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!