Sunday, December 5, 2010

Plant Cold Protection

I posted last week about bringing your houseplants in for the Winter. Hopefully you all did. What about the plants that are just way to big to move or are in the ground? Again, I am lucky in the aspect that I have a greenhouse and a fairly decent, healthy back, so I can move my 30 gallon pots. What if you don't have either of the above and want to protect that plant that is marginal in your neck of the woods?
Lets see if I can give you a few ideas.
Although I live in the Southern part of the United States, Winter sometimes brings cold temperatures that can cause severe damage to many of our landscape plants. Early frosts in the Fall can cause damage on plants that are normally adapted to our area. Plants need adequate time to harden off (adjust to outdoor conditions) before freezing temperatures occur. A plant’s ability to withstand cold temperatures will depend on the plant species, but also on how low and how fast temperatures decrease. When temperatures gradually decrease, a plant can acclimate, or adjust itself, to withstand colder temperatures better. Sudden decreases in temperature cause more damage in Fall or early Winter than similar low temperatures well into Winter. New growth that has been stimulated by late Summer pruning or fertilization is very susceptible. Hopefully you weren't outside in the past month or two feeding your plants or giving them a haircut. If you weren't, then you are already ahead of the game.
Ideally, The best way to prevent cold injury to plants is to choose ones that tolerate the cold temperatures in your area. We all know though, that pushing the envelope is sometimes more exciting and offers a wider range of plants.
So, in addition to right plant selection, you want to put the plant in the right place. During the Winter, the coldest spots are often found on the North and Northwest part of the property and in low areas where cold air settles. The warmest spots are usually on the Southern part of the property. Placing plants under non-leaf dropping trees will add some protection, as will using a fence, building, wall or temporary shelter made of plastic. Placing plants near a sidewalk or street can even add a few degrees of protection. These things absorb the heat and give it back off at night. For container grown plants, their roots are more exposed because they are above ground. Push together container plants that are left outside and mulch or cover them to decrease heat loss from the sides of the containers. Wrap the base of the containers in plastic, burlap, or blankets to reduce heat loss. Remember also, that plants that grow close to the ground are usually protected by heat radiating from the soil. Plants that are tall and more open do not receive this radiating heat and are not protected from the cold.
Covering your plants helps protect them from frost as well as from extremely cold temperatures. Covers that reach the ground and do not come in contact with foliage form a layer of insulation from the cold temperature. To prevent foliage breakage, avoid having the covers (sheets, blankets, painters drop cloth, etc) touch the foliage. This can be accomplished by using tomato cages for smaller plants and lumber teepees for larger ones. Don't forget to weigh down the sides so a gust of wind will not blow it off. When extremely cold temperatures are predicted, place a light bulb (60 watts or higher) or other heat source (spotlight) under the cover to provide heat. You can also use some of the old Christmas lights that we all had as children, you know, the ones that would melt your toy if it came in contact with? Check the lights before relying on them, the newer ones do not produce any heat. Be very careful when using a bulb or other heat source, which can be a potential fire hazard. Do not let the bulb or heat source come in contact with the plant (Christmas lights being a kind of exception) or the cover. Remove the cover and provide ventilation during the day, the sun can very easily burn the plant or at the very least, the extra warmth could possibly cause the plant to break dormancy. This precaution is critical when using plastic covers.
Watering plants before a freeze can help protect them from cold injury. Soil that is well watered absorbs more heat and gives it back to the plants at night. I don't have this problem here, but, Cold weather can also cause the ground to freeze. When this occurs, water is unavailable to roots; plants continue to transpire (lose water from their leaves) and dry out. Watering the soil to thaw the ground makes water available to roots.
Mulch will act as an insulator, so it too can protect plants, especially from wide swings in temperature. Sometimes the cold temperature is not what damages the plant, it's the freeze/thaw cycle affecting the soil and causing it to "heave" the plant. The roots basically get thrown out of the soil and break contact with it. Something to keep in mind about mulch, this "insulation" works both ways. It can prevent the soil from drastic swings,and getting too cold, but it can also prevent it from warming up when the time comes. You may wish to rake it back away from the plants as it starts to warm up in Spring.
Now occasionally, Mother Nature actually provides the best blanket of protection in the form of a light snow. Up to two or three inches of snow not only insulates the ground around your plants, but it also provides a blanket of protection over the leaves. If you have a situation in your area (I don't, usually) that this could happen, you need to think way in advance.
This is a picture of some Wax Myrtles, before the wet, heavy snow, hit us in February of this year. They were about 10 feet tall prior. If the homeowner knew this was coming, if ANY of us had known this was coming, they would have been pruned back in the middle of the Summer. This event caused considerable damage. Of the limbs that broke, some were as big as six inches in diameter. This picture was taken too late to do anything about it. What should have been done, other than the preemptive pruning? Someone should have gone out there and lightly shaken as much of the snow off as they could. The plants are fine and rebounded, but the shape of them is not one of beauty, Mother Nature is definitely not a landscape designer.  
To sum this all up, healthy plants are more resistant to cold injury than plants that are weakened by disease, by insect damage, or by improper care. Plants that are being grown in their proper growing zone will do better than one that is 3 zones off. With an ounce of prevention, and a little work, you too will produce a plant that can wear a tag that states "I survived the Winter of 2010/2011".
Happy Growing!

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