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WHAT IS WRONG WITH MY RHODODENDRON?


Rhododendron 'Schneekrone'
Picture by S & J Perkins
Rhododendron 'Schneekrone' in Salem, NH


WHAT IS WRONG WITH MY RHODODENDRON?

This spring has brought a flood of inquiries like this, all relating to a widespread and mysterious affliction. Our local landscape seems to be littered with brown skeletons of once-thriving evergreen rhododendrons.

Symptoms are brown and drooping leaves, either on the whole plant or on certain branches, and a shriveled, dried appearance of the newer twigs.

Why?

The consensus among experienced growers in our Massachusetts Chapter is that there is probably no single cause, but rather a combination of factors affecting many different plants in varying degrees. Here are the possible reasons that have been put forth:

1. Desiccation. When the ground is frozen, the plant cannot obtain water, yet is still transpiring water through its leaves. If the situation persists long enough, the leaves simply dry up and turn brown.

2. Sun. The desiccation problem is exacerbated if the plant is in a sunny location, and especially in late winter when the sun is higher in the sky.

3. Reflected sun. The desiccation problem is also exacerbated when the ground is snow-covered, as it was for most of this past winter, with reflected sunlight off the snow doubling the drying effect of direct sunlight.

4. Drought. The drought of the summers of 1998 and 1999 were stressful for plants that were not adequately irrigated, and loss of portions of the root system may have occurred. Thus, even in unfrozen ground these plants may have been unable to provide sufficient water to their leaves to prevent desiccation.

5. Breakage. Extensive breakage of limbs occurred from the accumulated weight of snow, especially during the snowstorms of February 5 and March 6. But breakage is not always obvious. Sometimes a branch can be severely bent to the extent that the bark and/or cambium layer is ruptured, reducing the flow of moisture to the leaves, yet the break is not noticeable.

6. Genetic adaptation. Some plants are simply better able than others to cope with the kinds of stress inflicted on them this past winter. One of the strangest aspects of the problem is the widespread damage sustained by supposedly reliable old "ironclad" cultivars like 'Roseum Elegans', 'Caractacus' and 'Lee's Best Purple'. One wonders how they achieved their hardiness ratings of -25F or -20F. Yet cultivars usually rated as too tender for our climate, or only marginally hardy, such as 'Butter' and 'Golden Star' survived unscathed.

7. Insect damage. A plant, or possibly a single branch, weakened by borer or weevil activity during the previous season may be unable to handle the additional stress imposed by severe winter conditions.

8. Rodent damage. The persistent snow cover of the past winter provided ideal conditions for rodent activity at ground level. In the past I have lost plants due to girdling of the bark by these pests.

So what can be done at this point to help insure the survival of an affected plant, or at least to improve its appearance? The first impulse of many gardeners is to seize the pruning shears or saw and remove the unsightly branches. But in so doing they may be removing perfectly sound parts of the plant. Even if the older leaves are brown, the wood may be alive. One way to check is to use a sharp knife to remove a sliver of bark on an affected branch. If the wood underneath is green, the branch is still sound and will probably put forth new growth and leaves. In any case, it's best to wait until mid-June, or even early July, before doing any drastic removal. If the plant or branch has not leafed out by then, it's safe to count it as a loss.


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