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Leveraging Ecology: Get a healthier, more productive garden by using ‘cover crops’

Welcome back to the Leveraging Ecology series; a collection of articles highlighting ways in which ecological principles can be applied to human practices and problems.

In the last article, I highlighted how the Festival de Cannes leverages the relationship between hawks and gulls to deter pest birds from festival events to avoid needing to using indiscriminate traps or poisons.

Today, I’m shifting the focus to the garden and showing how using cover crops can result in healthier, more productive growth and robust backyard ecosystems.

Cover crops blanket a field. Image courtesy of the USDA.

What is a “cover crop?”

Cover crops are densely-packed and (usually) non-harvested plants which are rotated in-between food crops. They are a great way to use natural processes to rejuvenate and enrich garden plots during times in which the garden would otherwise be left bare.

Benefits of cover crops

The benefits of cover crops (locally and beyond the confines of the plot) have been well-documented in both large-scale commercial agricultural operations and in small, urban gardens.

Research performed and/or analyzed the USDA and universities like Cornell have shown that allowing a garden plot to sit empty between food crop plantings has a host of deleterious effects for the soil, such as promoting the loss of nutrients, reducing moisture, and reducing soil stability.

Cover crops push those unwanted processes in the opposite direction and that translates to healthier soil, more stable garden ecosystems, and improved crop yields (which is really the primary goal of any gardener).

Here are some of the key benefits of cover crops:

Reduced fertilizer use. Using nitrogen-fixing plants such as hairy vetch or red clover in cover crops provides vital soil nitrogen that gardeners might otherwise need to provide through fertilizers. This means both more money in the gardener’s pocket and less excess nutrients making their way out into the surrounding environment.

Fertilizer run-off is extremely harmful to the environment. For example, fertilizers that enter waterways can create deadly, inhospitable conditions for aquatic/marine life by generating “dead zones” which can span tens of thousands of square miles.

The exact process by which that occurs could span an entire post, but the “quick answer” is that it involves blooms of bacteria and algae which feed on the excess nutrients. While alive, these overabundant algae and microbes can consume more oxygen than is available, causing other organisms that depend on that oxygen to die. Additionally, when the algae and microbes die in large numbers, their decomposition can quickly consumes all the available dissolved oxygen in the water.

Weed Prevention. Cover crops create a thick mat of vegetation that can outcompete weed species. They achieve this through both the sheer magnitude of their physical presence (which reduces the amount of light, soil, and nutrients available to weeds) and by allelopathic processes.

Allelopathic processes involve root exudates that actively inhibit the growth of weeds. In essence, cover crop plants like vetch or brassica species create their own herbicides that will prevent weeds from gaining a foothold in the garden during off-seasons.

Healthier garden soil ecosystems. The most-productive garden soil “alive.” It’s full of fungi, bacteria, and a diverse range of animals whose combined effects result in the conditions that plants need to thrive.

Cover crops, like many plants, are excellent environmental engineers when it comes to creating the conditions necessary for healthy, “living” garden soil. They effectively terraform empty garden plots; transforming barren ground into a fully-functional soil ecosystem.

Aside from exuding substances that inhibit weed growth and discourage the proliferation of harmful fungi and bacteria, cover crop plants release compounds into the rhizosphere (the environment around their roots) that attract beneficial nematode species (while deterring harmful species), stimulate growth and proliferation of vital mycorrhizae, and recruit and sustain beneficial bacteria. This, in turn, encourages beneficial worms, springtails, ants (ant species such as those that don’t farm aphids can be very beneficial), and other animals to take up residence and create the kind of moist, nutrient-rich, and well aerated soils that every gardener strives for. Without a healthy environment below the surface, your garden will never be healthy above the surface.

Improved soil structure. When soil is left barren, it begins to radically change in terms of structure. What was once a warm, moist, and flexible substrate held together by organic factors like biofilms and roots, soon becomes a dry, hard, and often nutrient-poor slab of dirt.

Water and wind erode the barren soil and rob it of vital materials like humus. Organisms like bacteria, fungi, insects, and worms leave the area, go dormant, or die. Previously aerobic soil environments, created by factors like insect burrowing activity, collapse as the soil compacts; creating anaerobic subsurface conditions. In essence, all the great work done creating healthy soil for bountiful harvests during the growing season is rapidly undone when the plots are left bare.

