Covered with bright blossoms, flowering shrubs can be a joyous sight in spring. Rhododendrons (Rhododendron spp.) are especially colorful, but pruning at the wrong time could put the annual show at risk. Prune at the correct time to ensure abundant flowers while also keeping the shrubs looking tidy.
A spring-blooming shrub, the rhododendron develops flower buds during the summer or early fall on shoots that began to grow the previous spring, after flowering. Because of this, pruning as soon as flowers fall off the plant is the best strategy to preserve the plant's flowering the following spring. You can also prune a rhododendron in mid to late summer without damaging the plant, but if you do, it won't develop any new flower buds on the branches you cut back. Avoid pruning in mid to late fall, because new growth might be damaged by cold winter weather. Most rhododendrons grow in U.S. Department of Agriculture plant hardiness zones 4 through 9, depending on the variety.
Your pruning strategy for a rhododendron depends to some degree on its age and condition. If the plant is healthy and only a few years old, pruning some of its branches back every year or two is a good strategy to encourage new branch points and help the plant develop a bushy form. Make each cut about 1/4 inch above a growth joint, where there's a radiating group, or whorl, of leaves -- this is the point at which the branch started to grow in a previous year. Avoid making cuts between growth joints on a branch because there are no dormant buds in that area and the branch responds to this type of cut by dying back to the next growth joint.
As a rhododendron grows and ages, some of its deeper, inside limbs become shaded by the canopy and may become weak or die. When these less vigorous, larger branches start to fail, you can remove them to stimulate new growth from the base of the plant. For an especially old plant with poor overall growth, cutting the plant back severely to leave only stumps with no leaves can rejuvenate the entire plant, with several new branches eventually developing at the plant's base. This is best done early in the season, to give the plant enough time to develop new leafy growth before winter arrives.
In addition to routine pruning, a rhododendron also responds well to dead heading, or removing spent blossom clusters, a process that promotes growth of several new shoots behind the point of flowering. You can either grasp the flower cluster, called a truss, with your thumb and forefinger, and snap it off the plant, or use pruning shears to cut each truss away about 1/2 inch above its base. If dead or damaged branches appear during the growing season, or you notice a branch that crosses and rubs on another branch, you can cut these away at any time to keep the plant growing well. Rhododendrons are poisonous, so keep curious children and pets away.