Dr. Curtis E. Swift

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August 16, 2012
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SWIFT: Assessing damage of summer's hot weather

The hot weather is hopefully over. It has been quite hard on our landscape plants especially those under watering restrictions. Lately, I've been seeing cottonwoods and other trees shedding leaves as if it were autumn. The leaves on other trees have scorched around the edge due to the hot dry weather. While this is a common occurrence with Linden and a few other trees, the extent of the scorch has been severe on this trees. Trees that develop shallow roots like the white birch and Norway maple have scorched leaves and suffered tip die-back of some of their twigs and stems. I expect more problems for these two trees to be evident next spring especially if their owners don't water them adequately this winter.

The rains we have been experiencing were greatly needed but an occasional rainstorm in the summer would also have been appreciated. When trees are severely stressed for water in the summer and then experience rains in the fall, they tend to pop new growth. The new leaves resulting from these rains are larger than normal and the stems are more succulent than they should be at this time of year. Some of this new growth may not have time to harden off for winter and will suffer winter damage as a result. Heavy rains while trees and shrubs are attempting to shut down for winter is like hitting the plant with a heavy dose on nitrogen fertilizer: a burst of succulent growth.

I'm surprised lilacs and other spring-blooming shrubs have not put on a show by popping flowers. These shrubs tend to bloom again in the fall when their leaves are stripped off by a hail storm or when they are severely stressed. Evidently, the stress of this summer was not adequate to create the hormone and protein shift in these plants that precedes a late summer bloom. The hot dry summer also created problems for the roots of many of our plants due to an increase in the salt content of our soils.


As moisture evaporated from the soil and was transpired by the trees and other plants, the soil salt content increased. This is similar to what happens when you allow water to evaporate from a pot of salty water, the salt content increases.

This increase in soil salt concentrations not only dehydrates roots reducing their ability to absorb water and nutrients but is also known to trigger oxidative stress in the plant by increasing reactive oxygen species within the plant's cells. These free radicals damage cell membranes and even DNA.

Plants create their own antioxidants to combat disease organisms and other stresses, but when stress conditions exceed the plants ability to produce antioxidants, extensive cell or genetic damage occurs. Water is critical to keep the soil salt concentration at a tolerable level.

This year, many local residents discovered they had problems providing adequate water to keep their plants stress-free. In many cases, the lack of adequate soil preparation restricted the deep root penetration necessary for their plants to obtain water from deep reserves. Poor soil preparation also hampered the owner's chances of wetting the soil to the appropriate depth even when water was available. Our soils should serve as a reservoir of water and nutrients in time of plant stress and permit the flushing of salts below plant roots when water is applied.

Providing adequate water is critical to plant health and in semi-arid regions is even more critical due to the tendency of these soils to be salty. Selecting plants tolerant of salty soils, hot and dry conditions, drought, and even UV stress is becoming more important as the world's arable lands become more salty. Currently, about 30 percent of lands previously used to grow crops are no longer usable due to salts and that percentage is expected to increase to 50 percent by 2050.

In addition to proper soil preparation and adequate watering, when water is available, be ready to spray the leaves and drench the soil of your trees and shrubs with aspirin.

The use of aspirin has been shown to enhance a plant's ability to withstand the increase of free oxygen radicals resulting from saline soils and other stresses.

Lay in a supply of aspirin now. We will have another drought and you should be prepared.

Dr. Curtis E. Swift is the area horticulture agent with the CSU Extension. Reach him at Curt.Swift@mesacounty.us, visit WesternSlopeGardening.org, or check out his blog at http://SwiftsGardeningBlog.blogspot.com.

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The Post Independent Updated Aug 16, 2012 02:39PM Published Aug 16, 2012 02:38PM Copyright 2012 The Post Independent. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.