Stressed Plants May Look
Like Early Fall
Many of our landscape plants are under stress from one or more factors
this summer, including excessive heat and humidity, insect feeding and
disease infection. Some trees are losing leaves already and may be turning
color before they drop. This early defoliation is common when plants are
under stress, and this season has been quite challenging for many trees,
both old and young.
The recent intense heat and humidity make it difficult for plants to keep
up with water and cooling requirements, even when soil moisture might
be plentiful. One of the ways that plants cool themselves is through the
process of transpiration, the evaporative loss of water from the foliage.
When relative humidity is high, transpiration is reduced, so plant tissues
may overheat. When the transpiration rate is low, there is also reduced
movement of water from the surrounding soil into the roots. So plants
can still be in moisture stress despite having sufficient soil moisture.
The rate of photosynthesis (carbohydrate production) is also reduced when
heat and humidity are excessive.
Stressed plants are often more susceptible to disease and insects. The
warm, humid and frequently rainy conditions have been perfect for many
fungal and bacterial disease pathogens. Such diseases may start out as
spots that then get larger and may lead to leaf drop. Insects, such as
borers, may be more likely to infest weakened trees.
Plants that are under such stress may initiate what appears to be fall
color change and eventual leaf drop. Generally speaking, most plants can
cope with early foliage loss, but other stresses may also take their toll.
Plants that were already in trouble before the excessive heat and humidity
may not fare as well or perhaps even succumb. But most plants that are
otherwise healthy will recover just fine.
There is still plenty of summer to get through yet, so we gardeners will
just have to wait and see what Mother Nature has in store for us!
8-4-06
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