Four benefits of using mulch in the garden

With the summer heat already here and water supplies severely limited, many of us are anxious about being able to keep our gardens looking beautiful during the long dry months ahead. Mulch can be a really good tool in this effort. But what is mulch and how should we use it?

Mulch is an organic material spread on top of the soil that can have many very useful functions for gardens and plants if you use the right materials. It cools and protects the soil from the drying and heating effects of the sun and helps retain soil moisture.

Mulch made from composted green waste and manure can build up soil organic matter levels over time, so soil can hold more moisture and plants can better resist drought. The right mulches can give all the fertility plants need, eliminating the need to fertilize. If you apply it thickly enough, mulch can suppress weeds. Mulch also dresses up gardens and highlights plants’ foliage. Mulch makes our gardens much easier to maintain.

Chocolate-cake consistency

Many materials are used as mulch. The best mulches for building healthy soil are organic materials that have been composted.

Composted green waste offered by local companies or municipalities are often good choices. Look for green waste that is mostly, but not entirely, broken down so there are coarser pieces as well as a compost-like component. Avoid green waste that has big chunks of wood chips or bark.

Green waste with manure as an ingredient is exceptionally good. Composted manure is excellent as a mulch but difficult to find.

Many people use wood chips as a mulch. These are not ideal, except used under mature trees like native oaks. Soil organisms break down wood chips very gradually. This long process uses a lot of nitrogen, depriving plants of vital nutrients. In areas susceptible to fire, wood chips around houses also may be a poor choice because they are flammable. Composted green waste has a texture similar to soil and has low flammability.

Mulches contain varying amounts of nutrients. If you have drought-resistant plants and native plants, composted green waste probably will contain enough nutrients. If you have more water-loving perennials or shrubs, you might want to look for composted green waste that includes manure or a fertilizer like feathermeal (ground chicken feathers). Bark mulch looks great but doesn’t break down for many years and has very few nutrients.

The organic matter in good mulches feed soil organisms that help develop healthy soil with good structure and good porosity so water percolates in and is retained. The goal is for soil to have a chocolate cake-like structure that allows water and oxygen to move through it as well as earthworms and other vital soil organisms. Water also adheres to humus and aids greatly in developing the water holding capacity of the soil.

A dressier look

Spread mulch over garden beds about 2-4 inches deep, depending on the size and age of your plants. The bigger and older the plants, the deeper the mulch can be. Small and young plants may just need a couple of inches of mulch initially. You can add more later, as needed. You want to keep mulch from touching plant stems and foliage. Applied a few inches thick, mulch helps cool the soil and protects against water loss. Plastic landscape fabric, on the other hand, acts as a physical barrier to soil organisms and prevents nutrients from cycling and soil organic matter from increasing.

Mulch is best used with drip irrigation. The surface of the mulch often is dry, so when you’re watering from overhead with a hose, it can be hard to wet.

Bare soil invites weeds. Weeds with seeds that blow in the wind like dandelion, tree of heaven, cottonwoods and wild lettuce, readily germinate on bare soil. Most weeds produce copious amounts of seed, and it can be difficult to weed plants from bare soil. Mulching soil creates good soil structure and friability and makes weeding much easier. Plants are easy to pull out. Over time, pernicious perennial weeds like bindweed will lose their strength and vigor as soil structure improves. Just make sure to pull plants before they set seed.

In terms of appearance, mulch, like a crisp tuxedo, really dresses up gardens. The brown color makes foliage and flowers pop.

The ideal time to apply mulch is in early spring, before plants have grown much. But you can mulch now, when plants are large, although it is a little more difficult. You may need a helper to hold up the plant’s foliage so you can spread compost on the soil around the plant. Or you could just spread mulch on any bare soil. Again, don’t let it touch foliage or stems. You also can apply mulch in the fall after you have cut back your plants for the winter.

With the water-saving and healthy-soil benefits of mulch, try to make mulching an annual practice.

Kate Frey’s column appears every other week in Sonoma Home. Contact Kate at:, Instagram @americangardenschool

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