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Fielding Questions: Leaf galls appearing on area maples

Don Kinzler, gardening columnist 1 / 2
Maple leaf gall. Special to The Forum2 / 2

Q: Our Autumn Blaze Maple tree has greenish-red bumps on many of its leaves. Are these insect eggs that will hatch and eat the dickens out of the leaves, or is it a disease? What treatment is recommended? — Bruce Johannes, Fargo

A: The bumps are maple leaf galls, caused by the feeding of tiny mites. Adult mites become active as leaf buds open in spring. As they feed on developing leaves, hormones cause abnormal plant cell growth that encloses the mites inside a gall where they're protected. Adult mites lay eggs within the gall and die. The eggs hatch and feed, turn into adults, and exit the gall. Then they head for the trunk and branches to overwinter in rough areas of bark.

Although this sounds scary and appears dramatic, the leaf galls cause little or no health problems to trees. Since it's mostly cosmetic, control isn't usually recommended. Research indicates that there's no adverse effect to the tree, unless possibly over 25 percent of each leaf is covered with closely spaced galls.

Control is difficult after the galls form because the mites are nestled safely inside, well-protected from insect spray applications. If gall reduction is desired for future seasons, the best time to treat a tree is early spring before leaf buds swell, as adults are leaving their overwintering sites on the bark and heading for new leaf growth.

Apply horticultural oil, available at garden centers, to the bark of trunk and branches before spring bud-break. This kills mites as they are heading for new growth after exiting their winter home in bark crevices.

Q: I always use fresh-cut grass clippings as mulch, but your past article recommends using dry clipping. Why dry? — Mark Dubord, Fargo

A: The reason for letting grass clippings dry out somewhat is in case they're applied in a thick mulch layer. Fresh green clippings can "heat" if layered very thickly. Letting them dry down a bit lessens the tendency. If they're put on the garden in a thin layer, they dry down quickly enough to avoid the problem. It's a thick layer that's a concern if used fresh.

Since it's easier for grass clippings to dry down if spread out in a thin layer, it can be done right in the garden, adding more as the previous layer dries.

Q: My tropical hibiscus plant bloomed beautifully last summer but quit over the winter. I repotted it this spring but it's still not blooming and there are white spots on the leaves. My patio is on the north so I placed it on the west side of my privacy wall where it gets full sun later in the day. Any suggestions? — Marlys Schenck, Moorhead

A: Hibiscus plants need plenty of winter sunshine indoors to continue bloom. The increased sunshine you've given it outdoors will help. Repotting plus water-soluble fertilizer should coax flowering. If the plant has grown a bit woody, pruning helps stimulate new growth. The white spots could be powdery mildew, which can be prevented from spreading by spraying with all-purpose fungicide. Or they could be caused by increased light, as unaccustomed leaves are exposed to more sunlight until they adjust. If the white spots are cottony, mealybugs would be the suspect. Feel free to send me a photo of the leaves, if you like. Best wishes for a well-blooming hibiscus.

If you have a gardening or lawn care question, email Don Kinzler at All questions will be answered, and those with broad appeal may be published, so please include your name, city and state for appropriate advice.