|TREES IN HOT WEATHER
|TREES IN HOT WEATHER
By M.W. Staples
Summer is the season when the shade of urban trees is most appreciated. It is also the season when trees display
stress signals, and heat-loving insects proliferate.
Stress may be indicated by off-color, undersized foliage, marginal browning, premature shedding, ragged or shredded
foliage or just a general tired or lackluster appearance. With conifers, a dull gray cast and early shedding of older
needles are certain indications of trouble. Causes may be as varied as the signals -- excessive heat reflected from hot
pavements, drought, compacted soils, restricted root area, winter injury, frost and wind damage, weed-killing
chemicals, air pollution and fungus diseases, or any combination of unfavorable conditions.
Insects common to the season include aphids, scales, lace bugs, mites, tussock, moths, cicadas and bagworms.
Insects, if present, call for appropriate controls. Watch for lace bugs on azaleas, rhododendron and hawthorn, and
look for mites on almost everything, especially conifers.
Feeding is always in order for stressed trees and there is no need to fear starting new growth late in the season.
Trees that are in low vigor now will not begin new growth this fall, no matter how well-fed.
In the past few years, liquid feeding with a pressurized hydrospear haws become a leading method for tree fertilization.
It offers several advantages. The soluble fertilizers are expensive, but the method is much faster than punch bar or
drill methods, and the total cost is reduced. The holes left in the lawn are not a hazard, and at the dry season it gives
a much-needed drink to thirsty trees.
Frost crack or trunk split is a wintertime hazard for many trees. It usually happens during wide temperature changes.
The sun warms and dries the surface of the tree trunk during the day and it freezes again at night. Cells in the center
of the tree remain moist and at a higher temperature. The stress of fluctuating temperatures on the two wood
conditions causes the bark to split. Even heartwood cracks in extreme conditions. The cracking occurs on the south
or southwest side of trees where the sun strikes, giving rise to the often-used name of "southwest disease".
This usually happens to young trees, but an eight-year-old "American Beauty" crabapple received a 30-inch-long bark
crack down the trunk when temperatures twice reached about 20 degrees below zero. It is not known when the crack
occurred, but it showed up in early spring. It started healing under the damaged bark, but by mid-August, 3-1/2 inches
of exposed heartwood still was left to be covered by the new wood.
The frost crack could have been prevented by painting the trunk white or wrapping it with some of the commercial tree
trunk wrap, or anything to break the sun's rays on the tree trunk. Newspapers, aluminum foil, a gunny sack, or a board
tied to the south side of the trunk would help. Even bare branches break the winter sun enough to help. This tree was
the tallest of a group with no low overhanging branches.
Other most susceptible trees are maples, lindens, oaks, horsechestnuts, beeches, London planetrees and apple.
Deciduous trees are more subject to cracking.