Fall, winter, and spring bring the danger of frosts and freezes to Texas gardens, orchards, and

landscapes. Although sporadic and unpredictable, these cold spells have left their mark on horticulture in Texas by wiping out peach crops, freezing pecans in their shucks, forcing the replanting of spring vegetables, killing valuable landscape plants, and necessitating the replacement of beloved avocado, citrus, and fig trees.

Home gardeners and commercial growers can minimize these losses by understanding how cold affects plants and implementing diverse strategies to protect them.

Plant freezing and hardiness

When the water inside plant cells freezes, ice crystals form that can pierce and damage the cell walls, killing the cells. As temperatures rise, fluids leak out of those cells, and they begin to decay.

Many ornamental and edible plants have mechanisms to resist freeze damage. Trees and woody plants that go dormant, such as pecan and peach, can tolerate very low winter temperatures. However, they can be injured if they are too slow to stop growth in the fall or begin growing too quickly in the spring. Many species of woody evergreens, such as hollies, can tolerate tremendous cold.

Some species of herbaceous (non- woody) plants are cold tolerant, enduring all but the most severe cold in Texas.

Advective freezes bring sudden, steep plunges in temperature, wind speeds of more than 4 mph, and masses of cold air from 500 to 5,000 feet deep. They may bring clouds and precipitation at the onset and can take 1 to 3 days to make their way out of Texas.

These freezes create uniformly cold temperatures throughout the plant canopy, sometimes damaging the plants from their lowest to highest points. The harm is caused by low temperatures and drying, sometimes relentless, winds.

Some of the most serious plant-damaging cold events recorded in Texas have been advective freezes. Most frequent in the winter, they occasionally wreak havoc in the fall as they usher in winter suddenly before the plants have time to acclimate. Freezes generally become less numerous and less severe as spring arrives, although it takes only one serious freeze late in the spring to damage tender transplants and spring-blooming fruits.

Radiative frosts occur when the sky is clear, and winds are less than 4 mph. During the day, the sun’s radiation heats the plants and soil; at night, they lose radiation back to the sky. Plants and other objects cool faster when skies are clear because of the unimpeded loss of radiation.

Depending on the amount of radiation that the plants gain during the day, they may cool steadily at night to the freezing point before sunrise. This can occur on clear-sky nights in the winter, spring, or fall. On cloudy nights, the clouds reflect radiation back toward earth, which slows plant cooling and reduces frost injury.

The most severe radiative frosts occur when the weather is cloudy during the day and clear at night. The clouds reduce the amount of radiation absorbed during the day; if they dissipate late in the day or early during the night, intense cooling and plant freezing may be experienced.

Because the tops of the plants are most exposed to the open sky, they are the likeliest parts to be injured by radiative frost. The leaves at the top of the plant slow the radiation loss from the lower sections, so the cold damages the plant’s outer and upper parts most on frost nights.

Under radiative conditions, the leaves, stems, and other plant structures that have full sky exposure can be as much as 5 degrees colder than the recorded air temperature. Therefore, some plants show frost injury even when the recorded air temperatures did not drop below 32°F.

Assessing damage

After a freeze or frost, the leaves of damaged herbaceous plants may immediately appear withered, and water soaked. However, the freeze injury to the twigs, branches, or trunks often doesn’t appear on shrubs and trees right away. Wait a few days and then use a knife or thumbnail to scrape back the outer bark on young branches. Freeze-damaged areas will be brown beneath the bark; healthy tissues will be green or a healthy creamy color.

Delay pruning until time reveals the areas that are living and dead and until the threat of additional frosts or freezes has passed.

Leaving dead limbs and foliage at the tops of plants will help protect the lower leaves and branches from night- time radiation loss.

Pruning after a freeze does not improve the outcome. Also, plants that are pruned tend to be invigorated more quickly, which may set them up for further damage in Texas’s unpredictable cycling of warm and cold temperatures.

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WAYS TO RECYCLE REAL CHRISTMAS TREES

What can I do to recycle or reuse my real Christmas tree after the holidays? Before taking your real Christmas tree down this year, consider using it outside to create a backyard habitat for birds.

To attract birds to your backyard, you must provide their three basic needs — food, water, and cover or shelter. Your old Christmas tree will provide excellent shelter for birds, providing protection from wind and predators. It can also serve as a feeding station, where you provide a buffet of food that native birds love.

Before taking the tree outside, remove all decorations and lights, including tinsel. To provide the most shelter possible for the birds, place the tree on the south or east side of the house, sheltered from winter’s harsh north and west winds.

Anchor the tree securely by setting the stump into the ground or a large bucket of damp sand and secure the top of the tree with twine to a nearby building, fence or trees. Here are some things you can add to the tree:

“Decorations.” Decorate your tree with strings of popcorn, cranberries, or raisins. Apples, oranges, leftover breads, and pinecones covered with peanut butter — then dipped in birdseed — can also be added. For best results, push the edible ornaments well into the tree. Popcorn is attractive to cardinals, finches, and grosbeaks. Cranberries and raisins attract cedar waxwings, finches and any robins wintering in the area.

If you decide to start feeding the birds, be consistent with your feeding. They become reliant on your feeders as a food source; empty feeders during a severe cold period or storm could result in the birds starving to death before they can find another food source.

Water. Even in winter, birds need water to drink and to keep their feathers clean. A birdbath with clean water will attract many birds if the water is not frozen. Commercial immersion heaters keep water from freezing. They are available from many nurseries or bird supply stores.

Providing for the winter needs of birds can result in many hours of entertainment, spent watching these beautiful creatures.

Other options. But if you don’t want to feed the birds, there are several other ways to put your old Christmas tree to good use. Again, remember to remove all lights, decorations and tinsel. Many communities collect trees to be chipped and used to cover walking paths, or as mulch for trees in city parks and arboreta. Check with your city offices to determine if this in an option.