Where there are plants, there is mulch. Leaves, flowers, and stems die and fall off trees, shrubs, and perennials, and whole dead bodies of annuals cover a lot of ground in natural areas. This natural mulch decomposes, returning nutrients to the soil, and also cools the soil and helps to slow down evaporation from the ground. It may also harbor pests and diseases, but that’s all part of being a natural ecosystem.
Our gardens, however, are not natural ecosystems so why mulch? Along with slowing soil water loss, mulch can help control weeds; it can make the garden look more uniform; it can reduce the amount of mud splashed on plants when you water; it can prevent a crust developing on bare soil.
Here are some pros and cons of a variety of mulching methods:
- Living mulch: Covering the ground with plants cools the soil and slows evaporation from the ground and from any drip irrigation that it grows over. Of course, the plants themselves require water so it’s a good idea to choose ones with low to moderate water requirements. Any leaves the plants drop will sift down to the ground and decompose, adding back their nitrogen and other nutrients. Dense planting adds to the lush look of the landscape but may take a while to grow in, and may take some maintenance. Once established, the plants tend to shade out and out-compete weeds.
- Leaves: Oak leaves, pine needles, elm leaves, etc-- these are free and add back nutrients as they break down. But just like in nature, they can support pests and diseases. Never use leaves from plants that have problems. Pine needles tend to make the soil a bit more acid so are good for azaleas and camellias. Shredding helps them break down faster.
- Plastic: This is cheap and certainly prevents water loss, and it can absorb heat that helps crops to mature. But it also provides a damp, snug home for pests and diseases to develop. It can cut off air to the roots and raise ground temperatures enough to kill good soil organisms as well as surface roots. It generally isn’t very attractive and will eventually decompose into shreds.
- Gravel, stones, tumbled glass: These are permeable to air and water and easy to use. However, in sunny areas they can absorb a lot of heat during the day and re-emit it at night, greatly increasing local temperatures. They can reflect light, heating nearby areas and making it hard for some plants to grow. They tend to migrate unless contained with some sort of edging and can be hard to get rid of if you decide you don’t want them. Weeds can grow through them unless you put weed cloth or newspaper under them, and they can be difficult to keep clean. They don’t add any nutrients to the soil but also are less likely to support pests and diseases. Too much gravel makes a garden look bleak.
- Bark and wood: Shredded bark, bark nuggets, coconut fiber, wood chips etc, are all easy to find and use. They come in many textures and colors, some natural and some dyed. Local cities give away free mulch and compost (check the Sustainable Claremont website for dates). These mulches do a good job of slowing evaporation from the soil and make garden beds look tidy. Dark ones can absorb lots of heat; light ones can reflect too much heat to nearby plants. All of them will decompose eventually and need replacement. Wood chips tend to remove nitrogen as they break down. Some of these mulches can pack down and interfere with air movement into the soil. Putting weed cloth or newspaper down under them will help prevent weeds growing up through them, but weeds may still grow in them. These mulches are absorbent so they can reduce the amount of overhead water roots receive, especially if the layer is too deep. On the other hand, too thin a layer won’t do the job—3-4” deep usually does the job. The soil should be wet as far down as you’d like it to be before you spread the mulch. Avoid creating a “mulch volcano” around tree trunks or shrub stems—the bark is meant to be open to the air. If you want to add new mulch, remove some of the old so the resulting layer isn’t too thick. Keep an eye out for pests and diseases.
No choice is perfect or totally maintenance free, but the benefits of mulch for your garden and your water bill can be great. Whatever you use, be sure water is getting to the root zone and that the bases of trees and shrubs are kept clear to prevent physical damage and fungal diseases. And do consider leaving some areas of bare, unwatered soil in the garden to provide home for our native ground-nesting bees!