Ron Kujawski | Garden Journal: Beware of lower temperatures and radiative frost

Though it is not unusual to see a coating of frost on plants at some point in September — usually late September — it did surprise me to find frost damage on many of the sweet potatoes and a few of the squash and tomato plants in my vegetable garden on the morning of Sept. 2. I know what you must be thinking, i.e., air temperatures were above the freezing mark on that morning. Yes, they were. In fact, the low air temperature that morning at my garden site was 36 degrees. Thus, how did my plants suffer from frost damage?

The answer is: "radiative frost." During the day, the sun's radiation heats the surfaces of plants and earth; at night, they lose radiation back toward the sky. On a clear, windless night, heat lost from the plants can occur quite rapidly, resulting in intense cooling of the surfaces of plants and other objects. On such a night, these surfaces can be as much as 5 degrees colder than the air temperature. In the case of my garden, that meant that the surface temperature of the plant leaves was actually at or below freezing. This resulted in some damage to plant tissue as represented by a darkening of the leaves, mostly the top or outer most leaves. The vegetables that showed any damage were species of tropical or sub-tropical origin. Plants that can tolerate light frosts were unaffected.

I am not about to predict the weather through this month, as erratic as it can be these days, but perhaps we need to pay attention to chilling forecasts a little sooner than usual.


- Shop early for bulbs to get the best selection. This is especially important with daffodils. Those with side bulbs attached are the most sought after and are selected first by the knowledgeable buyer.

- Become a root inspector! It's easy and you don't even need a badge. When your houseplants appear troubled, the cause may be below ground. Remove the plants from their pots and inspect the roots. Healthy roots will always be white near their tips. If houseplants need repotting, use pots which are opaque. Studies have shown that plastic pots that allow a significant amount of light through the pot wall can retard the development of roots of some houseplants.

- Begin exposing Christmas cactus to short day conditions to have it in bloom for Christmas. Keep your plant in total darkness for 12 to 14 hours each night until flower buds form. The temperature should be under 65 degrees during this period. At temperatures over 70 degrees, buds will not form.

- Dig gladiolus bulbs (actually corms, for those who want to be technically correct); allow them to dry and then cut off the leaves. Store them in net bags in a location where there is plenty of air circulation. Do the same for tuberous rooted begonias but wait after the first light frost to dig them.

- Entice birds to your yard by planting fruit-bearing trees and shrubs, especially those with fruits that tend to hang on during the winter. Some good choices are gray dogwood, red osier dogwood, bayberry, elderberry, snowberry, hawthorn, crabapples and viburnums including high bush cranberry.

- Pinch out the tops of Brussels sprouts to hasten the development and maturation of the side buds or sprouts. Also remove the lower leaves to allow the buds room to expand.

- Don't harvest grapes until they are fully ripe as they never improve in quality after they are picked from the vines. Proper time for harvesting can best be determined by an occasional sampling of a grape. If it causes you to pucker, leave the little sucker; if it causes you to smile, pick a little pile. (Ugh! Maybe I should skip the poetry next time.)

- Harvest fall-bearing or ever-bearing raspberries as quickly as they ripen. Frequent picking helps reduce insect and disease problems that often accompany over-ripe fruit.

- Make a large batch of potting soil by mixing equal parts of garden loam, sand and compost or peat moss. Store your mix in a large garbage can for winter use.

- Don't panic at the sight of browning of needles on evergreens, especially if it is the inner-most or oldest needles which are affected. This occurs in pines, spruces, arborvitae and junipers. It is a perfectly natural phenomenon since each year these trees grow a new set of needles in the spring and drop their oldest set in the fall.


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