A rotten knot

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

A: Your black cherry tree had black knot, a common fungal disease. When a tree is infected, black gall forms on the branches and sometimes on the trunk. A few galls do not harm the tree, but over time the tree may have hundreds, which will cause the infected branches to be damaged. Cutting the infected branch is the best control method. Frequent sprays are not recommended.

The changing flowers of hydrangeas

Q: We have some 18-year-old hydrangeas on the north side of the house. One of the bushes grows full, round flowers, and the others look like this. Is this a normal difference or is this a problem? [The reader sent a photo.] The leaves seem unharmed.

A: This is what your hydrangea flowers look like. There are two flower shapes on large-leaf hydrangeas — the round, ball-shaped flowers are called moped flowers, and the flat flowers are called lacecap flowers. The center of the lace cap flower is surrounded by a ring of beautiful flower buds that look like small and concentrated flowers, which give it a soft appearance, which is why it is called lace. Different species have different types of flowers.

When to prune a butterfly bush

Q: I have a really big, overgrown butterfly bush. It is blooming well but it is taking up too much space in the garden. I didn’t cut it this year when I should have. If I cut him now, will I kill him?

A: By cutting it now, you won’t kill the butterfly bush or budalia, but you will reduce the number of flowers it puts on. You have a few options. Now you can cut it a little bit or make it thinner, by cutting back each branch. When these branches grow again and start to bloom, cut the remaining half. Or cut back hard, water well and wait for fall for more flowers. The plant blooms on new growth, and even if it is fresh and dry, the recovery growth will not be fast. Pruning at the end of February next year.

Stump reader of hibiscus-eating insects

Q: Something is eating all my hibiscus leaves; I have a number of them that are completely ripped off. I have tried many types of bug spray and nothing seems to work. Tell me what insect does this and what stops it? [The reader sent a photo.] I enjoy your column and often find it very helpful. I would really appreciate any help you can give.

A: Your plant has a lot of insect damage. From the looks of it, I suspect the mallow sawfly is the culprit. The larvae of this insect look like caterpillars, but if you look closely, they have six or more pairs of legs on the front of their body. These insects are closely related to wasps but do not sting. The adult is an insect with black wings and an orange chest (the area behind the head). She lays eggs on the hibiscus, and the emerging larvae begin to feed. Over time, you can turn the leaves into lace. In Arkansas, they can have several generations per year. Check the plants and remove any larvae you see. Products containing spinosad can be sprayed, but use with caution if there is a lot of pollinating insect activity on or around the plant.

Janet Carson, who retired after 38 years with the Arkansas Cooperative Extension Service, is among Arkansas’ most prominent horticulturists. Her blog is at arkansasonline.com/planitjanet. Write or email her at PO Box 2221, Little Rock, AR 72203. [email protected]