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Index : Miscellaneous Gardening Topics
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- 31. Recent Warm Weather May Leave Plants out in the Cold - Top
- Unusually warm weather coupled with rains this fall have left plants flush with growth and vulnerable to an early cold snap.
Normally, autumn temperatures slowly decline, starting in late September. Plants need this gradual "cold conditioning" to prepare them for winter temperatures. This year our October temps have run as much as 15 degrees above normal. In response, plants continue to grow and the sap in the branches continues to flow. This tender growth and water-filled stems are much more susceptible to cold injury. Given the fact that our first freezing temperatures could come as early as Thanksgiving, these plants are set up for significant freeze damage.
We can't do anything about the temperatures but we can help toughen up soft, succulent growth by lessening the amount of water these plants receive. It's not a matter of watering for a shorter period of time. If your running your drip system, let's say for 3 hours, keep running it 3 hours. But if your watering once a week, then lengthen the time between watering, to once every 10 or 14 days. By lengthening the time between irrigations the plants will start to toughen up or what is technically referred to as "hardening-off".
As well as reducing the frequency of irrigation, plants, in general, should not be fertilized at this time of year. The exceptions are fall and winter flowers, vegetables and container plants. Fertilizing, especially with plant foods high in nitrogen, will only stimulate additional soft, leafy growth and further delay dormancy. Pruning plants should also be avoided. The action of pruning will stimulate a growth response in the plant, and this new, tender foliage will easily be frozen.
The plants most susceptible to the cold are the tropical and sub-tropical species. Citrus of all types will be vulnerable to an early cold snap. Among citrus, limes and lemons are the most prone to cold injury. Then comes oranges. But even tangerines and grapefruit may be injured. All newly planted citrus, regardless of the type, should be protected.
We grow many tropical and near-tropical trees and shrubs in Tucson. These plants have borderline cold hardiness for our area, even in a good year. Given our conditions this fall, they will be especially vulnerable to damage from an early frost. These plants include; Jacaranda, Indian Rosewood, Hibiscus, Cape Honeysuckle, Bougainvillea, Tropical Bird-of-Paradise, Allamanda, and Natal Plum.
Some desert plants may also suffer cold injury. Hybrid Mesquite trees, now lush and green, are the most likely to be damaged by an early cold-snap. As with other plants, cut back on the water and don't fertilize. Mesquites are much more tolerant of drought than they are of being over-watered or over-fertilized. Many types of imported cacti, such as Golden Barrel, Old Man, and Totem Pole are also cold-sensitive.
If you intend to cover up tender plants on cold nights, have plenty of sheets ready for the job. Plants can also be covered with newspaper or paper grocery bags for small plants. Frost covers, also called frost blankets are probably the best plant covers. They are made of light-weight, spun fabric like that used for the lining of coats and jackets. Cacti can be protected with caps of strafoam cups, paper grocery bags, or burlap. On these succulents, it's the growing point at the top that needs protection.
Used alone, plant covers provide several extra degrees of warmth. If additional protection is needed, a light can be placed under the cover. The best and safest to use is a shielded utility lamp with a 60 or 75 watt incandescent bulb. They can be found in hardware and home supply stores. The bulb will provide substantial warmth when placed under a cover.
On cold-sensitive plants, any pruning should wait until all danger of frost has past in the spring. Here in Tucson that's March 15th. If plants do freeze back some, leave the frozen branches on through the winter. The dead leaves and stems will actually provide some additional cold protection for the living portion of the plant beneath.
Finally, before cold weather arrives, gather and store any frost protection materials you might need. Some advance planning now will make it easier to cover and protect your plants when the time comes.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161. - Updated: November 2, 2003
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