You have seen the forecast. Are your garden, orchard, or landscape plants ready for the freezing weather?
Freezing weather kills plants just like ice busts pipes. 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.
First let us review at the USDA Plant Hardiness Zone Map. In Angelina County, we are classified as zone 8b, with annual low temperatures expected to reach 15 to 20 F each winter. Lower “zone numbers” are located to the north of us and are correlated with a lower minimum annual temperature.
North of us from Tyler to the Red River is zone 8a, with temperatures expected to be as low as 10 to 15 F. To the south in Beaumont and Houston, you will find the boundary for zone 9a, with minimum temperatures expected to be only from 20 to 25 F.
When you study plant tags in the nursery or in the catalog, anything with a number 8 or lower should be acceptable in our area.
So, if we are zone 8b, then the weather forecasters are certainly telling us that we will reach the expected low, if not lower. To be sure, we may lose some of the more tropical plants that we tend to use in the landscape. We are on the edge of survival for many topicals that get by for years only to be bitten back by a good long freeze like the one we are expecting. Tropical plants such as palms and citrus are the common victims.
There are two types of cold events can damage plants in Texas: advective freezes (“freezes”) and radiative frosts (“frosts”). Texans are familiar with the term blue norther, a windy cold front that moves south from Canada through the Great Plains. The technical term for these events is advective freezes.
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.
One weather forecast that I look at online says we will have wind speeds from 10 to 11 mph on Sunday and Monday. Now that may be nothing for folks living in Lubbock, but here in Lufkin, that is considerably more wind than which we are accustomed.
The coming advective freeze can be confronted with the following measures.
First, adding a source of heat beneath a covered plant can make a big difference, especially if you seal or secure the cover to keep wind from moving the warmer air out from under it.
Common heat sources include a mechanic’s light, a clamp-on floodlight with a heat lamp bulb, and a string of Christmas lights—the ones with large bulbs, not the small twinkling bulbs that emit little heat.
Another way to prevent or reduce advective freeze injury is to use semi-permanent frames, tunnels, or hoop houses. Made of lumber or PVC pipe, these frames can be covered in 4 mil or heavier polyethylene film (UV stabilized for longer life) and heated with lights, water barrels, or sprinklers. The cover and heat source create a greenhouse-like environment during harsh freeze conditions.
During severe advective freezes, your pre-existing windbreaks can offer some protection for plants. Useful windbreaks include buildings, natural vegetation belts, and purposefully planted rows of trees and hedges. They slow the wind, enabling the active measures to provide heat to be more effective. The south sides of buildings and densely wooded vegetative areas create slight microclimate advantages for citrus and other freeze-susceptible plants; often these sites are where we find survivors after a severe advective freeze.
The practice of mounding soil or other materials around the graft union or crown of plants is called banking. Although many people wrap tree trunks with quilts or pile up hay, leaves, or mulch around the trees, the most protective material for banking is soil. Whereas mulch or blankets must use their own insulating properties to slow heat loss, soil transfers the earth’s heat to the plant parts that it touches.
Mulch can protect perennials, strawberries, and other marginally hardy plants. The mulch’s insulating properties will enable the plant’s crown and roots to survive the freeze.
Commercial fruit growers have learned that water sprinklers applying the right volume and frequency of water droplets can protect leaves, branches, blossoms, and even fruit from frost and freeze injury. Because water releases heat quickly when it freezes, it must be applied continuously to maintain the heating process. If you apply enough water continuously, it will offset the cooling effects of the cold air, wind, and radiational cooling.
A common misconception among home gardeners is that the ice formed on plants from sprinklers insulates the plant against the cold. In reality, ice is a poor insulator, so once new droplets of water stop being applied, the leaves and branches will continue to cool below freezing and may be damaged.
Container plants must be moved inside if possible. Because the roots of plants growing in containers lack the insulation of the earth, they can get much colder than the roots of in-ground plants. Roots are often less hardy than the top parts of the plant; they may die if the temperature in the container drops much below 32°F.
A good way to protect valuable container plants during a killing freeze is to move them to a garage or other protected location. Another option is to move the containers close together on a protected side of the home or other structure.
Water the plants thoroughly to increase the amount of heat produced in the container as it cools or freezes. For added protection, pile mulch, leaves, or hay over and among the containers and cover them with a frost blanket or other cover.
Next week after the freeze, the leaves of damaged herbaceous plants may immediately appear withered and water soaked. However, the freeze injury to the twigs, branches, or trunks often does not 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.
Follow these steps and you should save more of your landscape from damage or total loss.
Stay warm in the days ahead!