;
  • Add acidity to the soil, for some organic mulches like pine needles, which is a boon for acid-loving plants; 
  • Make the landscape more uniform and attractive; and
  • Add organic matter and nutrients to the soil as it disintegrates (only organic mulch does this).

The mulch material you choose depends on the type of plants you want to protect. But for all plants, you'll always want to keep mulch a few inches away from plant foliage or trunks.

How to Use Mulch

The type and amount of mulch to use varies, depending on when and why you are using it. For summer flower beds and vegetable gardens, it is usually enough to apply a 1 to 3-inch layer of mulch. Use 1 to 2 inches for fine textured mulch, like pine needles, and thicker layers for bulky mulch like straw or hay. Mulch disintegrates over time, so renew it periodically.

For winter, use loose material for mulch such as straw, hay or pine boughs. This will provide insulation but won't compact under the weight of snow and ice. These mulches are all bulky, so 4 inches is a good starting point.

How not to Use Mulch

Once you learn about the benefits of mulch, it's easy to get careless or overly generous and apply too much. But this practice can be disastrous for your plants. Here are three for the DON'T list:

  • Don't allow layers of mulch to build up on garden beds. Add additional mulch only when the layers you already applied have disintegrated. Thick mulch can suffocate plants or prevent water from penetrating.

  • Don't bury perennial crowns in mulch. With perennials, too much mulch can be lethal. The top growth of perennials dies back in winter, but the plant crown, just below the soil surface, prepares the buds for the next spring. Piling mulch on a perennial keeps the crown so wet that it rots during dormancy. 

  • Never pile up a mountain of mulch around a tree trunk or shrub Tree trunks may rot from all that wet mulch, and bark-eating rodents and insects may move into the mulch pile and use it as a B&B. The trunk must be completely clear and the tree root area exposed for several inches around the tree. 
 

About the Author

 

Teo Spengler is a docent with the San Francisco Botanical Garden and a staff writer with Gardening Know How. She has written hundreds of gardening and plant articles for sites like eHow Gardening, Gardening Know How and Hunker. She holds a JD in law from U.C. Berkeley, an MA in English and an MFA in Creative Writing.