Hard Frost vs. Light Frost: Protect Garden Plants

A light frost is about 32°F. It can damage basil, tomatoes, and other warm-season plants, even when it only lasts a few hours before dawn. A hard frost is 28°F or colder and will kill most warm-season crops.

A radiation frost comes on a calm, clear night as heat leaves the soil. That is when a cover has its best chance to hold a little of that soil heat around the crop. When a cold front brings much colder moving air, it is an advective, or wind-driven, freeze. A light cover cannot make up for severe cold and steady wind. Protect a close call with a well-built tunnel, but harvest tender crops rather than trusting a sheet through a hard wind-driven freeze.

Basil is less forgiving than a frost chart suggests. Its leaves deteriorate below 50°F and can discolor below 40°F. When several chilly nights are coming, cut the basil while it is still in good shape instead of waiting for a visible frost.

At 26°F, do not expect a sheet or floating cover to carry a fruiting tomato vine through the night. Pick mature green tomatoes and fruit showing its first blush of color so it can finish indoors. Harvest mature winter squash and pumpkins before a hard freeze as well. Their vines may take a light frost, but temperatures below 28°F can injure the rind and cut storage life short.

A cover works by holding heat released from the ground after sunset. On the day before a light radiation frost, check the root zone. If it is dry, water the soil early enough for it to soak in before dark. Moist soil absorbs heat during the day and releases it more slowly overnight.

Water the ground, not the foliage. Skip this step when the bed is already wet or cannot drain well. Never water frozen ground. Moist soil can help a properly covered bed through a close call, but it is not a defense against a severe wind-driven freeze.

Thin insect netting is not frost fabric. A standard floating cover adds only a small margin, while a heavier fabric or framed tunnel can offer more protection. The fabric rating and its installation both matter.

Bring the cover all the way to the soil and weigh down the whole edge. An open side lets the warm air out and pulls cold air in around the stems. A dry bedsheet can help for one night, but a wet cotton sheet can cool the air beside it as water evaporates. Put plastic over hoops or a frame so it cannot touch the leaves. Vent clear plastic as soon as direct sun begins warming the tunnel. A bright morning needs faster attention than a cloudy one.

Cold-hardy fall vegetables can become sweeter after cold weather. As temperatures fall, they convert some stored starch into sugars that help protect their cells. A well-established kale plant can take a frost that ruins a summer crop beside it.

In an acclimated brassica, ice can form in the spaces around the cells while the leaf is frozen. Leave it alone until it thaws. Bending or harvesting a stiff leaf damages the frozen tissue, and failed cells show up afterward as dark, water-soaked patches. Once the leaves are flexible again, check the center of the plant before deciding whether it will recover.

Cold air moves downhill and settles in the lowest part of a yard. It can collect beside a solid fence or at the bottom of a foundation wall where it has no easy path away. That is why one bed may frost while another stays clear on the same night.

Put warm-season crops on the higher ground where cold air can drain away. Use the low pocket for tougher crops when the layout allows it. Keep row-cover clips close to the beds that need them, because the garden will show you where frost arrives first.

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