Mulch has been called the gardener’s friend—and for good reason. In winter, mulch protects bare soil, prevents erosion, and protects plants. In spring, mulch locks in moisture, suppresses weeds, and feeds the soil. Learn how to mulch, when to mulch, and about the many different types of mulch to use in your garden.
Mulching is a fundamental part of gardening to plants looking beautiful and productive through the year. If you don’t already know how to mulch, it’s important to learn when to mulch, the right depth for mulch, and the right type of mulch.
What Is Mulch?
At its simplest, mulch is any material that covers the soil’s surface. In nature, mulch is simply fallen leaves and plant debris. In the garden, mulch can also include compost, wood chips, rotted manure, cardboard, or even seaweed.
It’s only recently that we’ve come to appreciate mulch’s sustainable and ecological benefits. Done correctly, mulching feeds our soil’s living microorganisms with nutrients and the waste from these tiny microbes creates healthier soil structure for plants, limiting compaction.
Benefits of Mulching
- Reduces weed growth by keeping light from reaching the soil surface.
- Reduces water loss from the soil surface, which helps maintain soil moisture.
- Moderates soil temperatures, keeping it warmer on cold nights and cooler on hot days.
- Protects bare soil, reducing erosion and soil compaction.
- Protects plants from the harsh conditions of winter freezes, thaws, and winds.
There are many other benefits of mulch:
- In winter, soil under mulch will be warmer than unprotected soil. This protects plants from the cycle of freezing and thawing (which can heave them out of the ground).
- Prevents crusting of the soil surface. Water moves more readily into soil covered with mulch instead of running off.
- Keeps soil from splashing onto leaves; keeping soil off leaves reduces the chance of plants getting fungal and bacterial diseases.
- Breaks down and feeds the soil (if organic mulch).
- Improves the structure of clay soils and the moisture-holding capacity of sandy soils.
- Slowly increases soil fertility (if organic) and may make micronutrients already in the soil more available.
- Warms the soil in spring, allowing the gardener to plant days or weeks before the soil would normally be ready.
- Keeps plants clean and off the ground, especially tomatoes and melons, to avoid plant disease.
- Limits the chance of damaging trees’ trunks when mulch is placed around them instead of grass.
- Improves plant health and growth (due to fewer weeds and more consistent moisture and soil temperature).
- Makes gardens “spiffed up” and attractive, giving a uniform appearance and rhythm to garden design.
Disadvantages of Mulching
Although using mulch has many benefits, in some cases, its use can be detrimental to the garden:
- TOO much mulch (a layer more than 3 inches deep) can bury and suffocate plants; water and oxygen can’t reach the roots. A layer of 2 to 3 inches of mulch is ample. Do NOT overmulch.
- Mulch can contribute to rotting bark if piled up around the trunks of trees and shrubs. Keep mulch 6 to 12 inches away from the base of woody plants. No more “volcano” mulching around trees! Keeping mulch away from the trunk discourages wood-boring insects, gnawing rodents, and decay.
- Mulch near plant stems is the perfect place for slugs, snails, tunneling rodents, and more pests to reside. Sprinkle wood ashes or diatomaceous earth around the base of precious plants to keep the slugs and snails at bay.
- Mulch can bake your plants with excess heat in midsummer if not done properly. (See more below.)
- Light-colored, wood-based mulches, like sawdust or fresh wood chips, can steal nitrogen from the soil as they break down. Counter this effect by adding a nitrogen-rich fertilizer, such as soybean meal, alfalfa, or cottonseed meal, to the mulch. (Learn more about soil amendments.)
How Much Mulch Is Needed?
With most organic mulches, a layer of 2 to 3 inches is plenty. The finer the material, the thinner the layer needed.
Inorganic mulch is often more shallow. For example, a mulch of small stones usually only needs to be an inch deep.
|If You Want Mulch This Deep…||…You Will Need This Much Mulch to Cover 100 Square Feet|
|2 inches||18 cubic feet|
|3 inches||27 cubic feet|
1 cubic yard = 27 cubic feet
Dry mulches—including sawdust, woodchips, peat moss, and dry straw—can be a fire hazard. Keep them away from buildings to be on the safe side.
Types of Mulch
The ideal mulch should be dense enough to block weed growth but light and open enough to allow water and air to reach the soil. Factors to consider when purchasing mulch are cost, availability, ease of application, and what it looks like in the garden. There are lots of materials of various colors and textures to choose from.
Both organic and inorganic mulches can be used effectively in the garden.
Organic mulches are natural products from leaves, trees, grass, and other plant material, often from your own yard. They mimic nature, breaking down gradually over time. The advantage is that they are truly adding organic matter to the soil. The disadvantage is that they must be replenished periodically.
- Compost is readily available and breaks down rapidly to improve soil. If you don’t have your own, often towns make it available from their leaf composting facility. The disadvantage is that it must be replenished and can contain weed seeds.
