Keep Watering Those Woodies

In previous articles, I have stressed the importance of careful watering of recently planted trees and shrubs during July and August. Until a new plant is able to establish roots in the soil surrounding the original soil ball, it is critical to keep the soil ball moist. It generally requires frequent light applications of irrigation water to achieve this without drowning the plant in a clay soil bathtub. Regular irrigations in which a known quantity of water slowly drips into the root ball of a new tree or shrub is generally the safest and most effective approach for getting the plant through the first summer.

Although well-established landscape plants usually require little or no irrigation to keep them alive through our brutal summers, timely and intelligent irrigation can do wonders to reduce stress. It is important to keep in mind that roots of trees and shrubs proliferate where there is ample soil moisture, and they can move water some distance, back to the mother plant. Anyone who has made a flower bed near a tree knows that the soil in such a bed is soon filled with tree roots proliferating to harvest moisture and minerals meant for the flowers. It is often difficult to water the flowers near a tree often enough to keep them from wilting because tree roots compete strongly with the flowers for moisture. Caretakers of woody plants can take advantage of this opportunistic root growth to reduce drought stress using a relatively modest amount of irrigation water.

Anyone attempting to irrigate enough to supplement rainfall in July and August to total 1 inch per week over his or her entire property may have a very high water bill. Given that it takes 0.62 gallons of water per square foot to apply one inch of water, an entire 10,000 square foot property would require about 6200 gallons for each one-inch application. Over the course of July and August, we may have 6 inches less rainfall than the amount of moisture lost to the atmosphere through evapotranspiration. If so, to make up the deficit completely over the entire 10,000 square feet would require over 37,000 gallons of water. Needless to say, very few of us have the time and money required to irrigate an entire 10,000 square foot property with this quantity of water. Luckily, our trees, shrubs and turf can get by with considerably less water than this and remain in good condition. Furthermore, once established, most trees and shrubs can function well with only a small part of their total root volume remaining moist through drought conditions.

Let your plants be your guide regarding when they need help getting through a drought. Cool, breezy, low-humidity weather that feels pleasant to people may be quite stressful to plants if soil moisture reserves are low. Often, leaves on even well established trees and shrubs wilt and droop during such conditions. If wilting persists for more than a few days, some plants begin shedding older leaves to reduce leaf surface area. While this is a normal drought survival mechanism, it reduces photosynthetic potential and, consequently, the amount of carbohydrates available to the plant. Judicious watering can often prevent significant wilting and leaf drop, thereby helping the plant stay more vigorous. Usually two or three irrigations during an extended drought will greatly reduce drought stress and keep established plants in good condition.

Methods for watering established trees and shrubs are quite different from those previously suggested for new plants. The objective for mature plants should be to saturate at least 20% of the root zone of the plant at least a foot deep. Start by estimating the square footage of soil surface that is shaded by the plant with the sun directly overhead. Since the root zone of a typical woody plant goes well beyond the shade pattern (sometimes called the drip line), half of this area might be considered 20% of the entire root zone. Apply enough water to that area to provide about 3 inches of precipitation. For example, if a tree shades a circle with a diameter of 20 feet, the area of this circle (A = p r²) is 3.14 x 10² or 314 square feet. To apply 3 inches of water to this area would require 314 x 0.62 = 194 gallons. However, since the objective is to wet half of the shaded area, 100 gallons would do the trick.

The delivery method for mature plant drought stress reduction irrigation does not need to be terribly sophisticated. The key is to apply the desired amount of water to the designated area at a rate that does not exceed the infiltration rate of the soil. Given that a 5/8 inch garden hose with 40 psi water pressure delivers about 6 gallons per minute, it would theoretically take only 16 minutes to apply 100 gallons. However, most clay loam soils can only absorb 0.2 to 0.5 inches of precipitation per hour. "Soaker" hoses that ooze water from pores over the entire length are effective for this type of irrigation. A 50-foot soaker hose laid out to cover the area uniformly works well. To improve delivery uniformity of the soaker hose, attach a "Y" adapter to the end of the garden hose and make a "gender bender" consisting of a short piece of hose with female hose fittings at both ends. Connect the male end of the soaker hose to the "Y" adapter with your gender bender.

Regardless of the method used, it important to keep track of the amount of water applied. If an impact sprinkler is used, set out a rain gauge. For soaker hoses, coil the hose in a tub or children’s wading pool and collect the water that is put out over a 5-minute period. Measure the water and divide by 5 to determine the approximate delivery rate per minute. Then set the hose out and water until runoff is noted and estimate the volume of water applied. It may be necessary to irrigate for several short periods over a series of days to apply the desired amount of water without significant runoff. An automatic timer is a good investment for busy or forgetful waterers. When irrigation is complete, poke a metal rod into the ground to see how deep the soil is wet. The rod will stop when it hits dry soil (assuming that a rock does not stop it first).

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630