Helping Freeze Damaged Plants Recover

Don't be too hasty to cut back freeze damaged plants. The recent cold weather caused browning on a number of cold sensitive plants. Hardest hit were bougainvillea, oleander, hibiscus, annual flowers, citrus trees and queen palms. Brown leaves are obvious, but die back of stems may not be. For this reason it's best to wait to prune back cold injured trees and shrubs until spring growth begins. Prune too early and you may be removing live wood, thus reducing the plants size unnecessarily.

Leaving cold damaged leaves and stems on the plant until spring has another advantage. The dead leaves help hold in heat around the undamaged portions of the plant. This can reduce further cold injury if we have additional freezing weather.

If you must prune trees and shrubs now, carefully examine the wood to determine which parts of the plant are dead and which are alive. Start at the branch tips working inward. Scratch off a little of the bark to reveal the cambium layer. This is the living tissue which transports water and manufactured food throughout the plant. Healthy cambium should be white, creamy, light beige or green in color. If however the cambium is black or brown the stem or branch is dead at that location. Move further down towards the base of the plant, checking the cambium as you go. When you find healthy cambium wood, prune back the stem or branch slightly below the dead wood, into the healthy cambium. Check a majority of branches on each plant to determine the extent of die-back before pruning.

Cold tender flowers such as geraniums, begonias and calendulas, which were damaged, can be cut back to remove frozen growth. Removing the damaged portions of these plant will encourage new growth and flowering. Watering should be continued on a regular basis. An application of fertilizer will also help flowers recover. Apply ammonium phosphate or a complete flowering plant fertilizer at the rate of 2 lbs. for every 100 square feet of flower bed area. When placing the fertilizer, avoid getting it on the leaves. Watering after application will help wash any fertilizer off the plants and soak it into the ground for a quick response.

Plants which have been subjected to freeze injury should be watered to keep the soil moist. With the warm conditions that have occurred since early January, plants are susceptible to drying out if adequate watering is not maintained. Water should be done on an as-needed basis. Generally speaking, watering once every two weeks should be sufficient for cold injured trees and shrubs. Depending on the temperatures, flowers and vegetables will need watering 2 or 3 times weekly.

Fertilization of trees and shrubs damaged by the cold is not recommended at this time of year. Plants are in a period of slowed growth or dormancy in the winter, and do not need fertilization. In the case of cold damaged plants, fertilization is detrimental as it encourages new growth that is even more susceptible to freezing. Wait until the danger of frost has past and the weather has warmed sufficiently to encourage new spring growth before fertilizing.

Queen palms are the only cold-sensitive palm commonly grown in Tucson. They need special attention after cold injury. Unlike trees, palms have only one growing point. If it is killed as a result of cold injury, the palm is lost. The problem isn't so much direct cold injury, but the secondary infections that can set in on cold damaged bud tissue.

If some or all of the leaves of your queen palm have turned brown after the recent cold, steps should be taken to protect the bud. First, check to determine if a rot has developed in the crown of the palm. Pull upward on the spear leaf, the newest leaf of the palm which has yet to open. If it easily pulls out and brown discoloration is at the base, rot has set in.

The next step is to treat the bud with a copper fungicide, such as copper hydroxide or Kocide. The product should be mixed according to label directions and sprayed or poured down into the bud. This treatment will control bacterial bud rot, the most common infection resulting from freeze injury. To speed recovery, apply a palm fertilizer in March. This will encourage the bud to grow and produce new leaves.

Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161.
Material originally appeared in Arizona Daily Star gardening column, on January 26, 1997.
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