- Shredded or chipped bark. Softwood bark mulch is attractive, resists compaction, and breaks down slowly. Hardwood bark is attractive but breaks down quickly and needs to be properly composted to avoid sour mulch and nuisance fungi.
- Shredded leaves and leaf mold are readily available and, if chopped, eventually break down and feed the soil with beneficial materials. The disadvantage is that leaves can mat if wet which reduces the oxygen and moisture in the soil. Avoid matted layers of wet leaves.
- Straw and salt marsh hay are inexpensive and a helpful covering; however, they decompose more quickly, may harbor rodents, and are easily blown away by the wind.
- Grass clippings are ready available but should be dried first or spread thinly to keep them from becoming a hot, slimy, stinky mess. Also, you can not use clippings from grass treated with chemicals in a food garden.
- Pine needles are attractive and stay in place better than most mulches. They are slow to break down, so don’t worry about them adding to soil acidity.
- Local byproducts, such as spent hops from a brewery, cocoa hulls, ground corncobs, coffee grounds, newspaper, or cardboard can also be much. Get creative!
Image: Mulching around salvia. Credit: Mark Herreid/Shutterstock
- Black plastic mulch helps warm the soil in spring, reduces water loss, and is convenient. This can make a big difference in short growing seasons. However, it’s not permeable so it’s more difficult to water; it also breaks down when exposed to sunlight and the soil under the plastic becomes very hot in the middle of summer if not shaded by leaves or covered with another mulch.
- Silver plastic mulch excels at warming soil in spring but doesn’t control weeds; the soil becomes even hotter with clear plastic in midsummer and plants can be damaged if the plastic is not shaded.
- Crushed stone, gravel, marble, or brick chips provide a permanent mulch around shrubs and trees. That said, these mulches are expensive, hard to move, and can get into the lawn. Weed seeds and soil can still find their way into the stones; an underlayer of landscape fabric will help prevent this.
- Landscape fabric smothers weeds while allowing air, fertilizer, and water to move through them and into the soil. They are treated to resist decomposition and they help retain soil moisture. It’s important to fasten the fabric down so perennial weeds do not push them up.
How to Apply Mulch
Mulching in Autumn
We do not generally use mulch in the fall, except for in bare, unplanted garden beds to prevent erosion. If you did not plant a winter cover crop (which you would till under in the spring), you should spread a thick layer of soil-conditioning compost or well-rotted organic matter over the bare soil. You could also use shredded leaves. Lay it at least four inches deep.
Otherwise, do not apply mulch to your landscape in autumn. The soil will not cool down quickly and plants may continue to grow. New growth may not harden off and can be damaged by winter cold. Also, mulching in the fall keeps the soil wet, which can lead to root rot and plant death.
Note: If you’re setting out new areas, start by clearing the surface of any debris and any rocks larger than a hen’s egg. Mow down grass or cut back weeds to the ground. Fast forward a few months and any grass and weeds below will have rotted down, while earthworms will work to gradually incorporate the organic matter into the soil below.
Mulching in Winter
Once you’ve had several freezes (often around Thanksgiving or after), then apply winter mulch around the base of any tender perennial plants or new plants. Grafted plants, like hybrid tea roses, benefit from being mulched heavily.
Shredded mulch, straw, pine needles, or shredded leaves are all good winter mulch. Apply 3 to 4 inches. It’s important to apply enough mulch in winter to keep the frozen ground completely covered so the plant remains dormant until spring no matter what type of warm or cold spells occur.
Take care NOT to put mulch next to the trunks of trees or crowns of plants, as this invites bark-gnawing rodents.
Protect branches and buds of evergreen or semi-evergreen shrubs such as rhododendrons and viburnums by wrapping them with burlap or protecting them with a tree guard filled with shredded leaves for insulation.
WARNING: Do not mulch like this! “Mulch volcanoes” will encourage rot at the base of the plant.
Mulching in Spring
Remove winter mulch in the spring when all danger of a hard frost is past so that the ground can warm and new growth won’t be inhibited.
If there are lots of weeds on the ground where you want to grow, consider a permeable landscape fabric on many of the beds.
Image: Permeable landscape fabric.
Or, lay down a layer of cardboard before adding your organic matter. Thoroughly wet the cardboard to help it break down. The cardboard will serve as a further barrier to weeds, exhausting and eventually killing most of them off. Once the growing season gets underway, you’ll find that any weeds that do manage to make it through will be much easier to remove.
After a few spring rains, when the soil has warmed, we lay down soaker hoses in each bed.
Then we cover the hoses with a fabric to speed up the change in soil temperatures and warm the soil for earlier planting.
Planting holes are cut at different spacings for different crops. Watering is efficient, and maintenance of a large area is made much easier.
Once the plants get some size on them, the fabric is covered and does not look so bad! We also use organic mulch including straw, leaf mold, grass clippings, wood chips, and shredded leaves for crops that like it cooler.
Regularly mulch with organic matter. Replace old mulch as it rots down or becomes incorporated into the soil, so that the ground is being constantly fed and gradually built up.