Dead Heading Rhododendrons can be a Monumental Task

Question:  Root weevils are taking a real “bite” out of our rhododendron plants.  What is the current recommendation from WSU on the best way to control these insects?

WSU entomologists recommend the insecticide acephate, which is available in most nurseries and garden stores under the trade name of Orthene.   When used according to labeled directions, Orthene effectively controls Black Vine Weevil, Strawberry Root Weevil and Rough Strawberry Root Weevil on ornamentals. Orthene is not registered for use on edible food crops like strawberries. 

Although adult weevils cause a characteristic notching in rhodie leaves, this damage is relatively minor compared to damage done by larvae to the plant roots.  Larvae of all root weevil species are quite similar in appearance and habits. They are C shaped, legless and white with tan heads, up to half an inch in size. The larvae feed throughout the winter and spring on root systems resulting in plants that are stunted, grow poorly and may die.

In April through June, the larvae pupate and adults emerge. The adults are night feeders that remain in the soil or in debris at the base of the plant during the day, then climb up to feed on the leaves at night.  Adult weevils cannot fly, but walk or are carried from one location to another.  Also, all adults are females capable of laying eggs. Adults are slow moving and should not be confused with swifter, predacious ground beetles.

Eggs are laid in clusters in or on the soil from June to September. The eggs hatch, and the larvae immediately wriggle though the soil to begin feeding.  There is one generation per year. 

In order to effectively control weevils, you will need to begin Orthene applications when you first observe notching of the leaves. It is easy to confirm the presence of weevils by inspecting the plant foliage on warm, still evenings. Or you can look for adult weevils in the debris at the base of the plant. Laying a small piece of cardboard at the base of the plant provides a refuge that you can check easily in the morning. 

Options for the control of weevils on rhododendrons include beneficial nematodes, which attack the larvae. Unfortunately, they have not proven to be very effective in our cold coastal soils. Another option for those not wanting to use an insecticide involves putting a sticky, gummy substance like Tanglefoot on the stems of plants to trap adults.

Question:  We have several very large rhodies that have outgrown their space and become spindly.  Would it be okay to prune them?  When?

Large-leaved rhododendrons may be pruned anytime without harming the plant, but there are times that are more expedient. Most rhododendron plants retain their leaves two years. This means that on young plants there will be four rosettes of leaves and fewer on older plants. It is permissible to prune back to any of these rosettes, depending on how drastically the plant needs to be pruned.  Always make the cut about a quarter of an inch above the rosette. The flush of growth will burst out from the auxiliary buds of the rosette.