abc logo Shaping and Pruning Rhododendrons
Rhododendron ABC's #10

Previous discussions emphasized conditions and treatments that cause rhododendrons to respond well to their environments and grow vigorously. In some cases, vigorous growth can cause plants to outgrow their positions in the garden and cause problems. Overgrown plants can be especially troublesome for homes with limited gardening space or for someone who buys or inherits a garden filled with old (possibly or probably straggly) plants. In my years of association with rhododendron growing and culture, I am often approached for advice about rhododendron problems. What to do about overgrown plants is certainly one of the most frequently asked questions.

So just what should we do to alleviate this problem? The answer may well be--dig out the old, discard and replant with younger stock. In many cases, this is not necessary and, in fact, may result in a tragic loss of a valuable specimen. The answer is more apt to be, get out the pruning equipment!

Many people have the idea that rhododendrons should not be pruned. In fact, most can and respond well to judicious use of pruning saws and shears. It is important to make the pruning cut just above the leaf whorls so that branch stubs do not attract disease or insects. Even large plants can be cut back severely if the cuts are made at the nodes where leaves are or if there am adventitious buds (buds that are dormant as small bumps on the stem) below where the pruning cut is to be made. These buds spring into growth after pruning and regenerate new stems.

A few rhododendrons lack such buds and thus do not respond well to pruning plants related to the Thompsonii group are notorious in this respect. If such plants become too large, they probably must, alas, be sacrificed.

As for timing, it is probably best to prune just after the plant has bloomed, around late May or June. In many cases there is still time for the new growth to set flower buds for the next blooming season. Severe pruning often results in the loss of at least one blooming season and can be carried out just about any time of the year. The bloom loss is compensated generally by the ready regeneration of a nicely shaped, smaller plant that takes up far less volume than an unsightly and overgrown unpruned specimen.

It is well to keep in mind that the volume of a sphere (the general shape of a rhododendron bush) is directly proportionate to the cube of its radius. Thus if a plant is cut back to one half of its apparent size, the plant volume is reduced to 1/8 of what it was before pruning.

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Dave Goheen