Hairy Vetch

Hairy vetch is a cover crop grown for what it leaves behind. It's a legume, which means it pulls nitrogen out of the air and stores it in root nodules, and when you terminate it in spring and let the biomass decompose, that nitrogen becomes available to the crop you plant next. A well-established stand can fix between one hundred and two hundred pounds of nitrogen per acre — roughly what a heavy-feeding vegetable crop needs for a season. But the value of hairy vetch depends entirely on two things: sowing it with a companion grass, and terminating it before seed pods form.

Sow hairy vetch in late summer or early fall, about ten weeks before your last spring frost — which in most climates means late August through September. It germinates in cool weather, establishes a modest rosette before winter, then explodes into growth in spring. The standard practice is to mix it with winter rye at about half the usual rye seeding rate. The rye acts as a living trellis; hairy vetch is a vining plant with weak stems, and without support it sprawls on the ground and produces less biomass. The two together produce more nitrogen and more organic matter than either would alone.

Inoculation matters if you've never grown vetch or other legumes in that bed before. The nitrogen-fixing bacteria that live in vetch root nodules don't exist naturally in most soils. You buy the inoculant as a powder, coat the seeds just before sowing, and the bacteria establish themselves. Without it, the vetch grows, but it doesn't fix nitrogen — you get green manure with no fertility benefit. Check the roots in spring; if nodules are present and pink or reddish inside, the inoculation worked.

Terminate hairy vetch at full bloom, which in most regions is May or early June. The critical error is waiting too long. Once seed pods begin to form, the seeds mature quickly, shatter to the ground, and hairy vetch becomes a persistent weed in the following crop — one that's difficult to kill and competes aggressively with your vegetables. Termination at full bloom, before pods develop, gives you the maximum nitrogen benefit with minimal weed risk. The standard method is to mow or roll-crimp the stand flat; the dense mat of dead vines and rye stems makes an excellent mulch for no-till transplanting of tomatoes, peppers, or brassicas.

The rolled mat tends to suppress most annual weeds for several weeks and keeps the soil cool and moist through early summer. You can transplant directly through it by cutting small holes in the mulch with a hori-hori or trowel. As the season progresses, the mat breaks down and the nitrogen becomes available — peak release is usually four to six weeks after termination. If you're direct-seeding small-seeded crops like carrots or lettuce, the mat is too thick; incorporate it shallowly with a broadfork or light tilling instead.