Express News Weekly Article
Saturday, November 1, 2004
By Calvin Finch, Conservation Director, SAWS, and Horticulturist

Be Ready for the First Freeze

            The average first freeze day occurs about mid-November in San Antonio.  That is only about two weeks from now so even if it is warm now, gardeners need to think about it and make some plans.

            We rely on a number of tropical plants to decorate our patios.  Bougainvillea, mandevilla, plumeria and hibiscus come to mind.  Tropical plants cannot survive freezing temperatures and may even decline when the temperatures reach the mid 40’s F.  Their internal workings are arranged in such a way that cool temperatures disrupt the chemistry.

            The tropical plants should therefore be secured before freezing temperatures arrive.  Bougainvillea and plumeria are very undemanding in their requirements for winter shelter.  The only requirement is that temps must stay consistently above freezing.  No light or even water is required.  That means that you can trim bougainvillea back and pile the container in a corner of a shed or garage.  Some gardeners even take plumeria out of their containers and stack the ‘logs’ in a pile.

            Hibiscus and mandevilla have a better chance of surviving if they are kept in a greenhouse where they receive some light and a watering every few weeks.  A few leaves should be retained all winter.

            Citrus trees are intermediate in cold tolerance.  Many of our favorite citrus can survive a few hours of freezing temperatures.  We do however need to be prepared to protect them if temperatures are predicted to fall below freezing.  Plants such as Mexican lime and Meyer lemon in containers can be carted into the greenhouse before the first freeze or you can cover them.  Trees in the ground, usually Satsuma orange or Changsha tangerine, need to be covered if temperatures are going to fall below 28° for more than a few minutes.

            The provisions that gardeners make can be as simple as throwing blankets or sheets over the plant, or as elaborate as building a frame to hold a plastic cover. 

            Citrus trees are often planted on the South side of a building close to the structure.  In such situations a piece of plastic can be rolled off of the roof to form a lean-to shelter.  On particularly cold nights a mechanic’s light, Christmas tree lights or even a small electric heater can provide heat under the shelter.  If a plastic lean-to is used, it is important to open the shelter during the days when temperatures are above freezing and the sun is shining.  The clear plastic allows the sunlight to travel through the plastic where it is converted to heat energy.  The Satsuma tree that was threatened by cold temperatures one night can be defoliated by heat within the shelter the next day.

            If you have small plants and want to protect them by just covering them, use a cloth material or a cloth layer and a plastic layer, rather than just plastic.  Cold is transferred very quickly through plastic to solid materials such as leaves and stems that it touches.  Plastic over cloth is effective because the cloth insulates the space between the plastic and the foliage, and the plastic blocks the wind and helps trap air in the area under the crown.

            Fall tomatoes often benefit by protection from the first frost in the autumn if it comes in November.  The weather often warms up for 2 or 3 weeks after the first cold wave enough to ripen the tomato crop.  If the forecast is only for temperatures 30-36° F an agricultural fiber like GroWeb may do the job.  It provides 3 or 4 degrees of protection.  The advantage of GroWeb is that light and air can penetrate enough that it need not be removed immediately after the freeze threat passes.  Blankets or plastic must be managed to allow light to reach the fruit and to prevent the shelter from heating to the point it does damage.  The cool weather annuals like pansies, snapdragons, broccoli and spinach do not need winter protection.  Another group that you do not need to worry about are the root hardy plants that freeze back every winter.  Esperanza, firebush and poinciana are in this category.  Let them freeze back without protection. 

            The weather is great now but the average date of the first freeze is not far off.  If you are going to protect your tropical and subtropical plants it is best if you have a plan and all the materials ready.