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Index : Watering
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- 17. Recent Rains and Hot Weather Stimulate Plant Growth out of Season - Top
- This fall, expect to see some unusual growth and flowering on plants that are normally heading into winter dormancy. The reason for this is the substantial amount of rain we received from the remnants of Hurricane Marty and the unseasonably hot temperatures that followed.
Hot, wet conditions trigger plants to put out what is referred to as a ‘secondary' bloom. It's likely that spring flowering trees like Sweet Acacia and Palo Verde will begin to bloom again this fall. The flowering could even be heavy on some trees. The same condition may exist for spring flowering shrubs such as Texas Mountain Laurel and Desert Senna.
Other trees stimulated to flower out of season are fruit trees, especially citrus. Although usually not heavy, many citrus trees may have a significant number of flowers develop. Some of these will be followed by fruit, however, if we have an early cold snap the fruit will likely freeze. Young fruit are very susceptible to freezing injury, even from a light frost. If some fruit do survive and continue to develop, this will not interfere with the development and ripening of the mature fruit presently on citrus trees.
In addition to fruiting and flowering, many plants may push out a spring-like flush of new growth. This could be a problem, as new, succulent leaves and stems are sensitive to cold injury, even typically cold-hardy species. Our average first frost is not far away. Hard as it may be to believe now, our first frost here in Tucson usually arrives by Thanksgiving. New growth coming forth on plants now may not have enough time to mature and "harden" to the effects of cold. On mature plants, the freezing of new growth will have little effect, but not so on young plants. They could be weakened or even killed!
There's nothing that can be done to prevent this new growth or subsequent freeze injury from resulting. However, it should be noted that fertilization at this time of year will only exacerbate the problem. Applying fertilizer to landscape plants in late Summer and into Fall will stimulate even more growth which in-turn will make plants that much more prone to freeze damage.
The only plants that should be receiving continued fertilization at this time of year are container plants, lawns, flower beds, roses and cool season vegetable gardens. Fertilization of trees, shrubs, ground covers and vines should wait till next Spring. This is also true for those new landscape plants installed this fall. Wait until Spring to fertilize them, and then use the fertilizer at half strength of the amount recommended on the label.
In addition to inappropriate Fall fertilization, over-watering as days shorten and the weather cools will also prevent plants from going into their normal dormancy and cessation of growth. When daytime temperatures dip below 90 degrees, the frequency of watering may be reduced on all plants. Lengthen the intervals between watering. For example; if you've been irrigating your landscape every three days, water every 6 days for now. Later, as the temperatures get colder, set it to come on every 12 or 14 days. The point is, continue to lengthen the intervals between watering as the temperatures cool and the days shorten.
Some types of plants will require more frequent irrigation than others. Fall vegetables and flowers may need daily irrigation until establishment. Then watering can be gradually reduced to once or twice weekly. Potted patio plants and raised bed gardens will also need frequent irrigation, two or three times weekly, to keep the soil evenly moist. Desert-adapted trees and shrubs will need little or no supplemental irrigation through the winter.
The amount of water to put on per application should not change from summer to winter, only the frequency of application should change. Water long enough to wet the soil to a depth of 12 to 18 inches for small plants like flowers, vegetables, ground covers and small shrubs. For larger shrubs and trees wet the soil to a depth of 24 to 36 inches.
The best way to determine the depth of wetting is to use a metal rod pushed down through the soil. The rod will push easily through moist soil but stop when dry soil is encountered. Sometimes the rod will hit a buried rock, so it's advised to push the rod down in several locations to be sure the measurement is correct. If water isn't getting down far enough, adjust the irrigation run time up to get the water down deeper. A piece of concrete reinforcing bar, available at home supply stores, is great to use for measuring the depth of wetting.
Written by John Begeman, College of Agriculture and Life Sciences, the University of Arizona, 520-626-5161. - Updated: October 5, 2003
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