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IN THE GARDEN: Pruning a branch infected by "black knot" fungus is best method of control

by Janet B. Carson | July 16, 2022 at 1:31 a.m.
The galls on this branch of a black cherry tree are caused by a fungal disease called black knot. (Special to the Democrat-Gazette)


Q: Can you identify this for me? [The reader sent photos.] It was on two small trees I cut down.

A: Your black cherry tree had a case of black knot, a common fungal disease. When a tree is infected, black galls form on the branches and sometimes on the trunk. A few galls will not hurt a tree, but over time the tree could have hundreds of them, which can cause infected branches to be deformed. Pruning out an infected branch is the best method of control. Rarely are sprays recommended.

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Q: We have some hydrangeas that are 18 years old on the north side of the house. One of the bushes produces full, round blooms, and the others look like this. Is this a variant of normal or is this a problem? [The reader sent a photo.] The leaves seem unaffected.

A: Your hydrangea blooms are supposed to look like that. There are two flower forms on big leaf hydrangeas — the rounded ball-like blooms are called mophead blooms, while the flatter blooms are called lacecap blooms. The lacecap bloom has a center of tiny, clustered flowers that look almost like flower buds surrounded by a ring of showy florets, which give it a lacy appearance, thus the name lacecap. Different varieties will have different types of blooms.


Q: I have a really large, overgrown butterfly bush. It is blooming so well but it is taking up too much space in the garden. I did not cut it back this year when I was supposed to. If I cut it back now, will I kill it?

A: You won't kill a butterfly bush or buddleia by pruning now, but you will reduce the number of flowers it puts on. You have a few options. You can cut it sparingly now or thin it, cutting every other branch back. When those branches rebound and begin to bloom, cut the other half out. Or cut it back hard, water well and wait for fall for more flowers. The plant blooms on new growth, and as hot and dry as it is, rebound growth won't be quick. Next year, prune in late February.

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Q: Something is eating all my hibiscus leaves; I have a number of them that have been completely stripped. I have tried several types of bug spray and nothing seems to work. Can you tell me what insect is doing this and what will stop them? [The reader sent a photo.] I enjoy your column and have found it a great help several times. I really will appreciate any help you can give.

A: Your plant does have a lot of insect damage. From the appearance, I would guess a mallow sawfly is to blame. The larvae of this insect look like caterpillars, but if you look closely, they have six or more pairs of legs at the front of their body. These insects are more closely related to wasps, but they don't sting. The adult is a dark, winged insect with an orange thorax (area behind the head). She will lay eggs on the hibiscus, and the emerging larvae start feeding. They can turn the leaves into lace over time. In Arkansas, they can have several generations per year. Inspect the plants and dispose of any larvae you see. Sprays of products containing Spinosad will work, but use caution if there is a lot of pollinator insect activity on or around the plant.

Retired after 38 years with the University of Arkansas Cooperative Extension Service, Janet Carson ranks among Arkansas' best known horticulture experts. Her blog is at arkansasonline.com/planitjanet. Write to her at P.O. Box 2221, Little Rock, AR 72203 or email jcarson@arkansasonline.com



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