Michael Womack
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Saturday, December 11, 1999

Protect plants against winter cold snaps

Don't prune, fertilize plants; cover them during major freeze

 

The recent slight dip to 32 degrees in inland areas caught many gardeners off-guard. You should therefore consider preparing your yard for the cold weather before the next freeze scare comes along.
   Before you make those preparations, it's important to understand words like "frost" and "freeze" so you will know how and when to take action.
   First and foremost, it is important to remember that where you see ice, the temperature is 32 degrees or below. Furthermore, you should be aware that the National Weather Service takes its 'official' readings inside a shelter at a height of 5 feet, so an official reading of 32 may not reflect the actual temperature near the ground at your residence.
   Frost generally occurs on clear calm nights when the overall air temperature during a frost is above freezing. However, because the earth's heat dissipates into the atmosphere during the night, it keeps getting colder and colder near the earth's surface. It might be 40 degrees 30 feet above the ground while below freezing at ground level. Since cold air is heavier than warm air, cold air will slowly move into low-lying locations, referred to as 'cold pockets'.
   Freezes, on the other hand are generally characterized by massive movements of below-freezing air from the north. Strong winds associated with these arctic blasts makes temperatures the same regardless of height or site. During a freeze, tender plants have considerably reduced chances of survival unless they are protected from the winds. Although "wind chill" itself has little effect on plants, protection from the wind prevents plants from losing heat as fast.
   Watering before the cold
   Another wind-related factor is desiccation, or drying out. When cool, dry wind blows over the surface of a leaf, water is drawn from the plant through leaf pores called stoma. It is important to water plants before a cold front hits to counteract the drying effect of wind. Construction of wind breaks will also help; they could be made out of available materials such as plastic or plywood. Just remember two things when using this approach. First, you are only reducing the air movement around the plants, not trying to keep them warm. Second, the wind-break structure must be strong enough to withstand strong north winds.
   Your plants will naturally prepare for cold weather or "harden off" as temperatures gradually drop during autumn. You can assist your plants in this process in the following ways:
  

Don't prune. Major pruning should be done in late February. Trimming a few stray limbs to allow you to cover a plant is okay, but major pruning may force a flush of tender, young growth that is not hardened for winter weather.
  

Don't fertilize. Application of high nitrogen fertilizers will also cause a flush of growth and should thus be avoided. However, if your plants appear to have a nutrient deficiency, a light application of a specific fertilizer supplying that particular nutrient may be necessary. A nutritionally starved plant is in a weakened state, making it more susceptible to freeze damage.
  

Mulch. Applying a thick layer of mulch around the base of tropical plants such as hibiscus will help keep the earth's heat around the base of the plant. The branches will likely die back, but new growth will probably reemerge next spring from the protected trunk.
   Trapping the earth's heat
   The key to freeze or frost protection is trapping the earth's natural heat. In essence, we are constructing a tent or small greenhouse over the plant and using the warm soil as our heat source. Wrapping the trunk of a plant will give very little protection. Neither will wrapping the foliage with sheets and tying the ends around the trunk creating a "lollipop-effect." The temperature inside your sheet will quickly drop below freezing because the plants are not producing their own heat and the earth will not be supplying warmth either.
   In the event of frost, short-term protection is needed. Blankets and towels can help keep the earth's heat near the plant. Remember that cloth holds in heat and is much better than plastic. If you were stuck outside during a frosty night, would you prefer a fabric or a plastic coat to hold the heat next to your body? Again, note that it is always best to cover all the way to the ground whenever possible to better trap the heat.
   If a major freeze comes, you need to start with the same line of defense; however, consider using plastic on top of the blankets to keep them dry. Also, getting the edges of those blankets to the ground becomes a necessity.
   Adding heat
   In the event of a two or three-day Arctic Express, additional heat sources may be necessary for plant survival. Some gardeners use strings of large outdoor Christmas lights or incandescent lights under the blankets and/or plastic. This practice can raise the temperature a few degrees around the base of the plant. If a light bulb rests on a leaf, that leaf will likely be damaged due to the heat of the bulb. Remember that plant materials are combustible. BE CAREFUL NOT TO CAUSE A FIRE HAZARD! Never use open flames or electric heaters near combustible materials.
   One final note, don't forget about your garden hoses during freezing weather. Gardeners often spend so much time focusing on plants during freeze preparations that they forget about their support materials. Make sure you drain hoses completely and store them in an accessible place to prevent cracking and leaks next spring.
   Making your cold weather gardening preparations now will reduce stress later and allow you to enjoy the cooler temperatures without worrying about your plants.
  
1999 Caller-Times Publishing Company, a Scripps Howard newspaper. All rights reserved.