By using cover crops when garden plots would otherwise be left empty, those healthy soil ecosystems can be supported and maintained. Even in situations of extreme cold in which the ground freezes and compacts and soil ecosystem activity is reduced, the benefits of cover crops persist.

For instance, over crops that are left to die in the winter provide a thick insulating mat of organic matter which traps soil moisture that might otherwise escape. Below the surface, their roots bind to the soil and help deter erosion during rainy and windy conditions. They also provides an abundant store of nutrients and organic material which returns to the soil after winter when the plant matter decays.

Furthermore, cover crops like sorghum-sudangrass, which can be planted after the winter in compacted soil, have powerful roots that break-up hard material in preparation for soil ecosystem development and harvest-crop planting.

Improved ecosystem health outside of the garden. Besides creating healthy environmental conditions below the surface, cover crops help support and improve conditions above-ground; both in their immediate vicinity and well-beyond the confines of the plot.

Cover crops recruit all manner of insect and while pest species will undoubtedly be attracted to vegetation, so will beneficial insects like bees and ladybirds which will use the cover crops as vital sources of food (like pollen) and as overwintering habitat. This can result in improved biodiversity for the whole neighborhood as the life supported by the cover crops support other wildlife like the insectivorous birds and hedgehogs that you’ll want nearby to help keep pests under control when the growing season starts.

Viewing cover crops in this way also has positive ethical implications. Taking up space for gardening removes habitat that might otherwise be available for wildlife. Given the degree to which human settlements and activities have destroyed natural habitats and to which organisms are being forced to depend on our backyards for survival, providing habitats in the form of cover crops when a garden is not in use can help reduce the negative impacts of agricultural activities on the surrounding ecosystems.

Selecting cover crop plants

Cornell University has performed extensive research on cover crops in urban gardens in New York and recommends the following considerations when you’re thinking about planting cover crops:

  • Rotation planning. What seasonal niche will the cover crop occupy? What vegetable crops will precede and follow the cover crop?
  • Management goals. What do you want the cover crop to do? What cover crop function is most important — adding organic matter to improve soil quality, contributing fixed N to the soil for food crops, suppressing weeds, providing habitat for beneficial insects, or suppressing a soil-borne disease?
  • Environmental conditions. What species will grow well given your climate, soil, and light availability?
A cover crop in a garden in New York City. Image: Garden Ecology Project.

There are some very-detailed guides on cover crop selection and management that are available free of charge through the Garden Ecology Project, the lab of Thomas Bjorkman, and the USDA publication Managing Cover Crops Profitability, 3rd ed.

There is also this handy online tool put together by researchers at Cornell to help with cover crop decisions. I highly-recommend checking them out, but if you’re pressed for time you’ll find some of the key approaches for backyard gardeners below:

Winter-kill cover crops: planted in the late summer, these cover crops will die in regions that experience cold, frosty winters. While they won’t grow as much as overwinter cover crops, they still provide excellent fall weed suppression. Also, the fact that they die in the winter means you’ll have plenty of mulch ready to go in early spring from all that dead cover. Examples of winter-kill cover crop plants include field peas, oats, and brassicas.

Overwinter cover crops: these hardy plants will survive the winter and provide very good weed suppression and nitrogen retention and/or fixation. They also provide substantial habitat for organisms both above and below the soil until you need to prepare for planting. You’ll want to wait until May to cut and mulch these plants and allow the material to lay on the soil for two weeks before planting. Examples of overwinter cover crop plants include hairy vetch, crimson clover, and rye.

Summer cover crops: these plants fill a gap between early spring crops and fall harvest crops; keeping the soil healthy and primed. They are also great at suppressing weeds, meaning less work when you prep for fall harvest plantings. Examples of summer cover crop plants include buckwheat and cowpea.

Sound interesting?

Cover crops are great ecological instruments that all gardeners can use to cultivate healthy and nutrient rich soil, reduce labor time (e.g. tilling compacted soil), cut fertilizer costs (monetary and environmental), and enhance their overall garden ecosystem and that of the surrounding community.

Again, I highly recommend checking out the above-referenced resources from the Garden Ecology Project and the USDA which explain cover crops in detail. Additionally, if you’re interested in a very-accessible, easy-to-read overview of how a healthy soil ecosystem works and how you can use those ecosystems to create fantastic gardens, pick up a copy of Teaming with Microbes by Jeff Lowenfels and Wayne Lewis.

I picked up a copy at a garden show a few years back and it was worth every penny.

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