Pruning over-grown rhododendrons

by Clare Liptak published Mar 10, 2014 12:00 PM, last modified Mar 16, 2014 06:19 PM
Landscape plants are not supposed to block windows, or encroach on sidewalks or driveways. A reliable method of turning an old overgrown rhododendron into a full and valuable feature in the landscape works because rhododendrons have lots of dormant, or latent, buds on their trunks and branches. The best time of year to prune rhodies is soon after their blooms have faded.
Pruning over-grown rhododendrons

R.argyrophyllum leiandrum

As with any pruning job, you first remove dead and dying branches. Since we’re talking rhodies, look for the large trunks with rough, patchy bark with holes in it (caused by an insect called the rhododendron borer) and remove those first. But if there’s lots of bark with holes, then the plant has been stressed for some time --borers attack stressed plants after all. You’re probably better off starting new with a healthy plant.

 

But assuming your plant is healthy, just overgrown, check for new growth present on the lower parts of trunks. In this case, go for the drastic method, which is to prune every trunk at varying heights but generally within 12 to 48 inches from the soil surface. If there’s new growth on a branch, prune the trunk back to that new growth, with a slanting cut that sheds water. In 2 or 3 years, the you will have an attractive, rejuvenated plant in scale with your house and landscape.

 

Rhododendron blooming
Rhododendron blooming
If there are no new shoots on the lower parts of trunks, be more careful with the pruning shears or saw.  Again look for the largest diameter trunks, and prune back one-third of them. Opening up the canopy like this allows sunlight to reach the lower branches and stimulate them to grow in length and diameter. Repeat this process twice, next year and the year after, pruning back one-third of the trunks each time. You’ll have a rejuvenated rhododendron and by removing only one third of the branches each year, the stress or shock to the plant will not be too great.

 

Clare Liptak, retired Somerset County Agricultural Agent, is an IPM scout, horticulturist, and Certified Tree Expert #208.

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