As the drought of 2012 drags on, with no end in sight, it may be tempting to resign oneself to the idea that landscape plants are doomed. Certainly, it has been challenging to devise irrigation strategies that provide enough soil moisture to keep ornamentals alive, let alone attractive- looking. Although it seems likely that there will be considerable mortality of trees and shrubs before the current drought ends, it is still worth the effort to attempt to keep the most valuable specimens alive. As terrible as trees and shrubs may look, many people will be surprised at the recovery that woody plants will make if kept alive until the drought finally breaks.
Drought causes many different symptoms on plants. As soil moisture is depleted the first response may be wilting of leaves, indicating that there is insufficient moisture to supply the transpirational needs of the foliage. Leaves need a constant supply of water to remain turgid and to cool their surfaces through evaporation. Wilted leaves close their stomata (leaf pores) to conserve water and, therefore, can no longer cool themselves. As the leaf temperature rises, the tips and edges often turn brown (scorch) and dry up. If the drought continues, many plants drop some of their leaves to further conserve water. All of these changes are drought survival mechanisms which help plants conserve whatever water they may retain in their leaves, stems and roots.
Although a drought stressed plant may look terrible, it may be far from dead and will usually benefit greatly from irrigation. If left un-irrigated during hot, dry conditions, the soil in the root zone may become so dry that it actually pulls water from root tissue, which may cause mortality. Even wetting the soil in a small percentage of the root zone of a tree or shrub can lead to remarkable recovery. It is quite common to see trees completely defoliated by drought producing new shoots within days after irrigation (figure 2). Although the prognosis is somewhat murky this year because the drought began so early in the season, I am optimistic that most trees can be saved if irrigated before it is too late. Check the branches to see if they are still pliable before giving up. Even trees showing branch dieback may recover fully if kept alive. Nick the bark farther back on the branch so see if it is still moist and green underneath.
In general, evergreen trees and shrubs are fairly drought tolerant. However, once they begin to show severe drought symptoms, such as needle browning, it is sometimes too late to rescue them by irrigation. There was considerable mortality of arborvitaes last summer, during the flash drought and many arborvitaes, pines and spruces appear to have passed the critical point in Columbia within the past week of triple digit temperatures.
Given the extreme climatic conditions that we are experiencing, it is important to wet the soil to a depth of 8-12 inches in an area covering at least 20% of the area within the drip line (the shadow with sun directly overhead) 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 (2x30 feet). Plants can redistribute water once it is taken up. Any method that gets this amount of water into the soil without runoff on a weekly basis will greatly increase the chances that a tree will survive the current drought and live to recover when conditions improve.
One approach to tree rescue irrigation is to lay “leaky hose” type soaker hoses within the drip line. Soaker hoses are notoriously uneven in their distribution. To improve uniformity, make a “gender bender” (figure 1), consisting of an 8” piece of hose with two female ends. This allows both ends of the soaker hose to be attached to a garden hose using a Y-adaptor. Lay the loop within the dripline of the tree to be rescued and irrigate long enough to apply at least 100 gallons of water. Although the wetting pattern varies greatly with soil type, two inches of irrigation will generally wet the soil to a depth of about one foot. This is generally deep enough to access many tree roots. Assuming that a 50’foot soaker hose will wet a band of soil about 18” wide, it will cover an area of about 75 square feet. Since it takes 0.62 gallons of water per square foot of surface, it would take about 50 gallons of water to apply one inch. Most domestic water supplies deliver about 5 gallons per minute from the hose bib, so it would take about 20 minutes to apply 2 inches. Inserting a 25 psi pressure regulator (available from garden stores) upstream from the Y-adaptor will make the hose more efficient (fewer geysers), but will lengthen the time required to thoroughly wet the soil. Check regularly for runoff. Under “normal” conditions, a typical landscape soil may only be able to absorb ¼ inch per hour. However, under present conditions, irrigation water will flow freely into cracks in the soil. Keep the hose running during the day, moving it around to the trees showing the most severe drought stress symptoms. Consider using a timer so you will not forget to turn the water off when you are not tending the hose. A simple way to tell if you have put enough water into the ground is the “screwdriver test”. If you stick an 8” screwdriver in the ground and it hits hard soil at 2 inches, keep watering.
REVISED: September 29, 2015