Posted by Brent Wilson on 10/4/2016 to Pest & Disease Control Tips
We get quite a few questions about browning, curling and scorched leaves on Japanese Maples. So I thought I'd write an article on the topic.
Leaf scorch is identified by browning of leaf margins and/or yellowing or darkening of the areas between the main leaf veins. Due to environmental causes, leaves may dry, turn brown, and become brittle. Premature dropping of leaves and twig dieback may occur during the late summer. Symptoms usually appear after drying winds in conjunction with periods of hot, dry weather.
In mild cases of leaf scorch, the leaves remain attached to twigs or stems, and little damage results. In more severe cases, plants may drop many of their leaves prematurely, although such plants do not die.
Leaf scorch is a noninfectious disease or disorder. It occurs most often after prolonged periods of dry, windy weather or bright sunshine when the roots are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves. Unfavorable locations, such as sandy or gravelly soil, near obstructions or pavement that restrict root growth, or exposed windy slopes can promote scorch. Anything that affects the plant’s ability to take up water, including insect and disease problems, can result in leaf scorch. Herbicides and pesticides may also contribute to scorch.
Regarding this leaf scorch, there's some good news and some bad news.
THE BAD NEWS FIRST...
Bad news is, if the leaves on your Japanese Maple are looking scorched and raggedy towards the end of summer there's nothing you can do to make them look better during the current season.
GOOD NEWS IS...
If your Japanese Maple has leaf scorch, but the tree is still alive, there's most likely nothing to be too worried about. It could just be a temporary problem due to heat and sun stress from a long and hot summer. Japanese Maples usually recover fine from this but won't put out new foliage until the following spring. That said, if the "root" cause (pun intended) of the browning or bad looking leaves is constantly soggy or wet soil, your tree could have a very serious problem with its roots: root rot. However, if you followed proper Japanese maple planting instructions at planting time, this shouldn't be a problem. That said, if you know you planted it correctly to begin with, but the drainage has changed since planting time, you could still have a soil moisture issue.
Causes of Leaf Scorch
First, rule out a soil moisture problem.
Though they appreciate a moist but well-drained soil, especially during the first two summers or so after having been planted in your landscape, Japanese Maples HATE wet feet. Constantly soggy or wet soil can cause root rot and other harmful or deadly diseases.
If the leaves of your Japanese Maple have slowly turned brown or black, starting at the tips and working towards the base of the leaf, this could be an indicator of overly wet soil and that root rot could be setting in. You'll need to conduct a soil moisture test. If you determine the soil is too wet, you can either replant the tree in a raised mound, so that it's roots are above ground level, or move the tree to a new, well-draining site.
There are several ways to test for soil moisture in a planting site:
- Some moisture meters work, but cheap ones don't always provide accurate readings.
- Established trees with an extensive root system: Use a small hand trowel to dig a small hole maybe 3 to 4 inches deep near the roots. If the hole fills with water you know you've got a problem.
- New trees recently planted: Dig a hole 12" wide by 12" deep just beyond the root ball of your newly planted tree. Fill the hole with water and let it drain. Then, after it drains, fill it with water again, but this time clock how long it takes to drain. In well-drained soil the water level will go down at a rate of about 1 inch an hour. A faster rate, such as in loose, sandy soil, may signal potentially dry site conditions and possibly a need to add organic matter to help retain moisture. A slower rate indicates poor draining soil and is a caution you need to improve drainage, replant in a raised mound, or relocate your tree to a well-draining site.
If the soil is too wet, and the stems and branches on your Japanese Maple have begun to die, their might not be much you can do to save the tree. To check stems or branches for signs of life, use a knife or the edge of a coin to scrape away a very small section of bark on leafless stems and branches. If the underbark is green there is life, if brown or tan color the branch has died and should be removed.
Continue this process throughout the tree to see what is still alive. If over 50% of the tree is dead there isn't a good chance of long term survival. This doesn't mean you can't attempt to restore the tree to health by replanting it. In attempting to save your tree you'll need to "lift it up." This means replant the tree at a higher level in the soil. In some cases it will be necessary to replant the tree with half or more of the rootball above ground level, then gradually tapering your soil mixture from the top of the root ball to ground level. Essentially, as previously mentioned, you will be replanting your tree into a "raised mound." This will allow the roots to grow down to the water table instead of standing in it.
Other Causes of Leaf Scorch
If the leaves of your Japanese Maple are browning on the tips or are scorched looking and curled, but the branches are still flexible and alive, the leaf scorch could be caused from:
Too much sunlight. With the exception of a few Japanese maple cultivars that have demonstrated good tolerance to full or all day direct sun exposure, most Japanese maples prefer some shade. In their native habitat you'll find Japanese maples growing as understory trees in the dappled shade or filtered sun of woodland borders. Morning sun with afternoon shade or filtered sun is fine.
If your Japanese Maple is planted in all day direct sunlight, and year after year the leaves become scorched during the summer, chances are you have a variety that prefers some shade.
As for a long-term remedy for leaf scorch caused by over-exposure to sun, you might have to relocate your tree. The only other alternative would be to plant a larger tree to the west side of your Japanese maple that would provide shade during the afternoon hours.
Excessive heat. Most Japanese maples will tolerate the heat in USDA Zones 5 to 9a. In zones further south the summer heat is usually just too much for Japanese maples. That said, there are a few cultivars that have demonstrated good heat tolerance to zone 9b. Even in zones 5 to 9 there's not much you can do about an unusually hot summer, and, as previously mentioned, not much you can do about leaf scorch. Your tree will drop it's leaves in fall and fresh new leaves will emerge the following spring. Then say a prayer that next summer won't be quite so hot!
Water on the foliage. Water on the foliage of a Japanese maple, especially when the sun is shining during the hot afternoon hours of the day, can scald leaves. To avoid splashing water on the leaves water at the base of the tree or beneath the canopy. If you have a sprinkler system set it to run during the very early morning hours. Doing so will allow any water to dry from the leaves before the sun gets too hot.
Note: When your tree has leaf scorch, be careful not to over-water it. Leaves with leaf scorch are no longer drinking water. Just keep the soil moderately damp.
Chemical applications. Japanese Maples are very sensitive to many types of chemical insecticides and fungicides. Because they don't have any serious problems with insects or disease, chemical applications should be avoided. That said, if you must spray for insects, such as the Japanese Beetle, use only a mild solution of Sevin (Carbaryl) spray, or other insecticide listed for use on Japanese Maples. To reduce the possibility of leaf scorch, make sure to spray only in the early morning hours.
If you think your Japanese maple has a serious problem, and you're not sure if it's leaf scorch or some other disease, don't hesitate to consult with your local arborist, professional nurseryman, or local Extension service agent.
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