Cover Crops on Urban Farms

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by Brian Allnutt
Bee-friendly, nitrogen-fixing clover grows in the pathways of Fisheye Farms’ garden beds.

Jon Miller is a retired union representative for city workers who’s in his second career as an urban farmer in Detroit. Like a lot of growers, he’s excited about the possibilities of cover crops for regenerating soil and reducing his reliance on more cumbersome off-farm inputs, such as compost and fertilizer. However, as Miller says, “This whole field of cover crops doesn’t address urban farming,” and he’s had trouble delivering on the promise of cover crops on his farm.

Cover crops — also known as “green manures” — are plants that aren’t grown for harvest, but instead for a number of other benefits, including fixing nitrogen, controlling weeds, adding organic matter, attracting pollinators, and feeding soil organisms, which, along with plant roots, emit various substances that help bind soil particles together to build good structure. Miller’s primary attempt at using them in his own growing spaces — which are 4,000 and 11,000 square feet in size — involved seeding low-growing white clovers in the pathways between beds. Unfortunately, the clover invaded his planting beds and turned into a weed problem. And yet, compost has been unable to provide the soil improvement he needs on degraded urban soil. “I’ve laid down 20 to 30 yards of it a year,” he says. “But you look a couple years later, and it hardly looks like you put anything down.”

Miller’s predicament — desperately needing to improve his soil, but being unable to incorporate cover crops into a highly intensive system — is common on urban farms. Growers struggle with space constraints, lack of large equipment to efficiently manage covers, and the need to grow crops, such as salad mix and spinach, that aren’t easy to grow with cover crops. However, covers offer obvious benefits for urban farmers. Naim Edwards, a Michigan State University researcher beginning a project on urban soils in Detroit, says urban farmers are dealing with compacted soils that contain high levels of debris and low organic matter. There’s also the expense — sometimes thousands of dollars a year — and the logistical difficulty of bringing in compost or manure.

Cover crop management can include irrigating, mowing, weeding plants from areas where they’re not wanted, and then killing and incorporating them into the soil. “For some folks, it’s just too daunting of a mental task to try to coordinate any energy around that,” Edwards says. “And for others, there’s some skepticism around scale.” Many of these farmers handle complex operations that may also involve a second job. No wonder some decide urban cover crops aren’t worth the trouble and instead call in another load of compost.

Andy Chae has both delivered pizzas and washed dishes in addition to running Fisheye Farms with his partner, Amy Eckert, who works a side job as a waitress. Like Miller, they’re trying to use clover in the pathways of their main field, which measures about 7,600 square feet. Using low-growing Dutch or New Zealand white clovers in pathways is an entry point for many growers. These bee-friendly, nitrogen-fixing plants can provide a reservoir of moisture, nitrogen-enriched soil, fungi, and other soil biota that crops can potentially tap into from the surrounding beds. Farmers can also under-sow them beneath taller plants, such as brassicas and solanums, or simply allow them to invade beds where it’s not a problem.

Originally, Chae and Eckert tried using a mix of clover and ryegrass, but the rye simply lost out to the clover after repeated cuttings. Cutting the clover down with a weed wacker was also a problem, because it broadcast the trimmings onto the surrounding beds. This addition of nitrogen-rich material could be great for other applications. However, like many urban farms, Fisheye grows a lot of high-value salad mix that’s in demand at both farmers markets and restaurants. “It does slow down the washing of the salad, because of all the clover in there,” Chae says.

Undeterred, Fisheye plans to experiment with a narrow reel mower that will drop the clover debris directly back down into the aisles. They’ve also been paying a neighbor to weed the clover out from the beds, which takes about eight hours a year.

Inside the farm’s hoop house, Chae and Eckert use peas and oats to prepare for spring production. This combination of legumes and grasses is popular, because both plants fix nitrogen and carbon in the soil. Although cover crops can suppress weeds, Chae and Eckert first put down a black tarp for a year to kill the bindweed growing there. Along with quackgrass, bindweed is one of the most persistent weeds Detroit growers face. The peas and oats will smother the remaining bindweed, enriching what will be a high-production space. Chae acknowledges that the bindweed might survive, but he believes in “putting down organic matter that will make weed management easier in the future, even if it doesn’t kill the bindweed.”

