Prune Rhododendrons Now To Ensure Blooms Next Year

Q: When do I prune rhododendrons? And how?

A: Fortunately, the time is now. Rhododendrons, like all spring-blooming shrubs, respond best to pruning during bloom or just after bloom. Pruning during bloom yields bunches of flowers to enjoy indoors or offer to friends, and serves the plant's needs.

Pruning now also protects next year's flower show. In mid- to late summer, the plants form new flower buds for bloom the following spring. Pruning too late, such as in fall or winter, removes these new buds.

Should you remove dead flowers when pruning? Gardeners often ask if faded blossoms reduce next year's bloom on rhododendrons. Rhododendrons can set bloom for the next year even if the old blooms are allowed to go to seed on the plant. Taking off the blooms before they seed improves the overall appearance of the shrub so many gardeners do this simply for the aesthetic improvement it provides.

If you have to choose, it's more important to dead-head the larger-leaved, large flowered types. Cultivars such as `Anna Rose Whitney', `Blue Peter', `Bow Bells', `Gomer Waterer', `Jean Marie de Montague,' and `Virginia Richards' all benefit from dead-heading. These cultivars are classified as elepidote, one botanical description for rhododendrons with large leaves, lacking scales.

The other broad classification, lepidote types, such as R. augustinii, `Blue Diamond,' and `PJM' have smaller leaves and are scaly. In general, the lepidote types don't need dead-heading while the elepidote sorts do. In my garden, R. augustinii seems to drop its petals without any need for dead-heading. You can judge by the size of the flower truss.

Look carefully at the plants when snapping off large, old flower trusses. The new growth emerges quite close to the old bloom stalk. So that you don't break off new growth along with old seed heads, snap the old flower head off above the new leaves. While dead-heading, prune out all dead or damaged branches.

Shaping helps to keep rhododendron branches from becoming floppy and lanky. Prune back to a whorl of leaves or to an obvious growth point where new green growth is emerging. The plant will fill in rapidly if properly pruned. Wobbly, lanky branches may indicate that the area has become too shady. Rhododendrons appreciate filtered shade, but will not set bloom properly in dark shade.

When shortening a branch, don't chop it into a stub at a spot where there is no green growth bud. The buds far down on the stems are small and easy to miss; they are rounded and small, slightly larger than a heavy pencil point. If the stem is pruned back to one of the growth buds, it will put out a new whorl of leaves. You can often feel potential growth buds by running your hand up a bare rhododendron branch and sensing the little nubs.

If a very large plant must be rejuvenated or shortened, take out 1/3 of the wood each year. Thin the plant by selecting a branch and sawing or clipping it out at the base where it meets the trunk. Often a larger rhododendron will benefit by having two or three large branches removed, and other branches shortened slightly.

Rhododendrons need good cultural care after pruning, so don't neglect summer watering. Deep soaking of the roots at least twice a month during the hottest, driest periods helps them. More frequent watering will be needed for rhododendrons that are newly installed or have been transplanted. Grouping plants with similar water requirements helps to conserve summer water.

Extra watering also may be needed if the plant is in an exposed, hot position, particularly where it gets bright western sun in the afternoon. Plants that are located like this may show signs of scorching or stress even when they receive adequate water. If they do, move them to a more protected location when the plant goes dormant in winter.

Another reason for attention to rhododendrons during summer is that next year's bloom depends on the summer care and the supply of moisture the plant receives.

Mulch is beneficial for retaining summer water, but don't pile deep mulches over the roots. Rhododendrons are shallow-rooted, and need oxygen at the soil surface level. An inch of mulch over the roots would be sufficient. Don't scuffle under the plants with hoes or weeding tools, either. The surface roots are easily damaged. If moving or planting a new rhododendron, be sure the root ball stays at the same level it was in the container or in the previous location.

Fertilize moderately. One fertilization after bloom, with an acid-based fertilizer formulated for rhododendrons, will be adequate for summer.

Gardening runs Sunday in Home/Real Estate. It is prepared by Mary Robson, area horticulture agent, Washington State University/King County Cooperative Extension.