Mulch is a layer of material — like leaves, bark chips, or even rocks — that you spread across your garden soil.
Among its numerous benefits: water conservation.
In the average yard, half of the water used for irrigation goes to waste, trickling away as runoff or evaporating. Mulch helps prevent that loss — by as much as 83 percent, according to some research!
There are a few different ways that mulch conserves water.
It acts like insulation. Mulch keeps soil cool in the summer, which prevents water from evaporating. (As a bonus, it also keeps soil warm in the winter — great if you want to get an early start on springtime planting.)
It improves soil structure. Organic mulch, which includes any mulch made from dead plant matter, provides food to microorganisms and insects. These creatures tunnel through the soil and produce a rich black goo that causes soil particles to clump together. Both actions encourage a porous soil structure that water can move through, nourishing plants and preventing runoff.
It acts like a sponge. Mulch prevents heavy rain or irrigation from oversaturating the soil or hammering it into a hard, impermeable crust. Instead, organic mulch absorbs much of that water, then gradually releases it in the hours and days between waterings. The slow and steady source of water helps prevent compaction and runoff.
If you’re still not sold, mulch has benefits beyond water conservation: it introduces important nutrients into soil, keeps weeds at bay (which indirectly conserves water since more will go to the plants you want), and prevents topsoil from eroding or blowing away.
So how do you go about adding mulch to your yard? The first step is to pick the right one.
Leaves: When autumn rolls around, don’t throw out the leaves that fall from your trees — they make great mulch! Note that leaves, like any organic material, can become a fire hazard come late spring. If you choose to use leaves as mulch, keep them five feet from your home or other structures, and at least a foot from any trees.
An even better option would be to shred or compost them, which makes them more fire resistant and less prone to blowing away in the wind. Electric leaf mulchers are inexpensive and will do that shredding work for you. They can even handle small branches and other cuttings, making mulch easier and safer for use, and accelerating the decomposition process. Read more about best practices on leaving your leaves and when to start spring cleanup to avoid disturbing pollinators.
Compost: Spread in a layer atop soil, compost makes great mulch. Vegetable or flower gardens in particular benefit from this nutrient-rich cover. And good news: it’s firewise! Compost doesn’t easily ignite when exposed to flames. But there is a downside to this mulch: unlike leaves and wood-based compost, it doesn’t suppress weeds. Some compost choices may work better than others as ground cover and in terms of fire resistance.
Charisse Sydoriak, an expert on firewise landscaping and volunteer fire coordinator with Ashland Fire and Rescue, recommends Rogue Fine Compost, available for delivery or pick-up at the Transfer Station in White City. This compost passed the Ashland Fire Department’s flammability test. While other vendors advertise fire-resistant products, those composts are not tested, Syodoriak said. “If their products have a high pumice content and only very small wood bits, they are probably okay,” she added.
Inorganic Mulches: Believe it or not, rocks, pea gravel, and glass can all work as mulch. These ground covers won’t decompose and fertilize the soil, but they will protect your garden from soil compaction, provide insulation in the winter, and help suppress weeds. That makes them a great choice for ornamental shrubs, trees, and perennials.
Unfortunately, some research suggests that these mulches aren’t as effective at conserving water —they tend to heat soil, which accelerates water evaporation. In addition, some plants may get too hot in the summer months when planted with these mulches. Still, inorganic mulch is the most firewise option, and is attractive to boot. Consider using it within five feet of your house, or with plants that will tolerate warmer soil temperatures.
Wood-based mulches: There’s a reason that wood chips, bark, sawdust, and shredded wood are a popular choice for mulch. They’re relatively inexpensive, attractive, prevent weeds, and are great for building healthy soil. But these mulches are a major fire hazard. Firefighters working on the Almeda Fire found themselves constantly putting out burning mulch, only to come back to the same home and find the mulch on fire again. A study conducted in the wake of that disaster found that wood mulches of all types have the capacity to ignite nearby structures, but that bigger wood pieces — like large wood chips — burned the most rapidly and intensely.
“Burning mulch casts embers. It was like a river of blowing fire across the ground during Almeda, much of it from mulch,” said Chris Chambers of the Wildfire and Community Risk Reduction Division at Ashland Fire Department. If you’re going to use a wood-based mulch, opt for shredded wood or shavings, and keep it at least 10 feet away from structures or trees. Avoid bark entirely.
To learn more about the hazards of bark mulch, check out this video from the Ashland Fire Department that details the results of their flammability study.
After you select your mulch, it’s time to spread it.
Lay it on thick — three to four inches deep. But make sure to avoid the “mulch volcano” around trees and other plants. Mulching too close to the trunk or root zone can trap moisture and prevent oxygen from reaching the plants, causing decay and stress to plants.
For a more comprehensive list of different mulch options and more information on mulch application, check out Colorado State University Extension’s in-depth guide.
Happy mulching!