These easy, affordable methods will keep garden soil warm—and plants producing food—well into the autumn.
As summer comes to an end, no gardener is ever ready to say goodbye to the days of fresh-picked, homegrown fruits and veggies. But as temperatures dip, keeping plants producing can be a challenge. The best way to keep your garden growing well into autumn is to manipulate the air and soil temperatures to best suit the plants you want to grow—which in autumn, usually means keeping things warm. While a greenhouse is perhaps the most well-known season-extension tool, gardeners without the space or resources to invest in a greenhouse can still extend their season by relying on a variety of flexible, inexpensive materials. Whichever method you choose, remember that most plants will benefit from being kept warmer at night, and can be uncovered during the day to enjoy the sun’s warmth. Sometimes, extra warmth is welcome in the daytime, too, but be sure to allow adequate ventilation under the covers. Some readymade products come pre-slitted for ventilation.
A cloche is a transparent glass or plastic bell-shaped jar (or repurposed plastic bottle) placed over individual plants to keep them warmer than the ambient air temperature and protect them from frost. The lightweight plastic versions often have a vent on top, which helps regulate the temperature inside the cloche, but they must be staked into place with U-shaped metal pins. Traditional glass cloches are more expensive, but they do not blow over, and they look absolutely lovely over rows of garden plants.
You can also keep plants warm and protect them from frost using a hoop house. A hoop house is a frame that can be used to support a variety of covers for plants. You can build hoop houses over your garden and leave them up year-round, using lightweight fabrics in summer and during the daytime to protect plants from insects and animals, and heavier blankets in fall or at night to protect them from frosts and freezes. (Learn how to make your own in the article "How to Build a Hoop House.") Materials suitable for covering your hoop house include readymade, ventilated row covers; lightweight mesh cloth; and old sheets and blankets.
Premade row covers are typically spun polyester fabrics that let air and moisture in while protecting plants from pests, fluctuating temperatures, variable weather and frost. The covers offer varying degrees of protection based on their weight: Lightweight covers are perfect in summer to protect plants from pests, including insects, deer and groundhogs, and the heavier “frost blankets” protect them from severe weather and cold. Reemay and Agribon are two common brands you will find in garden centers.
Clear plastic creates a temporary greenhouse when draped over a hoop house frame. Using UV-resistant, perforated plastic such as Gro-Therm raises the air temperature within the hoop house an average of 10 degrees. Because this plastic is ventilated, air still circulates beneath the cover, which reduces disease problems and prevents the plants from overheating on hot days. You can also use regular four- or six-ply plastic sheeting, available in the painting section of hardware stores, but it must be vented on hot, sunny days.
A word about using plastic in the garden: Many gardeners do not like to use a lot of plastic in the garden, for both environmental and aesthetic reasons. However, using plastic to heat up the soil and cover certain warm-season crops can dramatically increase yields and allow gardeners to grow more food for a longer period, reducing the amount of food purchased from the supermarket. If you do choose to use plastic, carefully store it indoors over the winter and you will find you can reuse it for many years. If using plastic gives you pause, investing in glass cold frames—essentially miniature passive solar greenhouses; read more below—and cloches is a fine alternative.
Perhaps the best way to modify the soil and air temperature for your plants, in addition to protecting them from winter winds and fierce precipitation, is to construct an inexpensive cold frame. A cold frame is essentially a box with no bottom and a clear top that can be removed during the daytime or when it’s warm outside. The top of the cold frame can be made of glass, plastic, polycarbonate or fiberglass, so long as it allows light to penetrate. Recycled storm windows make perfect cold frame lids, and can increase temperatures inside the box relative to the outside by as much as 20 degrees. The sides of your cold frame can be made of almost anything, from old boards to straw bales.
It’s important to secure hoop house covers firmly to the ground to prevent pests from sneaking underneath and wind from whipping them off. Garden clips are designed to secure row cover fabric and ventilated plastic to hoop house frames. You can also use extra-large binder clips. Another option is to weigh the material down with rocks or 2-by-4s, or buy row cover clamps that snap onto the covers and can then be staked into the ground, which makes it easier to check your crops.