Answer: To renew an overgrown rhododendron, start by gradually thinning it our at the base over 3 years: up to 1/3 of the branches can be cut to ground level each year. This will encourage new sprouts Thinning cuts can also be made which will help to control height and width. With these cuts, branches are pruned back to side branches. Make sure to prune to another branch or bud that faces out. This will be the direction of the new growth. Never leave a long stub which will eventually die and invite insects and disease: always trim to a bud or branch. . It is often possible to do this pruning during the blooming season and have flowers for the house. 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.
Rhododendrons should also be deadheaded and pinched back after bloom. 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.
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