Welcome back to the Leveraging Ecology series; a collection of articles highlighting ways in which the principles of ecology can be used to address human practices and problems.
Today, I’m shifting the focus to the garden and showing how using cover crops can result in healthier, more productive, and robust backyard ecosystems.
What is a Cover Crop?
Cover crops are densely packed and (usually) non-harvested plants that are rotated in-between food crops. They are a great way to use natural processes to rejuvenate and enrich garden plots during times when the garden would otherwise be left bare.
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 several deleterious effects, such as promoting the loss of soil nutrients, reducing soil moisture, and reducing soil stability. Cover crops counter those processes and lead to healthier soils, more stable garden ecosystems, and improved crop yields.
Here are some of the key benefits of cover crops:
Reduced Fertilizer Use
Nitrogen-fixing plants such as hairy vetch or red clover in cover crops provide vital soil nitrogen that gardeners might otherwise need to provide through fertilizers. This means 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. 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.
Cover crops create a thick mat of vegetation that outcompetes weed species. They achieve this through both the sheer magnitude of their physical presence 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 Soil Biodiversity
The most-productive garden soil is 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 are excellent environmental engineers when it comes to creating the conditions necessary for healthy, garden soil biodiversity. They effectively terraform empty garden plots; transforming barren ground into a fully functional 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.
Better Soil Structure
When soil is left barren, its structure starts to radically change. What was once a warm, moist, and flexible substrate held together by biofilms and roots, soon becomes a dry, hard, and often nutrient-poor slab of dirt.
Water and wind erode that 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 spongy aerobic soil environments compact and become anaerobic. 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, those healthy soil ecosystems can be supported and preserved. Even in situations of extreme cold in which the ground freezes and soil bioactivity 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 that traps soil moisture. Below the surface, their roots bind to the soil and deter erosion during rainy and windy conditions. They also decay, providing an abundant compost that returns nutrientsts to the soil in the spring.
Cover crops like sorghum-sudangrass even have powerful roots capable of breaking up hard material to better prime the soil for harvest crop planting.
Benefits Beyond the Garden
Besides creating healthy environmental condition in their immediate vicinity, cover crops have beneficial effects well-beyond the confines of the garden patch.
Cover crops recruit all manner of insect. While pest species will undoubtedly be attracted to vegetation, so will beneficial insects like bees and ladybirds that use cover crops as vital sources of food (like pollen) and as overwintering habitat. That means improved biodiversity for the whole neighborhood as the life supported by cover crops subsequently support other beneficial wildlife like insectivorous birds and hedgehogs.
This effect of cover crops also has positive ethical implications. Taking up space for agriculture removes habitat that might otherwise be available for wildlife. Human settlements and activities have disrupted much of the planet’s natural habitats. As a result, organisms are being increasingly forced to depend on our backyards for survival. Providing habitats in the form of cover crops when a garden is not in use reduces the negative impacts of our agricultural activities on the surrounding ecosystems and fills at least part of the gap we have created.
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?
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
These crops are planted in the late summer and 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.
Overwintering 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 overwintering 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.
Cover crops are great ecological instruments that all gardeners can utilize to cultivate healthy soil, reduce labor time (e.g. tilling compacted soil), cut fertilizer costs (monetary and environmental), and enhance their overall neighborhood ecosystem.
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 or the whole set of The Organic Gardener’s Guide (3 Books). Purchasing those books through those links helps support this blog.