Backyard Gardener: Hard frost marks end of growing season
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Hello Mid-Ohio Valley Farmers and Gardeners! Most likely we will say goodbye to the growing season this weekend in the Valley as frost is in the forecast. Any late squash, tomatoes or other vegetables need to be harvested quickly.
Don’t forget to move houseplants back indoors before they get frosted. Many homeowners move indoor plants onto the deck or front porch to enjoy during the summer. To be safe, Extension recommends houseplants be brought indoors before nighttime temperatures dip below 45 degrees Fahrenheit. Keep in mind many of our houseplants are tender tropicals, and some will suffer damage even at temperatures below 50 degrees Fahrenheit.
Frost forms when air temperatures fall close to freezing (typically from 33 to 36 degrees Fahrenheit) while the surface temperature of plants, soil, turfgrass, vehicles, or other objects drops to 32 degrees Fahrenheit or below.
When this happens, moisture in the air condenses into liquid dew or directly forms ice crystals. Surfaces cool more quickly than the surrounding air, which is why frost can develop even when the recorded air temperature may not reach 32.
The dew point is an important factor in frost risk; it is the temperature at which air becomes fully saturated with moisture. When the dew point is above 32, frost formation is less likely because moisture condenses as liquid rather than freezing.
When the dew point is below 32, frost is more likely, especially in dry air, which cools faster than humid air. In open fields with bare soil, radiative frost can occur at the height of the plant canopy during clear, still nights as heat is lost from the soil surface, increasing the chance of cold injury.
Frost damage is dependent on species, hardiness, variety, plant age, the specific plant parts damaged and duration of surface freezing temperature exposure. Frost injury tends to be more superficial and localized, and less severe than that of freeze, with symptoms like light bronzing or bleaching on foliage, marginal burns and small scattered necrotic spots. A few minutes of exposure may cause little harm, while several hours can lead to significant injury.
Do not be discouraged by the coming frost. First, many cold-hardy vegetables such as swiss chard, kale, collards and other greens actually become sweeter after the frost. Kale and swiss chard can survive temperatures into the lower 20's. Second, my grandparents said two weeks of good weather (warm and dry for farmers to harvest crops) always followed the first hard frost.
A killing frost can result from canopy temperatures dropping 2-5 degrees below freezing for five to 10 minutes, or from a sustained temperature 31.5-32 F lasting three to five hours. Most of our summertime heat-loving vegetables such as tomatoes, peppers, sweet corn and beans will be killed by the hard frost or freeze.
Your best bet is to harvest warm season sensitive crops quickly before a hard freeze, especially tomatoes. A hard freeze will cause tissue damage, and fruit will deteriorate rapidly.
Tomatoes can be ripened indoors. Look over your plants and pick any tomatoes that appear to be partially ripe or light green. Fruit that is dark green will not ripen indoors; it hasn't matured enough. Check for disease or dead stems, ensure the plants are still healthy, to ensure a safe product.
Tomatoes being ripened should be stored at temperatures between 55 to 70 degrees Fahrenheit in well-ventilated areas out of sunlight. Green tomatoes should ripen in about two weeks, while more mature fruit will ripen in a few days to a week in ideal conditions.
Root crops such as carrots, turnips, beets and parsnips of course are below the surface of the soil and are not affected by frosts. However, they can be affected by freezing. Root crops can remain in the garden after a frost and still be removed in good condition later, but get them dug and stored before the ground freezes hard.
Keep in mind sweet potatoes are a little different. After a killing frost, immediately get those sweet potatoes dug and into storage. They will quickly begin to rot from diseases entering the plant through the dead top.
Be careful digging! They don't have a protective hard layer as do the tubers of Irish potatoes. Cuts and bruises on sweet potato roots are not good, as they often lead to rotting in storage.
Irish or cobbler potatoes can tolerate light frost, but when the first hard frost is expected, it's time to get out the forks (I mean potato forks) and start digging. Same story with scrapes and bruises with Irish potatoes. They will cause rot during storage. Damaged potatoes should be used as soon as possible.
After harvesting, all potatoes must be cured. Let them sit at temperatures of 45 to 60 degrees for about two weeks. Make sure potatoes are well ventilated and not exposed to sunlight. Exposure to sunlight will cause potatoes to turn green and produce solanine, which is a toxin and can cause digestive upset. The curing process will give the skins time to harden and minor injuries to seal.
Floating row covers, low tunnels and other protective covers can be used by home gardeners as a low-cost way to protect sensitive vegetables from frost. Floating row covers are lightweight synthetic fabrics laid directly on top of a row of plants to protect against cold and insects. Low tunnels are fabric or plastic covers supported above the crop using wire hoops, metal pipe or bent PVC hoops.
Material edges for both are commonly weighted with sandbags or simply buried with soil to prevent loss due to wind. Lightweight covers come in varying sizes and weights, providing different levels of frost protection.
Greenhouse film (plastic) provides significant frost protection while still allowing substantial light transmission. However, the primary drawback is that this material is not self-venting. Gardeners must remove the cover on sunny days to prevent overheating.
Contact me with questions at the WVU Extension Office at 304-424-1960 or at jj.barrett@mail.wvu.edu. Good Luck and until next time, Happy Gardening!