Why you probably don't have to worry much about growths on tree leaves

"Fielding Questions" columnist Don Kinzler also answers questions about when to plant lilacs and how long it takes for apple trees to start to produce fruit.

Maple leaf gall June 11, 2022.jpg
A reader wonders what these growths are on maple leaves.
Contributed / Special to The Forum
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Q: What are these growths or galls on our maple leaves? — Steve R.

A: The round, protruding bumplike growths on tree leaves are called galls, and they inspire frequent questions every year. Tree types commonly affected include maple, hackberry, poplar, oak and linden. The galls are caused by several types of insects and mites.

Galls are produced by tree leaves in response to early-season feeding from these pests. Then, the insect or mite uses the gall as protection — a sort of home — while it completes its life cycle. The time to use a pesticide to prevent the galls was when leaves were opening and the pests were still exposed.

Once the galls form, the insects or mites are shielded inside, so applying a pesticide when you see the bumps has little or no effect at controlling what’s there, or for preventing new infestations. The good news is that damage is minimal, if any, and mostly cosmetic. Trees can lose up to 25% of their leaf tissue without causing any major damage to the trees.

Many gall-forming pests survive winter in the rough bark and crevices of the trunk and larger branches. Populations may be reduced by applying horticultural oil, available at garden centers, liberally to the trunk and branches in early spring just as the tree buds are beginning to open.


Catch up with the latest "Fielding Questions" and "Growing Together" columns by Don Kinzler by clicking here.

Q: Is it too late to plant lilacs? I’d like to plant about 10 as a natural fence in the lakes area. — Terry A.

A: It’s not too late to plant lilacs. In fact, June is a great month for planting nearly all trees, shrubs and perennial flowers. If the garden center at which you’re shopping sells bareroot lilacs, they should be planted relatively soon. Potted lilacs, and other woody plants in pots, have a longer shelf life.

Q: I planted two apple trees last year and this year they were covered with flowers. I noticed on one of the trees that some of the white flowers turned brown. Is it from too much watering? Also, how many years does it take before you see apples? — David C.

A: The average time from planting until decent fruit bearing on apple trees is five to seven years, depending on variety. Some begin bearing a little earlier, while some varieties take a little longer. Trees will sometimes bear a few scattered fruits earlier.

Soil that stayed too wet this spring could be the cause of the flowers turning brown. A brush with frost or lack of pollinating bees can cause flower failure also.

Very young apple trees sometimes produce flowers that the tree doesn't yet have enough energy to provide for, so the blossoms turn brown and wither away. That's OK, because a young tree shouldn't be bearing a heavy crop of apples before it’s mature enough anyway.

In many commercial orchards, flowers that appear on young trees are manually removed until the tree reaches the age at which it should be bearing. Bearing fruit at a too-young age can divert energy into fruit production at the expense of tree growth. At a young age, the tree's energy should go into branch growth, creating a solid, well-grown tree.

So if the blossoms on a young tree turn brown and wither away, that's actually desirable until it reaches its normal bearing age. We’re usually so anxious for the first apples on a new tree that we don’t always do what’s best for the tree long-term.


If you have a gardening or lawn care question, email Don Kinzler, NDSU Extension-Cass County, at Questions with broad appeal may be published, so please include your name, city and state for appropriate advice.

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