Plants Get Stressed Out Too

What's this? Fall in early August? Have you noticed an abundance of leaves on the ground or fall colors up in the trees in your yard, favorite park, or perhaps at your local walking trail? What's up with that?

Even though Alabama's heavy, humid summers are far from the crisp, cool air associated with autumn, signs of autumn abound lately. Is it suddenly en vogue in 2010 for trees and shrubs to ready themselves earlier for the new fall line, or is this change indicative of something more? Foliage taking on the hues of autumn, drooping and even falling off, are indeed reactions to heat and drought stress, as evidenced by this American beech (Fagus americana) in our Kaul Wildflower Garden.

The cooler-than-normal weather this spring delayed new growth in plants. With new growth being especially vulnerable to heat and drought stress, the high temperatures and sparse rainfall are taking their toll on many plants. This new growth is not as efficient at regulating a plants natural cooling process – transpiration. Once leaves wilt and turn brown, they usually fall.

This foliar collapse is a plant’s defense against harsh weather conditions. In high temperatures and drought most plants are struggling to remain alive, rather than putting on new growth. During these periods photosynthesis may slow, or even stop, in order to conserve energy; therefore the production of chlorophyll slows …the mechanism that causes leaves to color. When a plant senses the stress, it also begins producing more of the hormone abscisic acid, which is normally produced upon the onset of lessening daylight and cooler temperature in fall. This event causes the connection between leaf and stem to seal over resulting in leaf drop. This phenomenon is seen more in some species than others. Tulip-poplar (Liriodendron tulipifera) and sassafras (Sassafras albidum), pictured above, are often two species that  show their stress early. The good news that as long as the plant is not exposed to extended periods of heat and drought, most of them will survive. One exception is that once most conifers (pines, hemlocks, etc) reveal significant browning and needle-drop, it is usually too late to save the tree.

Some of the preventative measures that may be taken to avoid, or lessen the effect of heat/drought stress are mulching around tree bases to help retain more soil moisture, and watering frequently, especially early in the morning or in the evening. Remember that infrequent deep watering is more beneficial that infrequent shallow watering – which tends to encourage shallow roots that are more susceptible to stress. Pictured left is a Japanese maple (Acer palmatum cv.) from our Japanese Gardens displaying some fall color on many of it's leaves.

Visiting Birmingham Botanical Gardens is an excellent way to observe these issues, as well as to observe which plants are better suited to handling heat than others.

--Katie King, Summer Intern