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Freeze damage and stress in landscape plants

@BR Ednote:Editor’s note: The following question and answer report was developed by Kyle VandenBrink of OLM, Inc., the company that monitor’s Lakewood Ranch’s landscaping. VandenBrink referenced University of Florida Institute of Food and Agricultural Science data.

Q: Many of my plants are brown. How do I know when they are dead and should be removed?

A: Brown leaves are just a hint of the damage that may have resulted from the cold. Wait a few weeks or more before making any decisions. As the weather warms up, plant portions often continue to decline. You may notice stems cracking and bark peeling away as further indication of the cold damage.

In about a month, the extent of the cold damage can be detected. Use a knife to scrape along the stems until you find green tissue. This is normally the point where the plant can begin new growth. For some, the green stems may be found only at the ground. Given time, even these plants can recover rapidly because of the well-established root systems.

Q: Our sprinkler system went on during the freezing temperatures. Most of the plants were coated with ice, and are turning brown. Did the water harm the plants?

A: Water can be used for cold protection only if it is applied in a sufficient quantity and uniformly during the entire time of the freeze. When a sprinkler system comes on during the freeze and turns off before the freeze is over, damage can often be severe.

The wet plants are super-cooled and temperatures drop well below the air temperature, causing stems and leaves - that would normally not be affected - to freeze. Expect to do lots of pruning to remove damaged portions. Some plants that would have survived may have been lost.

Q: I lost most of my flowers. When can I begin replanting?

A: If you pick the hardy types, you can begin replanting immediately. Pansies and Johnny-jump-ups were not affected by the cold and are almost freeze-proof. Other good cold-weather survivors worth the risk include delphinium, dianthus, dusty miller, ornamental cabbage and kale, petunias, shasta daisies and snapdragons.

Q: Due to the recent freeze damage, our crotons had to be cut back to the ground. Will the plants still produce new growth, or should they be dug out and new ones planted?

A: Scratch the stems just below the ground. If they are green, you can expect some growth from the buds insulated from the cold damage by the soil. This growth could be slow in developing, as the buds are often immature. A wait of six to eight weeks after warm weather returns is common.

Q: Frost has damaged the ends of our hibiscus branches in one area of the landscape. When can we prune the plants? Why weren’t plants in other areas affected?

A: Some areas of the landscape are always colder than others. Your damaged plants may be more exposed to the wind, or they may be growing in an open area where frosts form first. Perhaps the unaffected plants are near a building or under trees where they are protected from the cold.

Because more cold weather is predicted, don’t rush to cut back the damaged hibiscus portions. The brown leaves may insulate the plant from future frosts and freezes. A good rule to follow is when you can’t stand the brown foliage any longer, it’s time to prune.

Q: When can I start pruning freeze-damaged trees and shrubs?

A: Don’t prune anything immediately. Wait to prune cold-damaged oleanders and other tropical shrubs, such as hibiscus and crotons, after they begin to sprout new growth with the onset of warm weather, maybe by late February.

The damaged leaves help to insulate damaged plants from frost and further injury. Once plants begin to sprout, be sure to prune below them so you cut in to green healthy wood. The cold acts as natural pruning to overgrown shrubs. Several light trimmings through the spring and summer growing season will promote dense growth. New sprouts will form just behind the pruning cut, so if you want the shrub to branch down low, you need to cut some of the stems down low. Azaleas and camellias should not be pruned until after they bloom. Several light trims to shape wild branches should be all that is needed.

Crape myrtles are deciduous trees, meaning that they lose their leaves during winter. If you trim them too early, they sprout out tender shoots that are likely to be killed by frost or freezing temperatures. Wait until the end of February before they begin to leaf out naturally, but only trim branches smaller than your finger. It is not advisable to follow the common but incorrect practice of hacking into 2” to 6” limbs. It leaves large wounds, which are slow to heal and the resulting branches are weakly attached and likely to break off in the wind.

Palms damaged by cold should not be pruned until the new fronds fully emerge. Pruning off brown or desiccated fronds remove a source of bud protection, nutrient source and limit the plant’s exposure to disease exposure by cutting into petiole (the base of the fronds) closest to the bud (a palms growth point).

Q: Why were these plants used in the first place if they would freeze?

A: It is my opinion the plants are within the ‘coldest’ range for their tolerance. Recently, the USDA updated Plant Zones indicating the semi-tropical line (Zone 9-10) has migrated from Sarasota to Tampa Bay. This indicator of annual cold temperature cannot take into account local micro-climates. The plants are successfully used throughout Tampa Bay.

Many of the more colorful landscapes using tropical and semi-tropical plants like ixora, scheffelara arbicola, trinette, Hawaiian Ti plant and Duranta “gold mound”, have cold intolerance at 35 F or colder.

Again, people want “Florida” to be the tropics where much of the state is in a temperate climate range. With the reward of high color comes the risk of weather exposure. As an aside, we have used hibiscus and bougainvillea for years and it, too, was bitten. I do not believe damage is permanent or warrants replacement as many well established plants can recover in a season. If the plant is in a high visibility location and detracts from the quality of landscape, replacement maybe considered.

Conclusion

Florida is afforded a temperate climate and with so much great weather we sometimes forget the number of freezes recorded at Tampa International Airport. The statistics show an average of 3.3 days at 32 degrees Fahrenheit or below per year. With that, it would seem hardly worthy of discussing frost and freeze protection to these rare occurrences. But still we want to have flowering semi and sub-tropical plants to brighten landscapes, patios and lanais. Periodic cold events and the value of personal or community landscapes require attention and action to preserve and protect the sensitive landscape plants.

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