Pruning Rhododendron - Knowledgebase Question

E. Northport, NY
Avatar for Duganmmse
Question by Duganmmse
December 10, 1999
I would like you to compare the proper techniques for pruning Rhododendron catawbiense vs. Rhododendron maximum. Please include the reasons and season or seasons for their pruning.

Answer from NGA
December 10, 1999
The genus Rhododendron is one of the largest and most diverse genera in the plant kingdom. It consists of over 800 species distributed throughout the world. The name derives from the Greek rhodon, meaning "rose", and dendron, meaning "tree." Our native North American species include R. maximum, the rosebay, R. catawbiense, which provided European hybridizers with the hardy stock used to develop the now ubiquitous "ironclads" of the turn of the century, as well as R. carolinianum and R. minus, the Carolina Rhododendron, and many species of azaleas (which used to be classified into a separate genus, but are now considered to be part of the genus Rhododendron as well).

One bit of annual maintenance that will help your rhododendrons maintain flowering is called deadheading. You should remove the spent flower trusses after the plant has finished flowering. A great deal of the plant's energy can be diverted toward seed production, and by deadheading you divert that energy back into vegetative growth. Once the flower heads have dried a bit, you should carefully break them off at the base, being careful not to damage the small growth buds surrounding the base. Plants which are not deadheaded tend to flower well every other year, while those that are deadheaded should flower well every year.

Normally very little pruning is needed. If a plant grows out over a walk or needs to be restricted for some reason, it may be pruned back moderately without fear that the plant as a whole will be damaged. It is often possible to do this pruning during the blooming season and have flowers for the house. Light maintenance pruning at the time "dead-heading" is done, can help keep the plant in shape. Light to moderate pruning done at the time the plant is flowering or immediately thereafter will not affect flower bud formation for the following year. Sometimes, as the plant starts to grow, only one growth bud at the end of a stem will begin to expand. If this single bud is broken out just as it starts to enlarge, it will cause two or more dormant buds at the end of the stem to expand, making a bushier plant. Again, this will not affect bud set for the coming year. Old leggy plants may need pruning, but often these are better replaced with smaller newer varieties. Old plants, however, can be cut back severely and still recover, although it may be a while before they bloom again.

Since both plants in question are species rhododendrons, the culture is the same.

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