Weekly Express-News Article
By Calvin R. Finch, PhD, SAWS Water Resources Director, and Horticulturist
Saturday, December 1, 2007
“Protecting Cold-Sensitive Plants”

            With our long growing season and generally warm weather we grow a number of tropical and subtropical plants that require protection from the cold. 

 

            The most cold-sensitive plants such as bougainvillea, oriental hibiscus, plumeria, pentas, mandevillas, and other tropical plants should be safety sheltered in a greenhouse, sun porch or other location that stays well above freezing.  Plumeria and bougainvillea do not need light during the winter storage period.  Most other tropical plants do if they are going to survive the winter. 

 

            The second level of cold-sensitivity includes plants like citrus and geraniums that can tolerate temperatures that fall below freezing for short periods of time and survive more severe cold if they receive some protection.

 

            Sheets, blankets, and even agricultural fabric will add four – six degrees protection.  Enclose the plant in a plastic tent and even more protection is afforded as long as the plastic does not touch the plants.  Plastic without an air space or fabric for insulation between it and the plant is not useful as protection.  Another concern, the air in the plastic tent will heat up very quickly if the sun comes out after the cold spell.  It is not unusual for plants to fry in such a tent if it remains in place when the sun arrives.

 

            A heating source placed under the plastic tent or fabric adds several more degrees of protection and keeps the shelter warm if the cold spell lasts for more than a few hours at night.  Christmas tree lights, a mechanic’s light, or a poultry warmer work well. 

 

Tomatoes cannot tolerate freezing temperatures without protection.  For the first light freeze the protection of agricultural fabric or sheets is all that is required to give you three or four more weeks of ripening time.  Just to hedge your bets, collect all the full size fruit that is showing any color change (off white or orange) before the freeze arrives.

                       

            The broccoli, cabbage, carrots, spinach, and onions should survive the cold spells without much trouble.  Mustard, beets, and even English peas can experience top kill or at least leaf burn.  To maximize cold tolerance, every plant should be irrigated as soon as the soil dries to one inch.  Watering just before the freeze is not usually effective.  The plant must have the water integrated into its structure for it to be useful. 

 

            The misconception that watering just before a freeze helps plants survive the cold comes from commercial strawberry and fruit orchards where they water all during the freeze.  As water freezes it releases latent heat which keeps the plants at or near 32° F.  Unfortunately, a few drops of water before the cold temperatures arrive do not offer any protection. 

 

            The cool weather flowers also have varying degrees of cold tolerance. Pansies generally bloom all winter even when we have freezes. Petunias, snapdragons, stocks, and calendula may stop blooming during the coldest weather in December and January.  They resume blooming sometime in February.

 

            In the shade, cyclamen seems to have the same cold-tolerance as pansies, but part of that tolerance is more likely because cyclamen is usually planted in sheltered locations in deep shade.  Primulas and begonias may also bloom all winter, but seem to have less cold-tolerance than cyclamen and pansies.

 

            The lawn is also sensitive to extreme cold.  The grass has more cold-tolerance if it was fertilized in the fall and if it did not dry out severely before the cold arrived.  St. Augustine grass should be watered every two – three weeks with at least ½ inch of water if it does not rain in that period.  St. Augustine grass is slow to go dormant and the roots remain active all winter.  Use the two week alternative if the winter weather is sunny and mild, and the longer interval for cool, overcast weather.  Zoysia, Bermuda, and buffalo grass more readily enter into the dormancy and are less sensitive to winter moisture once they are dormant.  Irrigate lawns with zoysia, Bermuda or buffalo once or twice during the winter if the weather is mild and dry.

 

            Most shrubs, trees, and perennials can be treated as the dormant lawn.  The exception is plants that are actively growing.  Last year most Texas Gold columbines died in the extreme winter drought due to lack of water.  Water actively growing plants when the soil dries to one inch.