Mulch: The Fifth Principle of Xeriscape

Let’s discuss one of the seven principles of Xeriscaping that we get a number of questions about from customers this time of year – mulches! For clarity’s sake, we are talking about mulches that fit into a landscape with low-water plant zones. We are not encouraging anyone to replace their lawn with only wood mulch or rock mulch and no plants! We are talking about Xeriscaping— not “zero-scaping.” Learn more about the other xeriscaping principles here.

What is mulch?

Mulch is a protective layer put on top of bare soil. Soil amendments or improvements (the second principle of xeriscaping) on the other hand are incorporated into the top few inches of soil with the intention of improving soil quality, usually for non-native plants or cultivated vegetable gardens. When it comes to mulches you have two common options: organic or inorganic, which is to say plant matter or non-plant matter. Organic mulches include wood chips, bark, straw, grass clippings, leaves, pine needles, or even dead weeds. Inorganic mulches are usually some sort of rock layer or even plastics.

Mulching is not some new landscaping concept. Mulching is actually mimicking the natural process of an ecosystem’s lifecycle. Plants die back or drop foliage in fall and this plant matter decomposes over the winter providing much-needed nutrients to the plants again come spring. Mulching is natural and necessary for healthy soils, plants, and yards!

Why mulch at all:

Suppresses weeds
Reduces evapotranspiration. More of the water applied to plants gets to the plants rather than evaporating
Reduces soil temperature variations. It keeps the soil underneath cooler in summer and warmer in winter, helping to protect plant roots.
Controls erosion
Improves drainage of water from the soil surface to plant roots.
Adds texture and improves aesthetics
Plant-based mulch helps build organic content in soil and “organic matter in soil, largely carbon from decayed plant and animal matter, holds plant nutrients, improves soil texture and structure, and helps moisture levels stay just right for plants.”

What are my mulch options?

If you have ever googled “types of mulch” you may have noticed that there are some opposing views about the best type of mulch. Most commonly here in Colorado, there are wood mulches and gravel mulches. There is a concept in gardening of selecting the “right plant for the right location” and I like to think that both organic and rock mulches have their appropriate locations in the landscapes as well. Let’s get into the two types of mulch best suited for ornamental perennial gardens here in Colorado and discuss the pros and cons of each.

Ultimately, you as the gardener get to decide which mulch – wood or rock – works best for your garden, your plants, and the aesthetic you are going for. We recommend trying a bit of both and seeing how it goes! After all, the thing about gardening is that it’s a bit of an experiment.

A note about weed barriers

Weed barriers may include plastic, permeable landscape cloth, newspaper, or cardboard. We suggest no weed barrier at all beneath your mulch layer in home landscapes – no matter the kind of mulch – since it tends to create more problems than it solves.

Plastic is a no-go altogether for a number of reasons. It doesn’t allow plant roots to grow sufficiently, it heats up the soil, it keeps out ground-nesting beneficial pollinators and insects, and it prevents moisture and air from getting into the soil to support the healthy soil microbes. Landscape cloth, though initially a permeable cloth, over the seasons, fibers fill with sediment, dirt, and debris and it ends up creating a non-permeable layer between the mulch and the soil, again leaving soil beneath starved for air and moisture and creating an environment for weeds to germinate on top of the cloth. See this video about the issues with landscape cloth.

Even more natural materials like newspaper and cardboard, though they break down more quickly than the last two we spoke of, may also keep air and moisture from getting into the soil. After all, wood mulch’s main benefit is that it decomposes to build soil health and this will not happen if there is a barrier between it and the soil! A 3” layer of mulch should be sufficient for slowing weed growth while still encouraging great soil health and a healthy backyard ecosystem.

Weed barriers may prove beneficial in very small sections like lining the ground beneath pathways, but they shouldn’t be used in large areas where you want plant zones to thrive.