Stressed Out about Water Stress
By Catesby Suter
As temperatures stayed impossibly hot, and rain refused to fall, Master Gardeners throughout Arizona became all too familiar with symptoms of water stress. During a recent, socially distanced plant call, Oracle Master Gardeners inspected 13 fruit trees, and all but one showed serious signs of water stress.
Wilting is generally the first symptom, often accompanied by an overall bluish cast to the tree canopy. Often this is the only symptom and the tree rapidly recovers when temperatures drop at night and the tree receives water. But, if the tree receives too little water, the temperature climbs or worse, the hot dry winds begin, then signs of more chronic water stress become evident. Yellowing leaves, scorched leaf margins, and dropping leaves all signal the tree has begun to shut down to conserve water. As the stress continues, fruit shrivels and drops, and branches begin to die back. The remaining canopy appears thin from leaf loss and the few new leaves are small. Sometimes, water stress is so bad, even the bark cracks.
While the solution seems as easy as adding water, in most cases by the time a worried homeowner calls, water stress has “primed” the tree, making it highly susceptible to insects and diseases. When water stress weakens a tree, a secondary pathogen can more easily invade, and the result is often fatal.
The first line of defense is to actively maintain a healthy watering schedule. Expect to adjust your irrigation schedule to fit the seasonal extremes, by checking soil moisture and watching for signs of water stress. Long, slow waterings are preferred, and trees should be checked after irrigation to determine if the irrigation is effectively reaching the root zone, generally within a foot of the soil surface. Test the depth of water penetration by pushing a narrow rod (1⁄4 to 1⁄2” in diameter) through the soil. When a tree has received enough water, the rod will penetrate easily to the 12” mark or slightly lower. If the rod slips too easily through the soil, penetrates past 3’ and comes out wet, the soil is too saturated. When a tree isn’t receiving sufficient moisture, the rod fails to penetrate more than an inch or two, missing most of the root zone.
The best approach to beating water stress is to increase the duration of irrigation. Slow, steady water is what is needed, unless the soil is too compacted. When water won’t penetrate despite a slow application, additional steps are needed. By thinking about the differences between soils, you will remember than water penetrates most rapidly through sand, less rapidly through loamy soils with clay soils being the most difficult for water to infiltrate.
Quick relief for compacted soils can be accomplished by drilling small diameter holes of 1” or less to a depth of 12-18” just outside the dripline of the tree, at one-foot intervals. Backfill the holes with sand, and these “sand straws” will take water right to the root zone.
