Q&A – