How to save a viburnum bush from white flies

The viburnum plant, also know as the snowball bush, ranges in size from 2 to 20 feet tall and usually serves in the landscape as screening, hedge, or focal point.

Garden columnist Dan Gill answers readers' questions each week. To send a question, email Gill at [email protected].

We have a row of viburnums in our backyard and need some help. They have a black residue (moldy appearance) on the leaves along with small, white flying insects. Could you please advise what we should do to treat? They are about 3 years old and just started to have this problem. — Aleisha and Ricky

This sounds like a heavy infestation of whiteflies. The adults (the white insects you see flying around) and the larvae (small disks on the back of the leaves) feed on the sap of the viburnums. As they feed, they excrete a sugary liquid called honeydew. This accumulates on the leaves and provides food for a black fungus, called sooty mold, that grows on the leaves. That’s the black stuff you are seeing. It is not attacking the viburnum — just growing on the honeydew produced by the whiteflies. It is the whiteflies we need to go after.

I’d recommend treating the shrubs with a systemic insecticidal drench such as dinotefuran (Greenlight Tree and Shrub Insect Control with Safari and other brands) or imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control and other brands).

In addition, it would help to spray the viburnums with a light horticultural oil like Year Round Spray Oil or All Seasons Spray Oil, being sure to get thoroughly under the leaves. Make two or three applications following label directions.

A two-prong approach using a systemic insecticide drench and oil spray will ensure you control this outbreak of whiteflies effectively. The sooty mold will not disappear right away, but it will weather off the leaves over time. The oil spray can help dislodge the sooty mold and make it weather off faster.

My neighbor's St. Augustine grass is growing into my centipede lawn. I want to stop this before it goes too far. What herbicide would you recommend to selectively kill the St. Augustine and not hurt the centipede grass? — Wayne

You can use the herbicide sethoxydim (Over the Top and other brands). This herbicide is toxic to St. Augustine and grassy weeds, but it is safe to use on centipedegrass and will not damage your lawn. Just spray the areas where St. Augustine is invading your lawn. If you are spraying near the property line, be very careful the spray does not drift over and damage your neighbor’s St. Augustine lawn.

Because my plumeria was so large and top-heavy, I planted it in the ground. Can I take a cutting and start a new plant before winter comes? How and when should I do this? Thanks. — Mary

I think you are wise to propagate your plumeria since you planted it in the ground. They are not reliably hardy outside in the winter and are often killed by freezes. Should that happen, the rooted cuttings can serve as replacements.

Plumerias are easy to propagate. We can root relatively large plumeria cuttings. Simply cut off the end of a branch 12 to 18 inches long. You may do this now. It will bleed a lot of milky sap but don't be concerned. Let the cutting air dry in the shade for a few days to seal the cut end, then plant it in a pot of potting soil. Bury the lower one-third to one-half of the cutting in the potting soil.

Place it in a shady area to root and water when the soil surface is dry. It will be rooted by winter and can be brought indoors. The cutting should be rooted in about six weeks, and at that time, you can give it some morning sun.

The one in the ground may survive the winter if temperatures are not too severe and you protect it with covers. But it’s very iffy.

Louisiana irises can be dug up and divided now.

THE GREAT DIVIDE: You may begin to dig and divide Louisiana irises, Easter lilies and calla lilies this month through September.

PLANT NOW: Here’s a list of vegetables that can be planted in your garden in August. Plant transplants of tomato, pepper, eggplant, and in late August broccoli, cauliflower and cabbage. Plant seeds of broccoli, cauliflower, Brussels sprouts, Chinese cabbage, cabbage, collards, mustard, turnips, squash and cucumbers. Plant seeds of bush snap beans and bush lima beans in late August. Sets (small bulbs) of shallots and bunching onions may also be planted this month.

CHECK POTS: After a summer of vigorous growth outside, some containerized plants may be pot-bound. Check and repot into larger containers if necessary. In addition, plants in pots sitting on an old brick surface, gravel or soil may grow roots out of the drainage holes and into the ground. Prevent this by lifting the pots occasionally, or boost them up on pot feet or pieces of brick.

OUT WITH THE OLD: When a vegetable crop is finished, promptly pull it up and throw it in your compost pile (if it is not heavily infested with fungus diseases). If you don’t want to plant immediately in the area, mulch it with 4 to 6 inches of leaves, grass clippings, pine straw or other materials you have on hand to prevent weeds. Old crops left in place look untidy, allow weeds to grow and may harbor insects or diseases.

Dan Gill is a retired consumer horticulture specialist with the LSU AgCenter. He hosts the “Garden Show” on WWL-AM Saturdays at 9 a.m. Email gardening questions to [email protected].

“Now ‘tis spring and the weeds are shallow rooted. Suffer them now and they’ll o’ergrow the garden.”

— William Shakespeare