Ground Covers: Protect Your Soil

Most gardeners quickly learn that bare soil in a garden is an invitation for weeds to take up residence. Because few of us enjoy weeding, we cover up the soil as quickly as we can, often with bags of shredded wood. However, while mulch is a practical way to neaten your flower beds, it isn’t always the most eco-friendly option, nor the best choice for keeping soil healthy.

Purchased wood mulch takes energy to produce and distribute to your garden center. If it’s bagged in plastic, there’s an additional footprint for the packaging. According to David Nowak, senior scientist with the U.S. Forest Service, shredding trees for mulch hastens the process of releasing stored carbon into the atmosphere, so wood mulch — which we use in part to reduce the need for watering and herbicides — in some ways actually adds to your landscape’s ecological footprint. Jessica Gutknecht, a soil scientist with the University of Minnesota, says that while mulch is helpful when starting off for adding organic material to improve degraded soil, over time, it’s not as beneficial to soil communities and soil quality as living covers.

While some situations may call for mulch, if you want to create a healthy, eco-friendly yard, consider mulch alternatives.

When to Consider Cover Crops

Cover crops — temporary crops of annual grasses or legumes that protect and build soil — are most commonly used in vegetable gardens, where soil would otherwise be left bare after the harvest. Ajay Nair, associate professor of horticulture at Iowa State University, recommends using cover crops in your home garden for several reasons. In addition to holding soil together and preventing erosion, when cover crops are tilled in, they add organic matter and nutrients that help improve soil quality. They also provide habitat for beneficial insects, which can increase biodiversity and improve the health of your garden.

Belowground, the roots of cover crops break up compacted soil, which allows water and air to penetrate and creates paths that your next crop can follow to access nutrients deeper in the earth. Cover crops also provide food for soil microbes, which in turn feed your vegetables. Certain cover crops have additional benefits. For example, legume crops (such as peas, hairy vetch, and clover) fix nitrogen, reducing the need for fertilizer; and brassica crops (such as mustard and oilseed radish) help remove pathogens from soil.

Nair urges gardeners who are new to cover cropping to research the habits of the cover crops they choose, because certain types, such as buckwheat, grow quickly and can take over if you’re not vigilant. He recommends selecting cover crops that won’t overwinter, so they won’t run rampant in the spring garden.
You can till frost-killed crops in, or just fold them over, to create mulch that you can plant right into during the following season. Nair suggests trying a combination of cover crops to get different benefits from each, as long as you understand their habits and keep them from overtaking your garden. You must also consider crop rotation when choosing a cover crop. For instance, brassicas, such as broccoli and kale, shouldn’t be planted where a brassica cover crop just went in.

Phil Nauta, author of Building Soils Naturally, uses dead plants to perform some of the work of cover crops. Instead of removing plants at the end of the growing season, he leaves them in place, where their roots and stems provide habitat for beneficial insects and microbes.  He cuts them down in spring, leaving the root systems intact, and then plants his next crop. If you don’t mind a messier garden, he says, this method is helpful when you don’t have time for cover cropping and still want to protect and enhance your soil.

Living Mulches and Ground Covers

Many perennial gardens use large quantities of wood mulch. But Gutknecht’s research shows that soil without something growing on it has decreased microbial activity, meaning reduced nutrition available to your plants and more carbon released into the atmosphere. Instead of mulch, she says, “If you can have something growing there, it’s going to have a much more powerful benefit. When you have something growing, the roots leak a lot of good stuff for microbes,” which improves soil health.

Instead of wood mulch, Paula Westmoreland, a permaculture designer based in Minneapolis, underplantstrees, shrubs, and perennials with low-growing ground covers, such as thyme, clover, and violets. She views ground covers as far more “functional” than mulch, saying that living mulch not only suppresses weeds and cools soil, but it also supports microbes, encourages beneficial insects, increases diversity, and helps cycle water and nutrients efficiently. As an ecologically focused designer, she also likes that once these living mulches are established, you don’t have to bring in new mulching materials each year.

