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Help your plants when it freezes

Grass is greener...

POSTED: January 10, 2014 12:11 p.m.

The freeze of last week has left unprotected plants — especially the soft and juicy non-woody plants — limp, shriveled and starting to rot in the 70-degrees-Fahrenheit days of this weekend.  
The herbaceous plants, like lilies, can have the dead leaves removed and discarded. Woody plants, like pittosporum and even loropetalum, got bitten by the cold, but hold off on the pruning them until spring.  Let the plant show you this spring what is actually dead. What looks dead now actually may have live wood underneath.
Also, pruning off dead wood now will open wounds on the plants that they will not be able to close until growth returns in the spring. These new wounds become sources for water loss, which easily can kill more tissue than the freeze did.  
Prevention of freeze injury is possible on smaller plants. We commonly advise folks to completely cover their plants and possibly put an incandescent light bulb under the cover to generate enough heat to prevent freezing.
With the loss of the 100-watt incandescent bulb, there are other alternatives. One is to use the heat of conversion of water into ice.  It takes one calorie of heat to raise one gram of water 1 degree centigrade. When a gram of water is decreased 1 degree in temperature, it has to give up 1 calorie of heat. But to change 1 gram of 32 degrees Fahrenheit water into 1 gram of 32 degrees Fahrenheit ice, the water has to give up 80 calories of heat.
This autumn, I planted three small Satsuma trees at my home. In preparation for the freezing weather, I filled several 5-gallon pails with water and placed one or two of them next to each tree. I then covered each tree and pails with a cardboard box (good insulation) and a tarp (to weigh the boxes down and keep out the wind) and weighted the whole thing down with bricks. A 5-gallon bucket of water set outside uncovered had ice 2 inches thick by morning. The pails of water under the cardboard boxes had no ice and none of my Satsumas lost as much as a leaf. There was no sign of freeze injury on any of them. The heat of conversion of water both from the wet ground under the boxes and the water in the pails effectively protected these plants from freezing.  
The tricky thing about freeze injury on plants is it is not just how cold it gets that affects the potential injury to plants. It also is how long it stays cold, how much wind is involved with the event and the weather leading up to the freeze. If weather gradually cools down prior to a freeze, the plants can become acclimatized to the cold and survive colder temperatures without injury. Satsumas have survived 21 F freezes if they have had a gradual cool-down prior to the freeze. The same plants could be severely injured had the freeze been only to 26 degrees Fahrenheit with no cool-down leading to the freeze.  
The freeze we got last week did not give us the cool-down needed to acclimatize plants, so I am expecting to see a fair amount of freeze injury across the landscape. Also, we got down to 17 F in the Keller area, and even an acclimatized Satsuma would not do well without protection.
The next thing coming up on the lawn calendar is application of pre-emerge herbicide for summer weeds. We are expecting to have that applied around Valentine’s Day. If you do not have pre-emerge on hand, now is the time to start shopping.
Oryzalin (Surflan), benefin (Balan), pendimethalin (Halts), benefin plus trifluralin (Team), benefin plus oryzalin (XL) or Atrazine (Hi-Yield Atrazine) are all labeled for pre-emerge weed control on centipede and St. Augustine.
Atrazine is the tool of choice on St. Augustine, but be careful with it on centipede during green-up or brown-down.
Scotts Bonus S (contains atrazine on a dry granular fertilizer) works well on St. Augustine in the middle of the growing season, but should be avoided on centipede during green-up.  
Centipede cannot tolerate atrazine during green-up. It will turn your centipede lawn orange with injury. It usually will grow out of it, but what is worse — a few weeds or an orange lawn? Also, if you are trying to get rid of weeds, feeding them with fertilizer when the grass is dormant is counter-productive.  
Two weeks after the Super Bowl, we will start on the lawn for next year!

Gardner lives in Keller and is the UGA extension agent for Glynn County, serving South Bryan.

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