As the days get cooler and the sun starts setting a little earlier, it's easy to get depressed about the waning days of summer — and of your summer vegetable garden. But cool temperatures don't necessarily mean an end to free, fresh, local food. Now is actually the perfect time to plant your fall vegetable garden with cold-hardy greens such as lettuce, spinach, chard, turnip greens (a real star in my fall garden), and many of the Asian greens.
Even if you don’t have a garden, you can plant greens in containers on your deck or sunny porch. Larger containers will hold more warmth at night and therefore better protect cold-sensitive roots than smaller ones (old picnic coolers make great fall planters, for instance). Or you can plant greens in shallow, lightweight flats and bring them inside on cold nights.
In either case, the one thing you need to do that you don't need to worry about in summer is to add a little extra frost protection once temperatures start getting really low. Hardy greens will survive a few frosts and chilly days without any frost protection, but in most areas of the country, you will need to protect them against the cold to extend your harvest into late fall and winter.
The simplest way to extend your greens harvest is with a product called a floating row cover, made from a non-woven fabric that insulates but still lets in quite a bit of sun without overheating (as clear plastic would). It's a good idea to start using row covers when nighttime temperatures start dropping into the 30s and 40s. For cold protection, get the thickest, wooliest type you can find. It is very easy to use: Spread it over your plants, leaving a little extra material to allow for plant growth. Tuck the edges down against the soil all the way around, and weight the edges with sticks, rocks, or sandbags to keep the cover from blowing off in the wind. Floating row covers will protect your greens and allow you to harvest for a few extra weeks.
A row cover is easier to manage in the early fall, as it doesn't overheat during the day and you can just leave it alone once you've put it on. But once cold weather sets in in earnest, or the snow starts to fly, you will need a bit more protection to continue to extend your harvest.
Cold frames (unheated boxes with glass lids) are a simple and very effective way to extend your harvest. You can buy fancy ones or whip one up from an old window or patio door. Even a wooden frame covered with clear plastic would work. Set four to six bales of straw in a rectangle around your greens, rest your window or frame on the top, and voila! The size of your window will determine the maximum size of your cold frame and how much of your garden it will cover; all four sides of the window will need to rest on the hay bales with no gaps.
Cold frames require a bit more diligence than row covers, since they can easily overcook your vegetables on a warm day. The key is keeping the interior temperature around 60 degrees during the day. When daytime temperatures are in the 40s or higher, you'll need to slide the top open to vent excess heat. And remember to slide it closed in the late afternoon, to hold the warm air in at night.
The Cadillac of harvest extension is the hoophouse, a simple unheated greenhouse made from sturdy arched hoops and clear plastic. Heavy clear plastic from home-improvement stores works, but it will last only about six months before it starts to break into bits. UV-resistant greenhouse plastic can be reused for multiple seasons. You can cover a single bed of greens using five- to six-foot-long lengths of heavy-gauge wire. Insert one end of the wire into the soil about a foot deep, arch it across the garden bed, and insert the other end into the soil to make one hoop. Space the hoops every four to five feet along the length of the bed, and then cover them with a sheet of clear plastic wide enough to leave a foot of material on either side of the tunnel, and long enough to leave loose flaps that hang over either end. Securely weight the edges and ends with sandbags, lumber, and/or rocks; it helps to attach the plastic to the frames with staples or tacks to keep the plastic from blowing away. As with a cold frame, you'll need to open the ends of the hoophouse to vent excess heat on warmer, sunny days.
To learn more about building and using hoop structures, my favorite resource is The Winter Harvest Handbook by Eliot Coleman (Chelsea Green, 2009). And, if you're interested in installing a hoophouse in your backyard but you're intimidated by the DIY construction, you can order kits online from Hoophouse.com and other garden suppliers.