Protecting Frost Sensitive Plants

The effects of frost varies with plant species, stage of growth, age, general health and water content.

Plant growth slows in response to cold temperatures and short daytime length, providing some protection from frost. At the beginning of winter this hardening process may be in transition so plants can be more susceptible. Same goes for end of winter when daytime durations are increasing and the plants are transitioning to a growth phase.

The extent of frost damage is also related to temperature, length of exposure and how quickly the temperature drops. Note that frost damage often looks worse than it really is; new growth may emerge in spring from tissue that you believe to be dead.

Frost damaged areas will protect underlying part of the plant, so don’t prune until new plant growth begins in the spring.

Keep Plants Well Watered

Ice crystals on the leaf draw out moisture, causing dehydration. This damage will be less severe if the plant is not already drought-stressed. Also, moist soil absorbs more heat and loses it more rapidly at sunset (see Covering Plants below). Try to keep moisture levels as even as possible.

What Types of Plants Need Protecting?

In leafing plants, flowers are killed first, followed by new leaves, then older leaves, stems and trunks. Plants are more susceptible to frost damage when the weather first turns cold since they have not yet adapted to the change. The same can be true at the end of winter since plants are adjusting for warmer temperatures.

• Plants that are native to warmer climates. Examples include citrus, tropical plants and non-hardy succulents. Frost hardiness is listed for plants in the Garden Tags pages.
• Plants in containers are more vulnerable since their roots are more exposed to the colder temperatures. Consider moving these plants under cover for protection or wrapping the container with a blanket.

Creating a Microclimate

Your yard has microclimates with little areas of hotter and colder temperatures throughout the year. In winter the sun is to the South and warms south-facing walls, rocks/stones and hard surfaces which provide significant protection from frost. Elevation differences of even a few inches also make a difference, since cold are sinks towards lower elevations and even divots. Plants in low areas will be more exposed to frost conditions. If you have a wash in the yard, this acts as a great conduit for channeling cold air.

Shady areas will also be inherently colder since the sun does not warm the ground and windy areas are more frost prone because the ground heat is whisked away.

To learn more about your yard temperatures you can place a few Max Min Thermometers in strategic locations out of direct sun. For example, in winter, place a thermometer near a south-facing wall, one at a low spot and one that seems to represent the general conditions of the yard. Compare the readings on your general conditions thermometer with reports from your local weather stations to learn how your yard differs.

One way to protect your frost sensitive plants is to place them in areas of your yard that are warmer in the winter, whether by elevation or stored heat from a wall.

Covering Plants

Use paper or a fabric like sheets or row covering to cover plants to provide insulation and hold in ground heat. The cover should drape all the way to the ground to trap ground heat within the tent. Use stakes or rocks to hold down the ends of the fabric if wind is forecast. Ideally the fabric would not touch the plant because those areas of contact can result in frost damage. Properly applied, a cover can protect down to 30 degrees F, with some specialty fabrics protecting to 20 degrees.

Don’t use plastic since it doesn’t insulate and can actually make frost damage worse where it’s touching the plant. Also, it’s best to remove the covering during the day since daytime sun can turn the tents into greenhouses and inspire plants to begin new growth prematurely. Do not gather the drape around the trunk or stems – the goal is to gather ground heat for protection.

Adding Heat

For harder cold snaps you can place outdoor rated lights under the frost covers, although keep in mind that LED lights will likely not produce enough heat to be effective, so incandescent is a better option. 100 watts is a common recommendation. Oh, and don’t let the heat source touch foliage.

Be careful to keep any sources of heat away from flammables like the cloth or paper.

Resources

• UofA Publication AZ 1002
• Tucson