To state the obvious, many of our landscape plants really show the signs of excessive heat and extreme drought. Some trees are losing leaves already and may be turning color before they drop. Others have turned completely brown while still remaining attached.
Early fall color and/or defoliation is common when plants are under stress and this season has been quite challenging for many trees, both old and young. The intense heat made it difficult for plants to keep up with water and cooling requirements, even in areas where moisture was adequate. Combine extreme heat with drought, and it is a wonder any plants survive.
One of the ways that plants cool themselves is through transpiration, which allows water to evaporate from the foliage. Plant leaves have pores called stomata that can open and close to allow water vapor and gas exchange with the environment. During extreme heat and/or drought, stomata will nearly close, thus reducing transpiration and exchange of carbon dioxide and oxygen. The end result is repressed biological functions including the transpiration cooling effect, the making of carbohydrates through photosynthesis, and the processing of storing carbohydrates for growth.
Generally speaking, most deciduous plants can cope with early foliage loss, but other stresses may take their toll. Plants that were already in trouble before the excessive heat and drought may not fare as well or perhaps even succumb. But most plants that are otherwise healthy will recover as more favorable growing conditions return.
We can also expect that next year’s foliage and early spring flowers will be impacted by the stressful summer. The buds for next year’s foliage and early spring flower buds have already been formed with likely inadequate carbohydrate reserves.
Deciduous plants (those that lose their leaves each winter) may look brown or defoliated, but may still have viable buds that will leaf out next spring. Cut through a few buds to look for green tissue inside. If buds are brown and crispy, that branch is not likely to survive.
All evergreens shed needles at some time, but healthy plants do not shed all needles at once as deciduous plants do. White pine and arborvitae dramatically drop older needles in late summer or early fall, which might happen earlier this year. However if evergreens are completely brown now, they are not going to leaf out again. Note that there are a couple of deciduous conifer species, notably bald cypress and dawn redwood, which do normally drop all of their needles each year. They too may have browned and dropped early.
There is still plenty of summer yet to get through so us gardeners will just have to wait and see what Nature has in store for us!