Keith Reid, a soil scientist and author of Improving Your Soil, says there’s always a trade-off when we choose how to cover our soil. While mulching is often the easiest route, he says there’s a dark side of mulch, pointing out that a thick layer can prevent light rain from reaching the soil, and the continuously moist environment that mulch creates may encourage pests and fungal disease.

However, while cover crops and living mulches may accomplish some of the same things we’re after with conventional mulch, Reid says if not properly managed, these plants may compete for light and moisture. Gardeners should be prepared for more work until plants are well-established and fill in the garden bed. He suggests planting perennial beds with plants of varied heights to fill as much space as possible, reducing the need for mulch while maintaining aesthetic appeal. The extra plant life creates what Reid calls “a banquet for soil microbe activity.”

When and How to Use Mulch

There are times, however, when mulch may be a logical choice. In the garden, a covering of mulch suppresses weeds to keep them from competing with your vegetables while adding nutrients and organic matter to the soil.

In perennial beds, mulch around younger plants will suppress weeds, conserve moisture, and regulate soil temperature — and keep things tidy. As plants grow, your need for mulch should drop, and you can add ground covers to further reduce your use of mulch.

Nauta advocates for mimicking nature whenever possible, pointing out that we don’t find rocks, fabric, or bark chips in the woods. “They may look nice,” he says, “but they often aren’t doing what we need in our gardens.” In perennial beds, he suggests modeling your soil covering after a forest floor — with ground covers and taller perennials, and strewn with leaves and small twigs. He recommends dusting mulched areas with compost to add more beneficial microbes.

When choosing mulch, think about its larger environmental impact and the impact it may have in your yard. Some mulches may do more harm than good, while others have significant ecological consequences depending on how they were produced and shipped to you.

What about inorganic mulches,
such as shredded rubber and plastic? While there are times a sheet of plastic may be called for (such as to heat up the soil for a heat-loving crop in a cooler climate), organic mulches are generally preferable. Plastics won’t benefit your soil, and they may leach chemicals. Most rubber mulch is made from recycled tires, which often contain contaminants, and landscape fabric is a petroleum-based product that tends to make weeding more difficult in the long term. Though not having to replace your mulch each season may sound appealing, these options are generally not recommended for soil health or environmental impact.

Whichever soil cover you choose, keeping in mind its effects on the environment and your garden will help you make the best choices for your plants and the planet.

Why Cover Your Soil?

In addition to suppressing weeds, there are several important reasons to cover your soil. First, bare soil erodes in wind and rain, depleting it of nutrients your plants need. Second, uncovered soil requires more watering. Third, according to soil scientist Jessica Gutknecht, exposed soil pummeled by rain is more prone to compaction, which inhibits microbial activity. Compaction also prevents water from infiltrating soil, depriving your plants of moisture.

Covering your soil will thus not only benefit you and your plants, but by conserving water and preventing erosion, it will also promote environmental health.

This article covers how to decide which of the three primary choices for covering soil — cover crops, ground covers, and mulch — is right for you.

Reduce Your ‘Mulchprint’

• Keep mulch to a minimum. Use living mulches where possible, and use wood mulch only where you really need it.
• Experiment with local sources of mulch, such as shredded leaves, pine needles, and compost. Compost is an especially beneficial mulch that makes nutrients available to plants more readily and encourages microbial activity. (Note that weeds may grow on compost more easily than on other types of mulch.)
• If you choose wood mulch,
  seek sustainably sourced materials, such as chips from local street tree trimmers. Free wood chips are frequently available from utility companies that are pruning trees, or at city compost sites. 
• Watch out for mulch made from trees that need protection. Cypress trees that support critical wetland ecosystems have been
  overharvested for decades, to the detriment of wildlife and coastal communities threatened by storm surge. Avoid cypress mulch, and help reduce demand for this threatened tree.
• Minimize your use of wood mulch by first laying down a sheet of cardboard, which will form a weed barrier and cut how much mulch  you’ll need for effective weed suppression.

Susannah Shmurak is a freelance writer focused on health and sustainability. She blogs about gardening, herbal remedies, and green living at Healthy Green Savvy.