Protection from a visit by Jack Frost

December 30, 2004
Victoria County Master Gardener

A weather forecaster informs us the temperature is going to be 34 degrees and there will be little or no wind. Let's assume the forecaster is 100 percent correct. You may ask, "What should I do about my tender plants?" The air temperature is what he is talking about. Solid objects such as rooftops and plants can actually get colder and freeze under these conditions. They will freeze and perspire like a glass of iced tea, and this perspiration sitting on the frozen surface will also freeze.

We have just described what you would call a visit from our dreaded friend Jack Frost. Your tender plants have frozen and may be dead. They needed protection even though the air temperature was above 32 degrees.

A radiant freeze is when the temperature gets cold - whether it is 34 degrees or 28 degrees - and there is no wind. Under these conditions you should cover your tender plants during the day at a time of the highest temperature level for that day. A plastic covering should not be used if it will touch the plants as it will transfer the cold directly to plant tissue causing it to freeze. So people normally use the old quilt.

Guys, make sure you select the correct quilt - and not a family heirloom! Place the quilt over the plant and anchor each side with a large rock or brick. Plastic may be placed on top of the quilt to eliminate soaking of the quilt from rain. Enclose or cover the soil under the plant along with the plant. Why? The plant does not generate any heat, and if you just cover the plant it will freeze. With the soil exposed, the heat will radiate from the soil into the space covered by the quilt and help keep the plant a few degrees warmer.

What plants do we protect? Annuals planted in the spring will terminate their life within a month anyway, so no protection is needed. There are some annuals that are planted in the fall such as pansies and snapdragons that will survive our winters. For winter protection forget about the annuals. The beautiful cyclamens can be frost hardy and make a bulb for the next year's plant. Many herbs such as parsley, dill and pineapple sage grow well in our winters and need no protection.

With herbaceous perennials, just let them freeze because they will come up from the roots. Examples of the herbaceous perennials are esperanza, flame acanthus, lantana, firebush, mealy blue sage and autumn sage. In the event of a freeze, wait until the spring to cut off the dead wood and the plants will return to their natural beauty. You may want to mulch around these plants to make sure the roots do not freeze. During a mild winter, the tops generally will not freeze.

Perennial shrubs such as Indian hawthorne, holly, yaupon holly, etc. will accept most of our freezes. Most deciduous shrubs will survive Victoria winter freezes. All our common trees will normally make it through the freeze. There is one major concern about these plants, and this is a sudden hard freeze when the plants are actively growing. Then the hardy plants can freeze. Also, if winter starts but we return to summer type weather, tree trunks on the sunny side can start growing and a sudden freeze will kill the side of the tree actively growing. A Victoria example of this was shown to our 2004 Master Gardener class. But there is very little protection people can do to help these plants. To minimize winter injury from freezing temperatures do not fertilize and water plants excessively, if at all, going into the winter. Also, during the winter season, the plant's dormancy period can be interrupted by untimely watering and fertilizing, potentially causing freeze damage.

Tropical plants are considered tender and subject to freeze and should be protected especially when there is a radiant type freeze. So when you are working with tropicals be careful. Various Victoria homeowners have greenhouses just for these tropical plants. Plumerias, a common Victoria tropical, must be dug before a freeze, leaves stripped, and dirt shaken from the roots, and stored in a dry area above 35 degrees until spring has fully arrived.

Another type of freeze is termed advective freeze. It occurs when cold air masses move in from northern regions (a Blue Norther) causing a sudden drop in temperature. The high wind circulates around the plant tissue and causes instant cooling. With an advective freeze, radiant heat from the ground or other source generally will not overcome low temperatures and high wind. So place your tenders on a warm porch or in a greenhouse. Windbreaks such as a house (south-side) along with a light covering help protect plants from advective freezes.

For an advective freeze, professionals may spray water on the plant to form a 32-degree ice coat on the plant tissue. This is frequently true for citrus trees. But, homeowners usually apply too much water and the branches break from all the weight. Some water in the soil, so that the soil is moist, can help; but do not soak the soil because the roots need oxygen. The water helps to seal the warm air pockets in the soil.

From a plant's perspective a winter that starts gradually and gets cold in small increments and remains cold until spring is an excellent winter. This helps a plant to acclimate itself. Dream on if you plan for this to happen in Victoria. So on a chilly night with low winds, the covering of tender plants with a cloth material or Row Cover (a lightweight material used in the nursery trade) will permit the soil temperature to radiate up into the covering and keep the plant tissue above freezing. Some people will add Christmas tree lights for heat but the hot lights should not touch the bark or twigs. Use of electricity in rainy wet conditions can be dangerous, so be careful! Gallon milk jugs filled with warm water can also radiate heat. Advective freezes require strong structural protections to prevent the high winds from circulating among the plant tissue and freezing it. Thus, plants on the south side of the house have more protection from the direct force of cold north winds.

So, how did your plants fare last week right before and during Christmas Day? We experienced both radiant and advective freezes - which were a real test to plant survival. If your plants did not do so well this time, perhaps you will now know more what to do when freezing weather heads this way again.

Saving our beloved plants from a visit from Jack Frost is a challenge in our hot and cold winters; but if you do it well, you are rewarded accordingly with the spring beauty of your plants.