Cover Crop, Hairy Vetch

Hairy vetch is a cover crop that is an attractive option for many growers in Massachusetts. It is hardy enough to survive the harsh winters of New England and can add significant amounts of nitrogen to the soil if allowed to grow long enough.

Hairy vetch is a legume which means that it lives in a close relationship with rhizobia bacteria that invade and establish themselves in the roots of the plant as it grows. These bacteria take nitrogen from the atmosphere (which is about 80 % N²) and make it available to the plant in a usable form. In return for the nitrogen, the legume gives the bacteria carbohydrates.

It is very important that the appropriate rhizobia species is used for hairy vetch (the rhizobia for hairy vetch will work for all vetches and peas). Without the rhizobia the vetch will not give the desired effects.

The rhizobia can survive in the soil for up to five years in the absence of their host plant. If you are seeding a legume in a field where that plant has been successfully grown in the past few years, then the rhizobia might not have to be added. However, if you are in doubt it is always best to add the rhizobia since it is easy and inexpensive.

The easiest way to add the rhizobia to the vetch is to add a little water to the seed (enough to make the seed moist but not so much that it clumps) and then to mix in the inoculant so that it sticks to the seed.

When left to grow long enough hairy vetch has supplied over 100 lbs/acre of nitrogen. We have observed excellent yields from crops such as sweet corn, peppers and broccoli following the incorporation of hairy vetch with no added nitrogen fertilizer. The amount of nitrogen you get from this cover crop will depend greatly on the amount of time you let it grow.

It is recommended to mix vetch with either winter rye or oat. There are several reasons for this:

• Both oat and winter rye are very efficient in taking up nitrogen from the soil (remember, the vetch is getting most of its nitrogen from the atmosphere, so it does not need much from the soil). By taking up more nitrogen in the late summer and fall we are reducing the risk of contaminating surface or ground water and the nitrogen is recycled so that it can be used by next years cash crop.
• The oat and rye can produce tremendous amounts of valuable organic matter if allowed to grow long enough.
• Both of these cover crops will give better erosion control than vetch alone since they emerge and establish themselves more quickly than vetch.

Early Seeding of Vetch

Hairy vetch can be seeded in late July or early August. This is a practical time for many vegetable farmers since they have land coming out of production at this time. Instead of leaving this land fallow until the fall when cover crops are seeded, we are encouraging growers to take advantage of this time to seed cover crops that will be able to produce significant amounts of organic matter and nitrogen.

We suggest mixing the vetch with oat at this time of the year instead of rye since the rye can get unmanageable the following spring when planted this early (oat doesn't overwinter in Massachusetts).

We have been recommending 40 lbs/acre of oat with 30-35 lbs/acre of hairy vetch. If you are using a grain drill then you can use seeding rates as low as 30 lbs/acre of vetch. If you are spinning the cover crop on and lightly disking it in then a rate of 35 lbs/acre is suggested.

The combination of hairy vetch and oat seeded in late July or early August will produce a tremendous amount of biomass by late fall. As stated above, the oat will winter-kill with a hard frost, however the dead residue will leave a thick mat that will provide erosion control. Hairy vetch seeded this early has less of a possibility of overwintering than if it is seeded later in August or early September.

Timing of spring incorporation of these cover crops and planting of your cash crop will depend on several factors. Probably the most important factor is whether the vetch has survived the winter. If the vetch did not survive the winter than it is advisable to plant a cash crop early in the spring. As the soil begins to warm up in the spring, the nitrogen in the dead vetch tissue will begin to be converted to water soluble forms of nitrogen. This nitrogen can be taken up by plants, however it can also be leached from the soil.

If the vetch has survived the winter you may want to wait a few weeks and get more growth out of the vetch which will translate into more nitrogen for your next crop. Take a close look at the vetch before you pronounce it dead, sometimes it will fool you. You can assume it is dead if it has not begun to re-grow by mid-April.

Late Seeding of Vetch

Here we are recommending that hairy vetch be seeded in late August or early September to ensure that it will over-winter. We have been recommending the seeding rate of 30-35 lbs/acre of hairy vetch in combination with 40 lbs/acre of either winter rye or oat.

Many growers seem to prefer the use of oat rather than rye with this later planting because of the tremendous growth of rye that occurs in the spring. This can be desirable if you are looking for increased organic matter in your soils, however some growers find the amount of biomass created by these two cover crops too much to handle.

It is important that you let the vetch grow long enough in the spring so that enough nitrogen is accumulated in the cover crop tissue. There will be only a small amount of nitrogen in the vetch early in the spring because it is so small. Plowing down a stand of vetch and rye in late April that was seeded in early September the previous year will not be adding much nitrogen to the soil. However, allowing these cover crops to grow until mid May or later will translate to significant amounts of nitrogen being added to the soil.

By Frank Mangan (413 545-1178)
UMass Extension
Publ.# VEGICM 95-4