Uncovering Cover Crops

Sowing a cover crop this fall may sound like no more than providing a cozy blanket for the soil to snuggle under during winter.

But that’s only the start. Of far greater importance, cover crops provide a natural way to build healthy soil. They add organic material, replenish nutrients, cycle nutrients from the depths to the upper layers, loosen up compaction, help stem erosion and crowd out weeds. They also provide sustenance to beneficial insects during the cool season when other forage or habitat plants may not be growing.

The technique lost favor in the middle of the last century, when chemicals became the widespread solution to just about any ailment associated with growing food. Now, with an increasing number of farmers embracing organic and sustainable methods, cover crops are making a comeback. Moreover, cover cropping is just as effective in home gardens as it is on vast tracts of land.

Getting Started

Technically speaking, cover crops can be grown during both the warm season and the cool season. But most of us are reluctant to give up space during summer, when many veggies grow big and sprawl.

On the other hand, winter edibles often take less room and more of our planting beds lie fallow. Now’s the perfect time to sow cover crops to rejuvenate the soil.

Choose empty vegetable beds and any new areas slated for edibles. If you’re converting lawn or thirsty ornamental landscaping to drought-tolerant plantings, prep the soil by sowing a cover crop there, too.

For planning purposes, expect to devote your beds to cover crops from now through next spring.

What to Choose

Legumes, grasses and cereal grains are traditional cool-season cover crops, while thick-rooted radishes are relatively new to the game.

Cool-season legumes include clover, vetch, field peas, alfalfa and bell (fava) beans, and are essential for fixing nitrogen in the soil. Symbiotic Rhizobia bacteria gather within pink nodules on a legume’s roots, grab nitrogen from the air, then add hydrogen to convert the nitrogen into a form that the legume can use. In return, the legume provides carbohydrates to the bacteria. When the legume dies or is turned under, the nitrogen is released and available to other plants.

To ensure a good fix, apply a Rhizobia bacteria inoculant before sowing your legumes. Spread the inoculant, either in granular or powdered form, on a thick sheet of paper. Sprinkle the seeds with water or milk, then shake them across the paper to coat them. Sow the seeds immediately—the bacteria are alive and will perish in sunlight.

Grasses and cereal grains, such as annual ryegrass, barley and oats, add fertility and improve soil structure. These grasses and grains may not be as pretty as ornamental equivalents.

But their spidery roots dive deep to retrieve nutrients as much as several feet below the surface and to create tunnels that improve drainage and provide passageways for earthworms to easily traverse.

Meanwhile, above ground, they inhibit weeds and provide habitat for beneficial insects.

Radishes possess a remarkable ability to drill into compacted soil. Daikon radishes (also known as Japanese or forage radishes) grow single, stout taproots, while oilseed radishes are stubby and branched.

Unlike other cool-season cover crops, radishes are not turned into the soil at the end of their lives. Instead, they are left in place to shrivel and decompose. The remains add biomass to the bed, while the resulting labyrinth of underground channels improves drainage and tilth. However, be warned that those decaying roots smell like rotten eggs—especially during warmer weather.

How to Sow and Grow

Cover crops are sown relatively thickly from seed, rather than grown from transplants. Broadcast the seed in September or early October while the soil is still warm. Wait too long, and your crop may not germinate well or be as vigorous heading into winter.

Pull any weeds, rough up the soil with a stiff rake, pulverize any dirt clods with the back of a shovel, apply an inoculant if you’re planting legumes, and scatter the seed at the rate noted on the packet.

Rake in the seed to the recommended depth, then gently water. Add a thin mulch of straw, finely shredded leaves, well-aged compost or topper, then water again.

Keep the soil moist until seedlings have poked their heads through the soil and produced their first sets of leaves, which should be within a few weeks. Reduce irrigation to once a week to 10 days or longer. Stop completely if the rain begins.

In general, cover crops need more water early to become established, and again when temperatures warm up in spring.

When to Call It

While tillage radishes are left in the ground to decompose, you’ll need to chop down and work the remains of legumes, grasses and cereal grains into the earth.

The knock-down is typically four to six months from sowing, so likely to be in March or April. However, pay attention to the crops’ maturity. It’s critical that the plants have begun flowering, yet their roots are still plump and pliable.

During my first go-round with fava beans, I mistakenly let the plants flower, set seed, then begin to wither. Lo and behold, the roots were so thick, fibrous and woody that I had to yank them out. It was impossible to cut them up and leave them in the soil to decompose within any reasonable time period.

Instead, at the optimal moment, mow down, weed whack or trim the foliage to the ground, then hack it into pieces no more than 2 to 3 inches long. Let the bits dry out for a week or two, then till them into the soil. Wait another three to six weeks, and you should be able to plant new vegetable seeds or seedlings, just in time for your summer garden.