One challenge of weather is its unpredictability. Rain one day, sun the next, cool then hot and back to cool, and on and on. But if variability is challenging, consistency can be much harder to handle. Imagine life if it rained every single day. Or, even worse, if it never rained. When Mother Nature dries up and stays dry, it becomes a threat to every living thing — including trees.
In severe cases, this means a drought, which the National Weather Service says is “a deficiency in precipitation over an extended period, usually a season or more, resulting in a water shortage causing adverse impacts on vegetation, animals and/or people.” And drought conditions require a change in tree management practices.
Travis Evans, district manager in The Davey Tree Expert Company‘s Santa Cruz, California, office, got a firsthand look at the impact of the four-plus-year drought that California recently suffered through. “Over the first year or two, most of the mature trees in the landscape handled it. But by years three and four, we really did start to see an increase in certain trees being stressed out,” says Evans.
And it wasn’t only hard on the trees. “The last couple years of the drought were extremely challenging as an arborist, because there were a lot of times where you had to step back and let the client know that you have to do more research,” says Evans. “We started seeing new pests as a secondary issue for drought-stressed trees — things we had never even seen before.” So scouting for insects becomes more important during drought, including looking beyond the normal pests. For example, Evans says the drought led to an increase in borer activity on conifers in that area, “which we hadn’t really seen a lot of in the past.”
Drew Zwart, west coast technical representative with Bartlett Tree Research Laboratories, says that the California drought made good, basic tree care practices all the more important. “The first thing that I like to see when I walk on a landscape during drought is a large and fresh mulch ring around a tree,” says Zwart. “That is the number one easiest and most effective and impactful thing that you can do, both because it helps to conserve moisture in the soil, and it helps to regulate soil temperatures, especially with shallow-rooted trees.” One other benefit that’s often overlooked, he adds, is that when you have mulch around a tree, there is no grass or other understory plants “that will compete, and probably win, for the water that is available.”
Zwart says that the same rules for mulch in normal circumstances apply in drought conditions: Ideally the mulch will extend out to the dripline, “though that’s obviously not always possible with the rest of the landscape plan, so as much as you can do — and 3 to 4 inches deep is as much as I’d ever recommend … It’s a soil treatment, not a stem treatment … you don’t want to be piling it up volcano-style.” Another tip he offers: Be sure to stir the mulch up at least once a year, otherwise it can actually form a hydrophobic layer and start to work against you. Zwart also recommends, when possible, using fresh, out-of-the-box chips with some bark and twigs mixed in. If a homeowner wants to use a dyed or painted mulch product for aesthetic reasons, it’s especially important during droughts to avoid dark or black types that will soak up heat from the sun, he advises.
Of course droughts are not unique to California and can strike nearly anywhere. For example, extreme drought conditions were experienced in parts of the Midwest from 2012–2014. “And we are still experiencing those impacts,” says Lindsey Purcell, urban forestry specialist with the Purdue University Extension Forestry and Natural Resources.
Cultural practices — notably mulch and water — become even more important during droughts, says Purcell. Older trees with a more developed root system typically respond more slowly to drought, he explains. “But we lost some really big trees; I never thought that would happen,” Purcell adds. That’s why it’s important not only to provide water for newly planted and juvenile trees, but also “veteran” trees, he notes.
“The most important thing with water is being able to calculate the amount of water the tree needs,” says Purcell. He has calculated those figures for most species of trees in Indiana; when in doubt, check with your local state extension office to see if they can assist in providing or calculating this information.
To help care for trees during the drought, Evans says that Davey crews were undertaking supplemental watering during the driest months of the year. “From May to October, every four to six weeks we were offering a deep-root watering — actually injecting the water subsurface — because it’s a more efficient way to water as opposed to having clients just use a hose on top of the soil and having the water dissipate into the atmosphere.” So subsurface watering is not only better for the tree, but it also reduces wasted water, which is especially important during any drought, when municipalities are likely to have limitations on water use. “We can really utilize the limited amount of water that we have to use,” says Evans. For clients who didn’t want to opt for the deep-root watering service, Davey recommended the use of soaker hoses. “Then, based on the species and the size of the tree, we would tailor a plan for them and help them with the amount of gallons and how frequently they should be doing that,” he adds.
Landscape trees used to getting water from landscape irrigation are prone to damage when that water is typically shut off during droughts, says Evans. “A lot of our clients were doing their part by not watering their landscapes due to water restrictions. While that’s great, it had an additional impact on the trees. That’s why doing that supplemental watering helps.”
Zwart has observed a somewhat related problem that can take place, especially during droughts. “When people are replacing their lawns with synthetic turf, they forget that a large portion of the water that their trees were getting was from when they watered their grass,” he explains. The same situation is true when homeowners try to do the right thing by shutting off their lawn irrigation in a drought. “You’re also cutting off water to the trees,” he points out. “Especially with mature feature trees, we really like to see dedicated irrigation zones, so the tree has its own zone that can be programmed independently from the lawn. With most decent irrigation systems, it’s not hard to do, it’s just not frequently done.”
This brings up another misconception: that native trees will be fine during droughts. “But when it’s a true drought, that’s not the case,” says Zwart.
Even native trees need occasional deep watering. The amount of water a tree needs depends on the species and canopy size of the tree, among other site-specific factors, but Zwart says that as a general rule, during significant droughts, a deep watering — getting the water down at least 12 inches — at least once a month is a good rule of thumb.
And sometimes, when water use is strictly regulated, it’s necessary to prioritize which trees in the landscape to water. “It’s certainly a lot easier to replace a shrub or a citrus tree than it is a 400-year-old valley oak,” says Zwart.
“So, if either the volume of water or the water bills are a concern, then it makes sense to prioritize the large stuff that you won’t get back in your lifetime as opposed to the smaller stuff that you can just run down to the nursery and replace.”