Planting Cover Crops at the Potomac Overlook Organic Vegetable Garden

By Judy Salveson, Extension Master Gardener

As soon as September arrives, it is time to start planting winter cover crops in the vegetable garden at Potomac Overlook Regional Park (2845 Marcy Road, Arlington, Virginia).

We start with a favorite, the versatile and lovely crimson clover (Trifolium incarnatum), which makes a low growing carpet of bright green from fall through spring, until it sends up 18-inch spires of deep red crimson in late April. It will suppress the weeds and add needed nitrogen to the soil for the next crop. This year, we will plant crimson clover in a bed where we grew greens and herbs and where we will plant eggplant next year in early summer after the flea beetle infestations that plague eggplants have subsided. The crimson clover will have time to show off its blooms in late May and then be cut down to dry as a mulch for the eggplant.

The most-used cover crop in the garden is a combination of field peas (Pisum sativum subsp. arvense) and oats (Aveena sativa). The nitrogen-fixing contribution of cold-hardy field peas is enhanced by the nutrient-holding capacity of the oats. The appeal of this combination is that it is fairly easy to work into the soil in spring when we want to start planting our early crops. The field peas have stems that are readily dislodged, and the oats are helpfully injured by the cold of late winter, making them easier to incorporate into the soil. This year we will plant a mixture of field peas and oats in the beds where we grew greens and herbs and where we will plant tomatoes and peppers next year. We will have plenty of time, at least two to three weeks, for the field peas and oats to break down in the soil after we turn them under, before we plant the tomatoes and peppers in May.

We have used winter rye (Secale cereale) as a cover crop because it can add a great deal of organic matter to a depleted bed, especially when it is combined with a nitrogen-fixing legume such as crimson clover and/or field peas. Winter rye can germinate at lower temperatures than the other cover crops we use, and therefore can be successfully planted as late as the end of October. To reap its maximum benefit, it is better to let it grow until it reaches its full height and flowers, then cut it down to incorporate into the bed, but this is labor intensive and delays planting the following year. If not allowed to flower before turning under, winter rye will keep growing back and require repeated cultivation to break down. We now rarely use winter rye alone. When needed, we prefer to use it in a cover crop mixture containing several grains and legumes for earlier and easier soil preparation.

For more information on how to incorporate cover crops in your garden, see
Best Bets: Cover Crops to Enrich Soil.