Understanding summer leaf scorch: Causes and effects explained

SALT LAKE CITY — August is one of the hottest months of the year in Utah, which means it tends to cause leaf scorch in certain trees and bushes.

What causes leaf scorch?

Leaf scorch occurs when the edges of the leaves — and sometimes even in the middle of the leaves — turn brown, and it typically starts in late July through August.

The ideal humidity levels for trees that are native elsewhere are between 50% and 70%, according to KSL Greenhouse host Taun Beddes, “because they don’t have to get as much water out of the soil for their life processes.”

However, Utah’s humidity level around this time of the year is usually only around 10%.

“Especially as the tree gets bigger, even though there’s sufficient water in the soil, it can’t get it fast enough to replace what it’s losing,” Beddes explained.

The key to knowing if your tree is suffering from leaf scorch instead of a different disease is checking whether it leaf’s out healthy in the spring. If it’s leaf scorch, the tree will do just that.

What does it affect?

Several species of trees are prone to leaf scorch, such as Japanese maples, tricolor beech trees, Norway maples like the Emerald Queen, and lindens.

The horse chestnut tree is another example.

“Because (horse chestnut’s) leaves are so big by September, a lot of times they’re really quite scorched and turn brown, but they leaf out healthy again the next spring, and so it’s just a physiological condition that doesn’t harm the tree,” Beddes said.

Leaf scorch can also affect some bushes, such as the burning bush and the snowball bush.

Essentially, it can happen to any tree or shrub with larger leaves, especially if they are older.

“You especially see it in trees that are 15, 20, 25 years old because, as they grow, it’s harder for them to push water up to the top of a taller tree,” Beddes said. “When it was smaller and maybe 15 feet off the ground, the pressure it takes to push water up isn’t nearly as great as it is on a 50-foot tree.”

There are some trees that are less likely to get leaf scorch, including zelkovas, State Street maples, Queen Elizabeth maples, Pacific and Norwegian Sunset maples, and hackberries.

What can be done?

Beddes advised against overwatering the tree, as that will only worsen its condition. If the tree is established, it’s okay to water it once a week. If it’s on the newer side, water two to three times a week.

“It is normal. The trees come out healthy the next year, especially if you still have 70% or 80% of the canopy that is green, even with some leaf scorch. If that leaf is still 80% green, it’s functioning,” Beddes said. “Even though the tree looks a little shabby … if you still have 70% or 80% of the green intact, the tree is still photosynthesizing, and it’ll be perfectly fine.”

Read more from the KSL Greenhouse show:

• Here’s why the ends of your tomatoes look different from the rest
• Is spurge invading your lawn or park strip? Here’s what you can do
• Got lawn problems? It may be one of these fungal diseases

KSL Greenhouse is live on Saturdays from 8 a.m. to 11 a.m. Follow the show on Facebook, Instagram, and YouTube.