CORVALLIS, Ore. — No one wants to think of harvest’s end when the vegetable garden is at its peak, but now is the time to plant overwinter cover crops to improve soil for next season.
Cover crops can add organic matter and aerate the soil, protect it from compaction caused by rain, suppress weeds and reduce erosion, said Nick Andrews, organic vegetable specialist for Oregon State University Extension Service. As a bonus, if allowed to flower, their blooms provide nectar and pollen for pollinators and other beneficial insects.
Not a bad deal for an almost no-maintenance plant. To get started, prepare your soil and broadcast plenty of seed. Lightly rake it in and water as needed until fall rains begin. Then enjoy watching your cover crop grow during winter before incorporating it in spring.
Types of cover crops
Cover crops include grains such as winter oats and cereal rye, as well as a wide range of broadleaf plants. Legumes — including crimson clover, Austrian field pea and common vetch — are nitrogen fixers. Beneficial bacteria in their root nodules take nitrogen from the air and supply it to the plant. When the cover crop decomposes in the spring, some of that nitrogen becomes available to fertilize the following year’s vegetables.
If cost is a concern, nitrogen from legumes is much cheaper than organic fertilizer and can compete with conventional fertilizer prices.
Planting and timing
Timing is key, Andrews said. Plant seeds of overwintering cover crops by September or early October, before the weather turns too cold and wet for germination and establishment.
Make sure seeds have good soil contact. Larger seeds like peas, vetch and cereals should be raked in lightly. Mix small seeds with sand to make them easier to broadcast and then water them in. If the weather remains dry, keep the area irrigated.
For vegetables harvested after early October, consider interseeding into established crops during summer before they cast too much shade.
Managing cover crops in spring
Be prepared to manage your cover crop in the spring and kill it before it sets seed — unless you want to save seed from plants like fava beans. In Oregon’s climate, most gardeners incorporate cover crop residue to promote decomposition. Do this three to four weeks before planting vegetables so the crop breaks down well. Otherwise, it can encourage diseases and insect pests.
If you don’t have three to four weeks for decomposition, remove the stems and leaves and apply them elsewhere as mulch or compost, or dig the cover crop deeper into the soil. When to kill the crop depends on when you plan to plant vegetables.
"Big is better when it comes to cover crops," Andrews said. "If you can, let the crop grow until early flowering. Just be prepared to incorporate big crops if you grow them. You’ll see why organic farmers love cover crops."
When it’s time to kill the cover crop, till shorter plants directly into the soil. If plants are too tall, mow first or use a weed trimmer. Tough-stemmed plants can be cut and left to decompose above ground. Or put the tops in the compost pile and dig in the roots. Some experienced gardeners who prefer not to till are having success with tarping, Andrews said. No matter the method, allow the turned-under material to sit before planting.
Beginner advice
- Start with a cover crop that is easy to grow and manage, such as crimson clover or phacelia.
- Prepare your garden ahead of time and have sprinklers ready if the weather is dry. Water the soil before seeding if it is very dry.
- The first time you try cover crops, plant them in an area you can reserve for vegetables typically planted in May or June. This allows time to manage the residues in spring.
- After success with one cover crop, try another in a different part of the garden. With experience, experiment with mixtures, interseeding, tarping and other practices.
Previously titled Now’s the time to plan for cover crops
