Milwaukee has faced extreme heat waves this summer, with even nighttime temperatures staying above 70 degrees. These high temperatures have been uncomfortable for people and trees alike.
Trees depend on a cycle of heating and cooling in the summer — and when temperatures stay too high, that can cause heat stress.
“It’s always a battle out there with trees,” says Adam Passo, district manager for the Davey Tree Expert Company.
He spoke with Lake Effect’s Joy Powers in-studio to share how to identify and manage heat stress in trees.
This conversation has been edited for length and clarity.
Joy Powers: We have been having some pretty intense heat waves here in the Midwest, and we're not really seeing the kind of relief from the heat that some trees depend on. What does that mean for tree health?
Adam Passo: It can be a challenge, but they're really quite adaptive. Trees want to live. They're fighters. They do whatever they can to conserve and protect. Underneath the soil, if there are similar trees nearby, they're grafting roots a lot of times. If one tree is kind of struggling in the stand, other trees will help it. They'll push water and nutrients. Stands of trees help each other out quite a bit. We don't see it, but there's a lot of stuff the trees are doing.
JP: How can you tell if you have a tree that might be heat stressed?
AP: There are some really obvious signs: the leaves get pretty droopy, they just look flatter than normal, they don't look puffed up and happy. Dogwoods are a perfect example. Right after a rain, they're really happy and kind of pointing upwards. But then, when it's hot and there’s less water, they're pointed straight down. Looks real sad, but it just takes the smallest amount of water for those kind of sensitive plants, and they'll perk right up.
There's some that are from Japan that require a little bit more moisture and are a bit more susceptible. There's katsura trees that we plant. Japanese maples. Some of those borderline trees that are not as hardy. River birches are really susceptible. They're the first to show that there are some issues going on. Dawn redwoods too. I saw it on my dawn redwood the other day. The needles are really soft and really light green, and mine started turning a little bit brown just because of the lack of water and the heat.
JP: So, if you see that your tree is a little off or a little heat stressed, is watering it maybe once a day or once a week going to help?
AP: Yeah. Two times a week is usually sufficient. A nice deep root watering is really fantastic. 15 minutes in one spot. Let it kind of soak down in there. Move to the next spot, soak down. A lot of people put the hose by the trunk thinking that it's going right to the plant, but those are the least absorbent roots on the tree. If you move that watering hose out to the drip line, which is the very edge of the leaves of the tree, that's where all the most absorbent roots are. You want about an inch a week for most tree varieties. If you have a pie tin or something and there's a big rain and you get an inch, you don't need to water it that week. But if you don't get anything in there, you'll need the supplement.
I always preach having nice mulch to conserve moisture and keep the roots cool. A shredded hardwood bark is phenomenal. It breaks down and adds nutrients to the tree, unlike the ground-up pallets that take nitrogen to break down and take away from the tree. Those colored mulches that are just wood pieces, some of those dyes can a little harsh. Anytime you can go natural with your mulch, that's great. Anything that we can do to keep those fibrous roots happy and healthy.
JP: Now, if you have a tree that you're really concerned about – maybe you're seeing some of these signs of heat stress – at what point should you consult a professional?
AP: Preventative is always the best thing, but somebody who cares about their trees and wants to keep them nice should probably just have an arborist that comes by once a year to take a look at things. If you're seeing big dead branches, that's probably a little too late, you know? We’re seeing a lot of thinning canopies right now. That could be due to a number of things, but a lot of times it's human caused.
Planting too deep is the biggest thing that I see in urban environments – trees that weren't planted high enough, the hole was dug too deep, and then they start life at deficit. Roots grow up and grow around the trunk, and they start choking it off as it grows in diameter. So, if you're going to plant a tree, please do. But look at those instructions and make sure you're not planting it too deep. You're really going to see issues 10 or 15 years down the road. At first it'll be fine, but a little preventative maintenance goes a long way.
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