How to extend your gardening season, and plant winter cover crops to protect the soil
STORY BY JOSH VOLK
September is the month of the equinox, the transition from summer into fall, and the best time to start preparing the garden for winter. Here in the Pacific Northwest, our weather is still relatively warm, but the days are getting shorter, and by the end of the month the sun is up for less than half the day. No matter how warm the air stays, summer crops slow to a halt as the soil cools off. That cool soil favors germination of cool-season crops — and cool-season weeds — and as gardeners, we need to start preparing the garden soil to handle the inevitable winter rains.
Relatively speaking, our winters really aren’t that cold here, and there are still many crops that can be harvested through the winter months, especially if we have a mild one. Many root crops, like carrots and hardy leafy greens like kale, often improve their flavors with the cold. They can be at their absolute peak all the way into the winter, and that’s something to celebrate. By the time we’ve hit September, the planting window for most of these crops has passed, with July and August being the peak planting months for late fall and winter harvested crops.
But there may be a few crops you can still sneak in.
Quick-growing salad greens, like baby lettuce, arugula, and mustards can all be planted with success into early October, if they are given a bit of protection with floating row cover. I’ve used Agribon AG-19 as a blanket that lets air and water through but gives protection from the cooling breezes and night skies. It can be placed directly on top of the crops or held up with simple hoops and held down with rocks, sandbags, or scoops of soil. The row cover is light enough that the crops push it up as they grow, but if you use hoops to keep the fabric up off the crops, it gives a bit more protection from freezing conditions and extreme wet that can cause rots.
Any crops that stay in the ground through the winter help protect the soil from winter rains. Hard rains beat against uncovered soil, causing surface compaction and erosion. As water seeps through the soil, it picks up unused nutrients and can carry them well below the rooting depth of the plants. Crop plants, and cover crops that are planted specifically to feed and protect the soil, help minimize any damage from winter rains — and can even improve the soil. They create a physical barrier to the raindrops, depositing the rain more gently on the soil. Their roots help hold the soil in place and create pore space for the water to soak in. The roots also take up nutrients for the plants to store through the winter, releasing the nutrients only when the plants die and decompose the following spring, making those nutrients available again to the spring-planted crops.
On the farm, my go-to winter cover crops are a combination of cereal rye and common vetch for completely bare spaces, or crimson clover for seeding in with late-standing summer crops like peppers or overwintered crops like kale and collards. Both vetch and clover are relatively inexpensive and help fix nitrogen in the soil to be available the following spring. Cereal rye is a grass with a vigorous root system, and grasses are excellent at scavenging for nutrients, helping keep them from washing out during the winter.
To plant the cereal rye and common vetch, I prepare a seedbed and then broadcast the seed, raking it in and then firming the surface of the soil with the head of my rake. Firming the soil pushes it into contact with the seeds, which helps transfer moisture from the soil to the seeds. It can improve germination significantly. I use a heavy rate of about an ounce of rye mixed with four ounces of vetch per 100 square feet (approximately 1/3 cup of rye seed mixed with 3/4 cup vetch seed).
If I’m using crimson clover, I throw the seed out, as evenly as I can, under and around the standing crops, using just an ounce of seed per 100 square feet (approximately 2-3 Tablespoons). The seeds are quite small, so I lightly scratch them in — or just leave them on the surface to germinate.
I’ve found that the timing of seeding fall cover crops, as with the timing of seeding most crops, is important for best success. I try to seed my fall cover crops in the last two weeks of September — or at least by the middle of October. I find that if I seed them earlier, they are less winter-hardy, and they have more competition from summer weeds that are still germinating in the warm soil. If I seed them too late, the winter weeds are too competitive, and the cover-crop seeds get off to too slow a start to really fill in before the weeds take over.
I find it very helpful to irrigate the cover crop with a sprinkler at least once to get them to germinate quickly. Quick germination helps them outcompete any weeds.
In some places, I just let the natural weeds come in for the winter. Chickweed has many excellent features, and when I can’t get cover crop planted on time, it provides reliable winter cover for my soils. My good friend Frank Morton at Wild Garden Seed loves to point out that chickweed is one of the best early-season flowering sources of nectar for beneficial insects, and it’s also a delicious salad green.
In the spring, when I’m ready to re-plant beds, I strip the cover crop off the beds with a hoe or by hand. I either compost the plants and add them back to the beds or use the compost as mulch in the garden. I find it too labor-intensive and time-consuming to turn cover crops back into the soil directly without a tractor that has a mower to chop it up finely.
There are many other good options for winter cover crops, including “soil builder” mixes that include multiple species and varieties, pre-mixed. Some will more reliably make it through our cool, wet winters; some can be left to grow until it frosts, which will kill the plant in place, creating its own mulch.
The most important thing is to have something growing, even if it’s just weeds. Soil protected with mulch or plastic can also be quite good, but you won’t be encouraging nutrient cycling and supporting the growth of soil critters that interact positively with living roots.
Josh Volk is the author of the book Compact Farms. He grows vegetables in Portland at Cully Neighborhood Farm, experiments with fruits and herbs in his home garden, and travels around the country speaking to farmers and gardeners. You can find more about Josh at slowhandfarm.com.