Rhododendrons and azaleas are closely related plants noted for their beautiful flowers. When planted very close together or too near a building, these popular landscape plants often outgrow their space allotment.
To produce compact plants with more flowers, pinch the tips of the new growth. To increase the number of trusses or flower clusters for the next year, pinch off about 1 inch of the new growth when it reaches 4 inches long and before leaves expand fully. Two or three new shoots will sprout from each shoot pinched. Pinch again right after the new shoots emerge, but before the next season’s flower buds are formed.
Even with pinching, rhododendrons and azaleas eventually can become too tall and leggy; some azaleas reach huge sizes. Shorten long branches by thinning to a fork, a side branch, or a dormant or latent bud. Although difficult to see, growth buds appear all along the stems; cutting just above a bud causes a new shoot to emerge.
To renew a rhododendron or azalea, gradually thin it at the base over several years. The entire plant can also be cut off at ground level and allowed to resprout, but it will take several years for the plant to reach an attractive size and shape. This drastic, sometimes risky, measure works only for vigorous shrubs; weak plants may not resprout.
Rhododendrons produce flowers grouped together into trusses. They grow more vigorously and flower better the following year if the old flowers are removed just after they fade and before they set seed. To remove the spent flower trusses, hold the branch with the faded flowers in one hand and with the other hand carefully snap off the flower head with a slight sideways pressure. Take care not to break off the buds or shoots along the sides of the trusses, because these will grow into branches tipped with the next year’s flower buds.
Although most rhododendrons today are propagated by rooted cuttings, old specimens may be grafted. Grafted plants consist of two different plants joined together; a desirable cultivar forms the top growth and the roots are selected for their vigor. If suckers emerge below the graft union (visible as a swollen area at the base of the trunk), cut them cleanly away or snap them off where they attach to the understock. If left unchecked, suckers can overgrow the grafted cultivar and produce inferior flowers.