In The Garden: Cool idea helps garden survive frosty nights

By | November 01, 2011
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As fall transitions into winter we never really know what’s in store for the garden. Sure, the eventual outcome will be freezing temperatures. But between now and then anything can happen, so I always plan for some protection, allowing me to extend the season into December.

We had our first light frost and a temperature dip into the 30s at the farm in early October, which is about right, but the season isn’t over yet. Protecting frost-tender vegetables is reasonable because after frost we often have what we call Indian Summer.

Once I understood the varying degrees of frost, I better understood how to protect my plants from the ill effects. Frost occurs when the surface temperature, at ground level, drops to 32° or lower and dew is present. The crazy thing is that this can happen even if the ambient air temperature is as high as 40°. On cool clear nights with no wind, frost is more likely because the accumulated heat in the soil radiates back into the atmosphere more quickly. A rapid heat loss will cause surface temperature to remain at or below freezing for a longer period of time pre-dawn. We have been taught that cool air settles and warm air rises; when there is no breeze to mix the two together, frost is more likely. The difference between a light, hard or killing frost is as much about how long the surface temperature stays at or below freezing as it is about the temperature. A short period before dawn allows only single ice crystals to form; longer periods of time allow the crystals to connect like filigree, resulting in a hard or hoar frost. A killing frost is called black frost because it usually turns plant tissue black.

So, while we cannot fool Mother Nature out of a hard freeze, we can keep the frost off and the keep the radiating soil warmth at the soil surface. Cold frames help us capture the warm soil air, and you don’t have to get fancy. You can construct a mini cold-frame out of old concrete blocks (or straw bales, bricks, 1- by 8-inch planks, literally anything that is going to give you the clearance you need, then top it off with a few old storm windows.

With cold frames I have successfully harvested spinach into December and wintered said spinach through the snows and ice-storms of January and February, to have it grow again in March with the revived version ready to harvest by month’s end. This rudimentary contraption found a new life in the summer when I used old window screens to keep deer and rabbit from grazing on my greens and later to provide some shade in the heat of July and August. High tunnels (unheated, plastic-covered structures) are used by professional growers to extend their seasons and we can make our own version out of partially deconstructed crates (check local garden centers after bulb season and the Christmas holiday season because bulbs and greenery are often shipped in crates that are re-useable). The hinged A-frame of an old crate can be covered in clear plastic stapled in place and set over a cool-season crop like broccoli, chard, kale, lettuce, turnips or pak choi, and it will protect the vegetables from a frost or a light freeze by capturing the soil’s warmth within.

The light frost and 36° temperatures in early October were thwarted by my simplest creation: Two old wooden chairs were placed at each end of a row of zucchini (which ultimately bore more squash as the following weeks warmed up) and heavy clear plastic was draped across the expanse. It was removed in the late morning before the threat of being baked on the sunny day that followed. Picnic table benches and bulb crates are great here, too.

Creating mini cold frame cones over individual plants is as easy as flipping a small cone-shaped tomato cage and wrapping it in heavy clear plastic; in effect you have a plastic teepee that can be moved around, removed on a warm sunny day and used again in the spring of the year to keep tomato seedlings warm if we hit a cold spell after you have set them out. The options are endless, really: Cruise the basement, barn or garage to see what’s lying around that can be employed as a reusable and recyclable cold frame that will extend your growing season on both ends.

Garden Pallet

Reuse and recycle — found objects as homemade cold frames

Old storm doors and windows: Lay over rims of brick, cement blocks, or other items to create a box around the garden.

Old screens: Similar to doors and windows, but protect only from light frost. Can also deter rabbit/deer grazing in spring and summer.

Wood crates and pallets: Can be used to create A-frame or box structures to which clear plastic can be stapled to form a mini cold frame. (See photo below.)

Bricks and concrete blocks: Use to support materials like old windows to keep the warm air near the soil surface.

Picnic table benches: Place over rows of vegetables and drape with plastic; remove when the sun comes up so you do not burn your plants.

Heavy clear plastic: Use to protect vegetable plants from frost; useful for wrapping tomato cages to protect plants; drape over supports to protect plants from light freeze or frost.

Fahrenheit Thermometer

Indian Summer is a weather event that brings back summer-like weather after a hard or killing frost. The American Meteorological Society defines it as “A time interval, in mid or late autumn, of unseasonably warm weather, generally with clear skies, sunny but hazy days and cool nights … at least one killing frost and preferably a substantial period of normally cool weather must precede this warm spell in order for it to be considered a true Indian Summer.”

Is it freezing yet?

Light freeze temperatures fall between 32° and 29°, cold enough to kill plant tissue of tender plants that have high moisture content in foliage.

Moderate freeze is 28° to 25°, enough to blacken foliage of most plants.

Hard freeze is 24° or colder. By the time a hard freeze hits, most hardy and deciduous plants have dropped foliage and gone dormant for the winter.

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