News release: Trees and Drought

Tim Baker, MU Extension Horticulture Specialist
102 N. Main, Suite 1, Gallatin, MO 64640
660-663-3232, bakert@missouri.edu

Release Date: July 26, 2012

Title: Trees and Drought

My rain gauge seems to be permanently stuck on “dry”, with only an occasional shower to measure. I sure hope we get relief soon, but the forecast is not hopeful.  This week, I thought I would give a few pointers about drought and trees, and measures you might take to help your trees out.

In traveling around the area, I can see a lot of trees that are under major stress, and some may have even died from a lack of moisture. Under these circumstances, many homeowners are understandably concerned about trees around their house. They don’t want to lose valuable trees in their yard, which offer not only beauty, but with proper placement, provide significant savings on utility bills.

Drought, of course, is a significant stress on any plant, including trees.  And all stress is cumulative.  That means that if a tree starts out with a diseased root system, for example, adding the stress of drought may be enough to kill the tree.  Other stresses include insect pressure, hot temperatures, or perhaps trees that are not well-adapted to the zone they are growing in.  Certainly the combination of heat and drought such as we have seen this year is especially hard on trees, taking some of them.

A large, mature tree normally uses a phenomenal amount of water.  A large oak, for example, may use several hundred gallons of water per day.  The roots take up the water, and it is transpired out through the leaves.

When a plant cannot obtain enough water, its leaves may start to wilt. Leaves have pores (stomata) that transpire water, and under drought conditions, these pores may close, to conserve water. Evaporating water has a cooling effect, and once the transpiration process is shut down, the leaf temperature will rise.  This may result in brown, crispy leaf edges, which we call leaf scorch.

Some plants may even drop their leaves, if the drought is bad enough.  For woody species, this may help them survive the drought.  Don’t assume that your tree is dead and cut it down.  Wait until next year to see if new leaves emerge.  You may be surprised.

As you evaluate the trees in your yard, you may be tempted to think all is lost.  Dr. Chris Starbuck, MU Extension Woody Ornamental Specialist, says “Don’t give up.”  His advice is to water your trees and shrubs, if they appear stressed from drought.

If the soil is bone-dry, it can actually pull moisture from the trees roots, possibly killing the tree.  By irrigating your tree, you can prevent this process, and hopefully save your tree. Dr. Starbuck has even seen trees completely defoliated by drought recover when given water.

When watering the soil around your trees, add enough water to wet the soil to a depth of 8-12 inches.  Dr. Starbuck says to be sure to cover at least 20% of the area within the drip line about once per week.  For a tree with a 20 foot diameter drip line, this will usually require about 75 gallons of water applied over a 60 square foot area. 

As you water, be sure to avoid runoff. Water not going to the root zone is wasted.

It’s also important not to over-do your watering. Saturated soils combined with high temperatures can actually kill a tree.  I will admit that as dry as it is, it will be hard to saturate your soil, but some people do get carried away, so I thought I better include this warning.

In all, watering should help, but it is not a guarantee that your tree will survive.  Trees that were already stressed or in marginal condition may not make it.  On the other hand, watering may just be what your tree needs to survive, and your help may make a difference.