Patrick Crouch, the program manager at Earthworks Urban Farm, Detroit’s first Certified Organic farm, has also struggled with making urban cover crops work. He tried low-growing white clover for years, even going so far as to allow it to take over beds, where he planted brassicas directly into cover using a bulb planter. He mowed down the cover beforehand, weeded around the transplants for several weeks until they were established, and then let the clover go.

“It worked well for the first season,” Crouch says. “The plants seemed really healthy, and we were reducing our tillage.” He also hoped the clover would reduce pressure from flea beetles, which he’d read would have difficulty finding the plants if they couldn’t see their silhouettes on the bare ground. However, he experienced a new problem when the farm’s vole population exploded. “The unforeseen consequence was that the constant cover made it so that hawks couldn’t see the voles well enough to hunt them and suppress their level. I was just seeing tons and tons of damage on crops, such as my overwintering carrots.”

Hairy vetch (Vicia villosa)

Crouch found a more workable use of cover crops by under-sowing hairy vetch — another nitrogen-fixing legume, and one that produces more organic matter — beneath tomatoes, peppers, and eggplants. He cultivates the soil for several weeks after transplanting, and then broadcasts the vetch by hand, counting on either rain or the moisture from the drip tape to germinate the seed. “And then I pretty well forget about the fact that the hairy vetch is there,” he says. “It does germinate, but it stays really short because it’s too shady.” In fall, he simply cuts back the solanums without disturbing the soil, and the vetch takes over in the cooler weather. “Come next spring, it starts flowering, at which point I cut it down and incorporate it.”

The timing is crucial. Crouch waits until the hairy vetch is on the verge of flowering and has maxed out its nitrogen-fixing potential before he kills it and incorporates it into the soil. Timing is especially important on urban farms, where growers often work in tight production windows and can’t afford to let a quarter of their growing space lie fallow under cover for a season.

Crouch has also used buckwheat. This plant doesn’t have the big payoffs in nitrogen fixation and organic matter addition that other covers do. But as a fast-growing, heat-tolerant crop, it’s easy to work into breaks in the summer growing schedule. It also suppresses weeds, traps nitrogen, accumulates phosphorous, and, in Crouch’s estimation, does a good job of breaking up compacted soil — a concern for Detroit’s urban growers. Crouch has used it between spring crops of salad mix and with the farm’s fall garlic planting.

Yet, the promise of cover crops can remain elusive, even for an accomplished farmer, when dealing with weedy soils. “Oftentimes, cover crops are presented as this magical item, and one of the ways in which they’re talked about is this idea of being ‘smother crops,’ ” Crouch says. “And they’ll work on some things, but I certainly don’t think they’ll get the nastiest of the nasties, the quackgrass and the bindweed.” The persistence of perennial weeds such as these has kept him from maintaining permanent covers under some fruit crops, such as grapes and raspberries, where they might be easiest to maintain.

However, cover crops will do things compost and fertilizer can’t: enter into the life of the soil; break up compacted earth; and produce exudates that nourish fungi and in turn build soil structure. “Anytime you’re talking sustainable soil improvement,” Edwards says, “you want plants, and you want roots doing the majority of the work; you don’t want humans to be adding a lot of inputs.”

For organic farmers, making urban cover crops work could mean fitting them in when and where they’re able. “For a lot of urban farmers, taking things out of production isn’t a very enticing idea,” Crouch says. “The solution is just to throw more compost down, but that solution, from my experience, isn’t adequate. You’re not really improving overall soil health in the way that you are when you’re utilizing cover crops that are able to really feed that root biome.”

Brian Allnutt is a Detroit-based writer who has covered food justice, urban agriculture, and environmental issues in both Flint and Detroit, Michigan.

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