Browning leaves can form on a Japanese maple tree at any stage of its life, from saplings to well-established specimens. Several factors like water and fertilizer levels, pesticides, or other environmental factors can lead to or intensify leaf browning. This condition of browning leaves is called leaf scorch. Read on to take a closer look at this problem, including details about the causes, what to look for, and some possible remedies.
Leaf Scorch and Its Causes
Japanese maple trees in their native habitat often live as understory trees that often have an added level of protection by towering taller trees that shelter them against the beating sun, high winds, and extreme weather. It is best when these trees are situated in a place that gets some shade (especially during the worst of the heat in the afternoon).
Leaf scorch looks like browning or greying of the leaf margins. Leaves may appear dead and brittle. You may also notice yellowing or darkening of the areas between the main leaf veins. Leaf scorch is most often caused by over-exposure to the sun.
This leaf condition can vary in terms of severity. In less severe cases, the leaves will remain on their branches, and just a few leaves in the upper branches will brown up and only along the edges of the leaves. As the condition worsens, browning extends throughout the leaf, affecting more of the leaves. In severe cases, the leaves become brittle and eventually drop off, and the tree may die back or appear to enter dormancy in summer or early fall.
Several factors can aggravate or worsen leaf scorch, including:
Too Much Sun
Most Japanese maple trees prefer dappled sun or some shade during the day. Bright, direct sunlight every day, all day, is usually too much for a Japanese maple tree. If your tree is suffering from leaf scorch every year, you might consider digging up and relocating it to a spot that gets some shade protection or planting a taller tree beside it that will eventually shelter it from the sun.
Lack of Water
If your Japanese maple tree is underwatered, then your tree might get a worse case of leaf scorch. You can mitigate leaf scorch by making sure to water them adequately during dry spells. A deep watering done less frequently is better than more frequent, shallow watering. The goal is to have the soil evenly moist, not soggy or overwatered. There's no such thing as a foolproof watering schedule since the environment is constantly shifting, but you can check your soil in between waterings to ensure it's not drying out.
Chemical Burn From Fertilizer or Pesticides
Following directions carefully when using chemical fertilizers or pesticides is crucial since they can cause leaf scorch. Generally, the proper time to fertilize Japanese maple trees is late winter or early spring (using a slow-release fertilizer). Similarly, you should only apply pesticides during cool, dry conditions below 85 degrees Fahrenheit.
Damage from excessive fertilizer application generally appears as the browning of leaf edges. Soluble salts from fertilizers also can pull moisture out of root tissues and cause wilting, marginal yellowing, and stunting.
To prevent damage from fertilizers, avoid feeding during the hottest summer months or enrich the soil with compost as an alternate way to feed the tree. Compost is safe to apply at any time and will not harm the plant. As for pesticides, only use them as a last resort and according to package instructions.
Exposure to Intense Environmental Factors
Harsh winters can damage, injure, or destroy roots. The salt from winter snow plows can desiccate roots, and intense winter weather and cold snaps can also kill roots that are not hardy enough. High winds can also draw moisture out of plant tissues drying them out. To protect your tree, you can do a couple of things:
- Proper location: When planting a Japanese maple tree, select a sheltered spot so it will not be exposed to high winds.
- Mulch protection: These trees benefit from mulch applied around the tree's base to help protect its root system.
Leaf Scorch Prevention
Knowing the factors that lead to leaf scorch can help prevent it. Remember, a tree that has leaf scorch is not a death sentence. It may appear worrisome that your beloved specimen has shed its leaves after a hot, dry spell in the summer and now stands naked, but do not take it as a sign that it is dead.
Leaf scorch is the tree's defense mechanism. Your tree has secondary buds that will produce a second set of leaves. However, when in desperation, do not reach for your fertilizer since you now know it can make the situation worse.
If you find that your Japanese maple tree still gets brown leaves in the summer, even after correcting the situation, consider experimenting with cultivars that are known to be more sun-tolerant, such as 'Crimson Queen' and 'Bloodgood.'
Leaf Scorch. Colorado State University Extension
Fertilizer or Pesticide Burn on Vegetable Leaves. University of Maryland Extension.