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Bumps on the Leaves of Maple Trees — The Shocking Truth!

Why are there bumps on the leaves of maple trees? Maple trees are considered to have some of the most gorgeous leaves out there.

Their broad surface area, unusual shape, and gorgeous autumnal coloring have made them an icon of North American fall. 

Maple leaves have come to represent everything from the nation of Canada to the unique, sticky sweetness of maple syrup –– and indeed, many people with maples on their land produce maple syrup

The trees themselves are also very regal. An old maple can reach up to 148 feet and its crown makes quite an impression.

If you have a maple in your yard, its gorgeous foliage will surely provide you with summer shade for decades to come. 

However, sometimes the usually sleek maple leaves develop bumps. This can be alarming at first, especially when those bumps are pink or black in color.

Don’t worry though; there is a simple explanation for these leaf growths, and they are not harmful to the health of the tree.  

 

Bumps on the Leaves of Maple Trees 

Bumps on maple leaves are called plant galls. They occur when eriophyid mites feed on young, unfurling leaves, emitting chemicals stimulating cell growth when they react with the leaves’ hormones. These bumps aren’t harmful and only need to be treated by snipping off the affected leaves.

Bumps on the Leaves of Maple Trees — The Shocking Truth! 1
Bumps on the Leaves of Maple Trees

The long and short of it is that while the brightly colored bumps that develop on the leaves of your maple tree in the late spring and early summer aren’t necessarily the most beautiful addition to your local foliage, they aren’t actually harmful. 

Galls may be cosmetically unattractive, but they aren’t detrimental to the tree’s health.

You won’t wake up one day worrying about saving a dying maple tree because of plant galls. 

 

How to Identify Plant Galls

If your maple tree’s leaves have started growing what looks like warts, you would be forgiven for being a little alarmed at first. 

In addition to leaves, galls might also show up on the twigs, roots, and buds of trees.

Though the galls typically affect the maple leaves, they can also appear on the twigs, roots, and tree buds
Though the galls typically affect the maple leaves, they can also appear on the twigs, roots, and tree buds

They can vary in color too, and can present as red, pink, green, yellow, or black –– or as some combination of these!

If you see bumps appearing on your maple tree’s leaves but aren’t quite sure what it is, look for the following telltale signs of bladder galls to determine whether that is what the bumps are.  

Maple bladder galls are miniscule and stop growing at about 0.125 inches. They grow on the tops of maple leaves. 

When they first appear, the galls appear yellow in color or even light green. As time goes on, they become pink and then red. Older galls turn black. 

Initially, the galls appear yellow, then they turn pink, red, then eventually black as time passes
Initially, the galls appear yellow, then they turn pink, red, then eventually black as time passes

If these are the symptoms your tree is displaying, it may be time to accept that it has a mite infestation. 

If these criteria don’t describe your problem at all, you’ll need to look into other conditions that affect maples until you find the one that best fits what is happening to your tree. 

For example, if what you have are black spots on maple leaves, your tree likely has tar spot. Treating tar spot requires an altogether different approach to dealing with plant galls. 

Black spots on the maple tree leaves are not plant galls but an infection called tar spot
Black spots on the maple tree leaves are not plant galls but an infection called tar spot

 

What Causes Plant Galls on Maple Tree Leaves

Plant galls are caused by Maple Gall Mites (Eriophyidae), which get to work as soon as the winter thaw is over.

They gorge on leaf tissue and the chemicals they emit while doing so react with the maple leaf hormones to create unusual-looking growths. 

When the leaves have unfurled in the spring, the mites use the galls they have made to lay their eggs inside.

The galls on the maple leaves are used by the Eriophyidae to lay their eggs into
The galls on the maple leaves are used by the Eriophyidae to lay their eggs into

Then, they die. When their orange-colored offspring break out of the galls a few weeks later, the damage to the leaves has been done. 

Mite infestations can be severe or light. It is not uncommon for one tree in a grove to be affected worse than the others. 

In the most severe cases, leaves may become deformed and fall off the tree, but this is very rare. 

 

How to Treat Bumps on Maple Tree Leaves

As much as you might dislike the look of the mite galls on your maple leaves, most arborists would not recommend having the problem controlled. 

The reality is that the problem will not affect the health of your tree unless it is a very, very young one. 

In the case of young, newly planted maples, it is worth it to remove affected leaves by hand.

When the Eriophyidae affect the young leaves of the maple tree, the best way of treating it is to remove the leaves manually
When the Eriophyidae affect the young leaves of the maple tree, the best way of treating it is to remove the leaves manually

This is because younger trees have fewer leaves and so having their surface area covered by something that reduces their ability to photosynthesize causes stress to the organism. 

By removing heavily affected foliage, you can reduce the size of the mite population on the tree.

If possible, dispose of the leaves by burning them or disposing of them in a trash bag so that the mite population does not spread to neighboring trees. 

 

Frequently Asked Questions about Bumps on Maple Tree Leaves

How do I rid my maple tree leaves of the bumps appearing on their surfaces?

If you want to get rid of the bumps even though they’re a cosmetic problem, cut off the most affected leaves. You can also release a gall mite predator, such as ladybugs, onto the tree. If the infestation is really bad, spray your leaves with a diazinon-based pesticide that kills spider mites. 

 

Should I hire a professional to examine the bumps on my maple tree leaves?

Hiring a professional arborist to come examine a maple tree with plant galls isn’t a requirement. Once you have diagnosed the problem using this guide, decide whether you can live with the cosmetic imperfection. If it bothers you too much, you can take the steps described in the previous answer. 


Author Bio

Daniel Iseli

Taking care of houseplants and gardening are my greatest passions. I am transforming my apartment into an urban jungle and am growing veggies in my indoor and outdoor garden year-round.