Tips for Beginners: Pruning Large-Leaved Rhododendrons
Reprinted from the Rhodireview, Vol. III, No. 3, 1991, and from the Baldsiefen Nursery catalog
Any rhododendron is suitable for woodland or informal use where a reasonable amount of space is available. Here it is only necessary to arrange the planting so that the robust growers do not over-crowd those of relatively restricted growth. It is around the foundation of the home, spots of limited area or the rockery that the choice is limited to the smaller-leaved and dwarfed types. Wise selection in the beginning will eliminate the need for most of the pruning. There are, however, times when pruning is justified and essential.
Large-leaved rhododendrons are the most difficult to prune but only for the lack of understanding. These plants put out their first growth in a flush that terminates with a rosette of leaves. Generally in the colder areas of the East and North the flush varies from 4 to 10 inches but more often around 6 inches.
On young plants, two flushes per year are the rule and often the second flush a flower bud. On older plants that set flowers freely, it is the rule that one flush is made followed by a flower bud. This latter is due to both a natural reduction in vitality after the juvenile stage is passed and the use of tremendous energy in blooming which delays the commencement of growth.
Large-leaved rhododendrons may be pruned anytime without harming the plant, but there are times that are more expedient. These plants, with few exceptions, 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 it is desired to prune. 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.
The most favorable time to prune when there are no flower buds is in the early spring just before growth begins. If done at this time, the growth cycle is least disturbed, and bud set the same season will be uninhibited. In so pruning, it is important to cut back every terminal or watch when growth begins. The terminals left uncut commence growth earlier usually with a long single shoot. These single shoots should be pinched off when the new growing tips are about an inch long. This will encourage multiple branching from that terminal. This step is important in shaping or reshaping a plant. Cutting back develops a heavily branched, sturdy shrub. If this is done when the plant is about 3 feet or less, it can eliminate future need for pruning.
Pruning old rhododendrons presents a different problem. When old rhododendrons become unthrifty in appearance, the cells in the vascular bundles have become hardened or plugged and no longer function normally. Any revitalizing to be accomplished must be done on new wood, supplied by drastic pruning. This is best done in the spring or early summer but spring preferred.1
One method is to cut the plant down to several inches above the ground. This is more successful with those having multiple stems coming up from the ground. In hybrids, which often have one stem, they may sometimes die from this. A less drastic means is to reach down into the shrubs and cut out about one-third of the old wood. Do this for a period of about three years. In this way, there remains sufficient leaf surface to supply nutrients for reestablishment of new shrubs.
Bibliography of Articles on Pruning
Cox, Peter, The Larger Species of Rhododendron, B.T. Batsford Ltd., London, 1979, pp. 74-77.
Delp, Weldon, "Pruning for Bushy Growth," Journal American Rhododendron Society, Vol. 41, No. 3, 1987, p. 137.
Knapp, Fred E., "More About Pruning Rhododendrons," Journal American Rhododendron Society, Vol. 41, No. 2, 1987, pp.96-98.
Reiley, H. Edward, Success with Rhododendrons and Azaleas, Timber Press, Portland, Oregon, 1992, pp 113-118.
Smith, Cecil C., "About the Habit and Shape of Rhododendrons," Quarterly Bulletin of the American Rhododendron Society, Vol. 12, No. 1,1958,pp. 14-16.
1 Editor's Note: The best time is right after flowering when new growth is beginning.