Dr. Grow: Is it dry, or what?

Drooping leaves on a viburnum shrub are the result of moisture stress.

I've heard several old-timers (anyone who is older than me by one day, or anyone who looks older than me) say that they have never seen it as dry as it is now. They must be right, because crabgrass in my front yard, usually the only plant growing during a dry spell, has put a halt to cell division and cell elongation. When weeds quit growing, it must be dry.

Earlier in the summer I was not too worried about the drought because it always rains during the Sikeston Bootheel Rodeo. When it didn't rain during that event, I began to get more than just a little concerned. Then I was sure it would rain during the SEMO District Fair. As you know, it didn't rain last week. This drought is really serious.

Most of the questions that I have fielded during the last few weeks have to do with the effects of the drought on plants in landscapes. People want to know if they should water, when to water, how much to water, what happens if they don't water. The list of questions goes on and on. Perhaps if we look at drought from a landscape plant's perspective, we can answer some of these questions.

Under normal weather conditions, plants lose water through transpiration. Water in the form of a vapor exits the leaves through small pores, called stomata. These pores, usually found on the under surface of leaves, are surrounded by two guard cells. The guard cells control the opening and closing of the pore.

If a plant is not under moisture stress, it can lose large amounts of water through transpiration. A large tree can lose up to 50 gallons of water a day. A corn plant can lose up to one half a gallon a day. If you do some calculations, you can see that an acre of corn can lose up to 13,000 gallons a day through transpiration.

During moisture stress, the guard cells around the stomata change shape and close the pore. This change in shape is due to the loss of turgor pressure. The guard cells do not have enough water in them to keep them totally inflated, and they become flaccid. Because the pore is closed, very little transpiration occurs, and water is conserved.

As moisture stress continues, physiological changes begin to occur. Enzyme activity is reduced and normal chemical reactions in the plant slow down. Photosynthesis is drastically reduced. Growth slows down because cell expansion and cell division cease.

If the drought continues, morphological changes also occur. First, leaves in some plants begin to roll. If the drought is severe enough, then the edges of leaves bum because of lack of moisture. Some deciduous plants begin to think it is fall, change color, and begin to lose leaves. They go into a drought-induced dormancy. Without leaves, deciduous plants do not lose moisture due to transpiration.

Lawn grasses go through several physiological changes. Leaves begin to roll and finally they turn brown. Moisture is conserved because the dead leaves no longer function. Grasses remain in this drought-induced dormancy until rains begin again.

Perennials will react somewhat like lawn grasses. As drought stresses the plants, leaves start to die. The root system goes into dormancy. These roots may return to life the following spring, if stress was not too severe.

Broadleaf evergreens, such as azalea and boxwood, cannot lose their leaves and then put on new growth the following year. They must be kept alive during a drought by watering.

At this point in time what should you be doing about the plants in your landscape? If you have done nothing so far, I would suggest you give all of your landscape plants a good watering. Let water trickle into the soil around the plants very slowly for several hours.

If you don't want to water all your plants, it is extremely important that you water both your broadleaf evergreens and other evergreens before winter. Because these shrubs and trees do not lose their leaves in the fall, they may lose moisture through transpiration all winter. If there is no moisture in the soil to replace lost water, foliage can dessicate to the point that it dies.

Fertilize any stressed trees and shrubs this fall only after you water them adequately. If fertilizer is applied during stress, it can increase the stress and cause damage and death.

If your lawn was in good shape prior to the drought, it may be able to withstand the stress and come back to life on its own after rains begin again. You will need to fertilize when rain comes so that the turf can develop a good root system prior to winter. Use a high nitrogen low phosphorous and potassium fertilizer, such as 28-4-4, on your cool season grasses. Use the same fertilizer in the spring for your warm season grasses.

If your lawn was in bad shape before the drought, you may want to reseed it this fall. Get it done as soon as possible. Plan on doing a lot of watering if the drought continues. Do not wait for rain to come. It may come too late, and then you will be seeding too late in the fall.

Because this drought has been so severe, evidence of the damage that may have been caused may not show up for the next one or two years. Your best insurance is to thoroughly water all plants in your landscape this fall. Pray we get some rain soon.