Rhodies Vulnerable To The Root Weevil

TODAY WE CONTINUE "The 10 Most Wanted" list of common pests and problems that assail Puget Sound-area gardeners during the garden season. Every Friday and Sunday through the Green Gardening Program's June 26-27 free tour of area gardens, we'll discuss the least-toxic control of another of "The 10 Most Wanted." To obtain a free tour map, call 547-7561.

Pest No. 6: Root weevils

By late May and early June the leaves of many cherished garden rhododendrons may have notched all around and look as if they were cut with pinking shears. The chewing damage is done by one of several species of root weevil.

In general, the damage probably doesn't endanger the plant's survival. But if the plant is in a container, it may be severely damaged or even killed. Since rhododendrons spend about 11 months of the year out of bloom, it's desirable to have the leaves whole.

Hand-picking adult weevils is one way to control them. The insects hide in ground litter during the day and feed at night. Like many insects, they prefer young tender leaves and will be chewing primarily on the newly developing foliage. Go out with a flashlight and knock them into a can of water. Getting rid of the mature insects in May and June will help control future infestations.

A biological control, a nematode, is currently available for use as a soil drench to control immature root weevils; it's being tried with some success here.

Some gardeners have had success with wrapping a sticky band of material like Tanglefoot, which traps the insect so it can't crawl higher, around the lower trunk of the rhododendron. That won't work if there are branches brushing the ground or other rhododendrons close by that can be used as a pathway.

A great line of defense is to choose and plant rhododendrons that are not as susceptible to chewing. Both leaf shape and the nasty-tasting chemicals produced in the leaves of certain rhododendrons resist weevil damage. Some good choices are "PJM" (pinky-purple, 100 percent resistant), "Cilpenense" (white with pink tinge), and `Virginia Richards' (a deep rose-throated cream flower).

Participants in the Green Gardening Program tour will receive a list of "Trouble Free Plants." Or mail $1 to King County Cooperative Extension, 612 Smith Tower, 506 Second Avenue, Seattle, WA 98104, requesting EB0970 "Root Weevil Control on Rhododendrons."

Q: How can I prune back my rhododendrons?

A: Extensive pruning of rhododendrons is best carried out in late winter or early spring. Less radical pruning is usually carried out after bloom so as not to lose the flowers. Rhodies can be pruned essentially like any other plant. Always cut back to a side branch, a whorl of leaves or just in front of where a whorl of leaves used to be. Dormant buds are often in evidence in the bark at such locations. Deadheading spent flower clusters and pinching our the center vegetative buds before growth starts are also forms of rhododendron pruning. This latter procedure can help keep rhodies bushy and more compact.

Q: My new grass seedlings are coming up red and burnt. Why?

A: Poor fertility and drought might be the cause of your grass problem, but without a good deal more information, it may be impossible to zero in on exactly what's happening. A fungus disease called red thread causes small pinkish or reddish growths to grow from the tips of the grass blades and eventually the plants may turn brown and burnt looking. WSU Extension plant pathologists and turf specialists recommend fertilizing your lawn adequately with nitrogen using a 3:12 ratio formulation (21-7-14, 15-5-10, or 12-4-8). Mow frequently, remove the clippings and throw them in the garbage or your yard waste program. Daconil 2787 (Funginil) also is labeled for control of this common springtime lawn disease.

Have a gardening question? You can reach us 24 hours a day by dialing 464-8470. Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant, and volunteer Master Gardeners. If you need more immediate assistance, call a Master Gardener, 296-3440, weekdays 10 a.m.-4 p.m.