Killing Frosts: A Letter From Joel Rosen

The effects of frost are often misunderstood by gardeners. Generally, climatologists and agronomists classify frost as light, moderate, or hard. Light frost (29-32) will usually kill or damage warm weather annual foliage and stems (basil, cucumbers, and melons are especially sensitive). Moderate frost (25-28) will damage or destroy the fruits of the warm weather annuals like squash, tomatoes, etc (especially if their protective foliage has already been destroyed in a previous frost). Hard or severe frost (24 or below) may cause damage to some of the less hardy coles like cauliflower and may induce at least temporary dormancy in grass and clover.

Of course most root crops, cole crops, and hardy greens will survive and in many cases improve in quality following temperatures in the 18-24F range. Further complicating the issue is the level of available potassium in the soil, which will increase even tender plants' frost resistance. I have on numerous occasions seen squash, pumpkin, tomato, and pepper foliage frozen as stiff as cardboard, only to thaw out in the sun with little significant damage. The amount of dew frozen around the foliage (acting as insulation) also provides some protection, while wind in sub-freezing temperatures will increase the level of damage. The heat stored in the soil also can make quite a difference to growth within 6" of the ground, so a late August frost when soil is still warm will often show less damage than a late September frost with the same temperature. Duration of frost is also a significant factor. Finally, the placement of a gardener's thermometer may cause confusion. A thermometer attached to a house wall or inside a porch will nearly always register significantly warmer temperatures in calm conditions. An accurate reading of 37F from a porch thermometer may be less than 100 yards away from a garden where everything is frozen stiff. The nearby unprotected garden in a low area could easily be 10 degrees colder than that porch reading on a still morning, enough frost to damage quite a variety of garden crops.

One final caveat. While warm weather fruits and vegetables may survive light frosts, gardeners need to decide if they are worth covering and saving. Basil begins to blacken and deteriorate after the first night or two of temperatures in the 30's and the quality of vine-ripened tomatoes deteriorates markedly once nights begin to dip into the 30's, epecially when the days remain cool. Due to its higher sugar content, a ripe tomato will "survive" more frost intact than a green one, but it may not taste a whole lot better than those green tomatoes many gardeners wrap in newspaper and ripen in warm places in the fall. Once the weather turns cold, I like to focus on enjoying the things that are starting to taste really good, like carrots, broccoli, cabbage, fall apples, etc.

Joel Rosen is a gardener and frequently contributes his weather information to John Latimer's Phenology Show. Joel lives on Park Lake near Matowah in Carlton County. The growing season and phenology in Joel's neck of the woods can be different from other parts of the KAXE listening area due to the influence on weather from Lake Superior