GARDENING IN GWINNETT: Preventing cold damage to landscape plants

In winter, many homeowners become concerned about the potential damage of the cold weather on their landscape plants. However, most of the woody ornamental trees, shrubs, groundcovers, vines and herbaceous perennials sold at local nurseries are capable of surviving very cold temperatures.

The cold hardiness of the plant material and how the plants have been cared for during the growing season -- proper watering, pruning, fertilizing and pest control -- can help in minimizing damage caused by freezing temperatures.

Cold injury can occur on both roots and above ground plant parts, water inside the plant freezes and expands causing tissue damage. Leaves and shoots will appear water soaked and wilted, turning them black in a few days. The extent of cold damage can be difficult to ascertain until the plant fails to come out of dormancy in the spring.

The parts of the plant most vulnerable to cold injury are the roots and the crown (the part where the roots meet the stem). The bark will often split at the crown near the soil line. The tissues in the trunk of the plant will freeze, then rupture when exposed to the morning sun.

Azaleas, camellias, gardenias and similar plants are especially vulnerable to bark splitting. You can observe the damage on the lower stems and branches near the soil surface. Injury from split bark can appear later in the year as dead twigs and branches. Always install these plants in areas where they will receive adequate protection from winter wind to reduce the likelihood of bark splitting.

Following a hard freeze, a simple way to determine if the plant material is dead or alive is to scratch the bark with your fingernail. If the stem tissue is green or white where you scratched, then that wood is still alive and it should put out new growth in the spring. If, however, the stem tissue is brown or brittle, then that branch is most likely dead.

Do not be in a rush to remove any plants that appear damaged. Before removing dead plants or pruning out dead wood, wait until springtime when the plants start to come out of dormancy. If a plant does not leaf out, then remove the dead plant or prune out the dead wood.

What are some ways to reduce the risk of freeze damage? Properly cared for plants in the home landscape during the growing season have a greater chance of surviving a hard freeze. Nevertheless, do keep these tips in mind in order to reduce the risk of cold damage:

  • Purchase and install plant material that is tolerant of the cold weather extremes in our area. Gwinnett County is in USDA hardiness zone 7b, meaning the winter temperatures have the potential to drop to 50 to 100 degrees. Any plants purchased should be hardy for this zone.

  • Maintain a two- to three-inch layer of mulch around plants at all times. Mulch helps to insulate root systems and protect the soil from rapid temperature fluctuations.

  • Never apply a high nitrogen fertilizer during the fall. Fertilizing may stimulate growth that cold weather may injure or kill. Delay fertilizing until late March or April when all danger of freezing has passed.

  • Avoid intensive pruning in the fall as it may also stimulate late season growth, increasing the risk of cold injury.

Timothy Daly, MS, Agricultural and Natural Resource Extension Agent, Gwinnett County Extension. Tim may be contacted by phone at 678-377-4010 or by e-mail at timothy.daly@gwinnettcounty.com.