In 1816, dubbed “The Year without a Summer,” a massive volcano erupted in Indonesia and left a sulfuric haze floating around Earth’s stratosphere. The layer of smog partially blotted out the sun in the Northern Hemisphere, resulting in a sharp decrease in temperatures and crop failures across Asia, Europe, and North America. Snow fell in Albany, New York, in late June. Thomas Jefferson even applied for an emergency bank loan after frost wiped out his summer crop at Monticello. Many people without access to “emergency bank loans” starved or survived on wild plants hardy enough to withstand the freezing summer weather.
Luckily, this was a once-in-a-millennium event. Besides the slow upward creep of global average temperatures, cold weather and frost dates are fairly predictable from year to year. These regional average temperatures limit gardeners in what we can grow and how long we have to grow it. However, with some advanced techniques and a bit of modern technology, we can extend our growing seasons earlier into the spring and later into the fall.
Row Covers The simplest form of season extension is to blanket your plants in row cover when temperatures dip down below desirable levels. Modern row covers are made of mesh polypropylene plastic which allows sunlight and water to pass through to the plants while providing insulation and up to 8 degrees Fahrenheit of frost protection. If you’re going to leave a row cover on your plants for longer than a day or two, it’s best to use some type of arch to hold it up off the plants. This adds a little bit more insulation from air between the plant and the cover and prevents the row cover from crushing your plants if it rains or snows.
Low Tunnels Low tunnels are semi-permanent structures that essentially act like unheated greenhouses that can be plopped right on top of your garden beds. To build a low tunnel over your garden beds, create a series of arches using PVC pipes, drape a thick-gauge plastic sheet over the arches, and clamp it down with clips that fit over the PVC pipes or a crisscrossing pattern of garden twine that holds the sheet to the arches. Because the plastic sheeting does not allow water through, remember to water your garden beds periodically. Also note that low tunnels do such a good job of providing insulation that if you get a warmer, sunny day it’s best to lift up the sides of the tunnel for part of the day to avoid overheating your plants.
Cold Frames Like low tunnels, cold frames are semi-permanent structures that can be placed over your garden beds. Instead of hoops and plastic, cold frames are built from old windows or glass doors held up off the ground by a wooden frame and angled towards the sun. They’re excellent for providing that extra bit of warmth to encourage seeds to germinate in the early spring, but because they are low to the ground, they don’t provide much room for larger plants to grow. If you don’t already have a raised bed to modify into a cold frame, straw bales are an affordable and good insulating material to use for the walls. Like with low tunnels, it’s a good idea to prop the roof open for ventilation on unseasonably warm and sunny days.
Misting it may seem counterintuitive to protect plants from frost by misting them with water before a cold night. But the freezing temperature of water is 32 degrees Fahrenheit, which is a tolerable temperature for many borderline hardy plants. The layer of ice formed around the plant provides a small amount of insulation and actually protects the plant’s vulnerable cells from freezing. This technique only works in marginal situations like light frosts and will not protect vulnerable plants from a hard freeze.
Incandescent Christmas Tree Lights Due to their wildly inefficient design, much of the energy used to light up incandescent Christmas tree bulbs is emitted as heat. If you’re expecting a frost, stringing up your fruit tree or decorating your garden beds with string lights can help stave off the coldest temperatures and protect your plants. Using this technique in combination with row covers, low tunnels, or cold frames can provide a compounded level of protection because the covers and tunnels will insulate and hold in some of the heat that the lights emit. We used this technique this past spring to protect our cherry tree from a late frost that threatened to kill off the opened blossoms and we hardly lost any flowers.
Although we aren’t likely to see any freezing cold summers in our lifetimes (knock on wood), every vegetable gardener should have a few tricks up their sleeve for an unseasonable frost or simply to extend their growing season. These season extension methods used in conjunction with home vegetable storage can help get us all one step closer to the dream of eating from our vegetable gardens all year.