Too often people think the best thing to do to help trees during times of drought is to fertilize, according to Lindsey Purcell, urban forestry specialist with the Purdue University Extension Forestry and Natural Resources.
“But that is the worst thing you can do with trees under stress,” he explains. “It takes an enormous amount of resources for the plant to metabolize fertilizer; the tree is already trying to survive because of the lack of water, and when it has to metabolize all of these additional nutrients, it throws it further into stress.” Plus, Purcell points out, fertilizer is a salt, and that fact alone can actually exacerbate the effects of drought stress.
Drew Zwart, west coast technical representative with Bartlett Tree Research Laboratories, agrees that, as a general rule, it is advisable to avoid fertilizing trees during a drought. “Most fertilizers are salts, and any salt you put in the soil is going to affect the osmotic balance,” he says. “But if a plant needs nutrients, then it needs nutrients,” Zwart adds, pointing out that there are low-salt index fertilizers available and that any fertilization should be done based on a nutrient analysis (Bartlett takes samples at least every two to three years, if not more often) rather than a calendar schedule. “That way you’ll be adding only the nutrients that are necessary, as opposed to a fast-release nitrogen salt,” he says.
And it’s important to minimize stress not only on younger trees, but also more mature specimens. Oftentimes there’s a belief that a mature tree has developed a substantial root system and therefore will be able to hold its own when things get dry, “and a lot of times, that’s true,” says Zwart. During an unusually dry summer, there may be signs of scorched leaves or a premature color change in the fall, but nothing like what is experienced during historic water deficits, says Zwart. After the historic drought in California in recent years, for example, “We’re seeing 300-, 400-, 500-year-old trees in native areas dying. They’ve made it through droughts before, just nothing this intense,” he says.
Travis Evans, district manager in The Davey Tree Expert Company’s Santa Cruz, California, office, says that during drought, it’s especially important to protect trees against any kind of construction impact. When the trees are already stressed, this can be lethal, he notes.
Another stress that can sometimes be eliminated is pruning. “There were trees that were so stressed that we recommended skipping pruning until the tree could make a recovery,” Evans states. “Of course, there were exceptions — we had to look at safety factors … In certain situations, we did have to prune to eliminate excessive weight so the tree could remain standing.” In cases where pruning was skipped, the tree was monitored more frequently than usual, and the service plan often switched to plant health care, either supplemental watering and/or fungicide or pesticide treatments to prevent further disease or insect stresses on the tree.
“You don’t want to prune any live, green tissue, because it takes resources for the tree to heal that wound,” says Purcell. “So you’re adding additional demands on the tree’s food resources.” Instead, he advises focusing on the “[unbeneficial] plant parts” — namely anything that is dead or dying. “Removing some basal sprouting is OK; those don’t really contribute to the canopy of the tree,” Purcell adds.
Tree-planting practices also need to change during a drought, says Evans. He works in an area where it’s normally possible to plant a wide range of tree species at nearly any time of the year, but during the drought it became necessary to plant only specific species and to do so during the dormant months or in early spring. Evans adds that the drought made clients think more about the tree species they wanted to add to their landscape. “A lot of times they would ask, ‘What can we plant that is drought-tolerant?’ It started a whole different conversation,” he says. While they may have specified a certain tree of a certain size with a certain type of flower, they became more concerned about how the tree would be able to handle droughts, at that time and also in the future. “We definitely started recommending a lot more natives and trees that wouldn’t need a lot of extra water just to make it through an average year,” says Evans.
Another lesson of drought is that for the trees, it’s not over just when the meteorologists say it’s over. While California overcorrected a bit with an extremely wet winter to end the drought, Evans says it will take “consistent rainfall over the next several years to nurse a lot of these mature trees back to health.”