Frost 101: it’s easy to prolong the harvest
After such a long, hot, dry spring and summer, cooler weather and frost seem hard to imagine. But if you plan and prepare now, you’ll be ready at a moment’s notice when frost is predicted.
We’re just assuming you want to protect the tenderest plants like tomatoes, eggplant and annual flowers. Because an early frost event can happen anytime, and then be followed by weeks of warm weather, it seems worthwhile to fool Mother Nature to prolong blooming annuals and the vegetable harvest.
And it doesn’t take much effort or expense. Here’s the primer on frost and keeping your garden growing.
• The average first fall frost along the Front Range is Oct. 7, but we’ve seen frosts as early as Sept. 8 back in 1962.
• When temperatures dip below 32 degrees, ice crystals develop and cause water in plants to freeze. A light frost, from 29 to 32 degrees, will kill very tender plants like annual herbs, beans and cucumbers. A moderate freeze, between 25 and 28 degrees, will destroy most annual vegetation and fruit blossoms. And a hard freeze, at 24 degrees, is considered a killing frost and the close of the gardening season.
• Many factors influence frost’s effects, such as bed location (south on a slope has more frost protection), wind (a breeze is good, but not a 32-degree wind), humidity (higher is better) and soil moisture (it helps keep plants warmer, so water plants before a frost event).
• Falling afternoon temperatures, along with clear, calm skies, can be signs of incoming hoarfrost, an old English term that compares frosted branches to white hair. Your odds of safely protecting your tender plants are actually higher with this type of frost.
• When a cold front moves into the area under clear or cloudy conditions with gusty winds, the air gets very cold from the ground up. Plant protection from this type of frost (called advective frost) is more difficult.
• The key to protecting plants is to cover them with the right materials and place them correctly to trap the warmer air from the soil near the ground. Protection must be draped and spread to the ground and then held in place by boards, clips, pins or rocks.
• Blankets and sheets work well as long as they remain dry. Cover them with plastic if moisture accompanies frost.
• Floating row covers made of Reemay fabric (gauze-like) placed directly over plants do an excellent job of frost protection (plus they are great for keeping insects out and starting seeds in the spring). This material allows water to pass through without damaging plants, including very large tomato plants or beds of annual flowers. Thicker floating row covers can protect plants down to 24 degrees. Find floating row covers (from thin to thick) at local garden centers or mail order companies. They can last several seasons, so they’re worth the price if you want to prolong harvest.
• Plants must be protected from direct contact with plastic covers or the plant will freeze. So use plastic over a frame, hoops or tunnels, and be sure to extend the cover and secure it to the ground to trap warmer air.
• Other methods of cold protection are available including space blankets (found in camping stores), cloches, or the use of holiday lights under cold frames. In a pinch, use what you have on hand. An emptied plastic garbage can or 5-gallon paint bucket will cover a pepper or eggplant.
• Remove all the covers once the air temperature rises above 38 degrees. More: cmg.colostate.edu/gardennotes/722.pdf or check out Jeff Ashton’s book, “The 12-Month Gardener” (the 2002 classic from Lark Books can be found used online).
• Continue to harvest summer-growing vegetables.
• Harvest winter squash and pumpkins before frost, when they’re fully mature and a deep color. You should see no green veins on butternut squashes, for example. A great indicator: Harvest when a fingernail cannot easily puncture the rind.
• With some protection, hardy, cool-season crops like kale, spinach, chard and arugula actually improve in flavor after a light frost. Mature broccoli and cauliflower tolerate some frost (down to the 20s).
• Parsnips and carrots love cool weather and taste sweeter after a frost, so mulch them well with leaves or weed-free straw. Dig and use them as needed all through the winter until early spring.
• Collect the seeds of heirloom varieties, dry thoroughly and place in sealed containers for next year.
• Sow some seeds of spinach and lettuce, then mulch them. They’ll sprout this fall and go dormant with cold weather. Next spring, they will be your first crop to harvest while other gardeners are still dreaming of a crop.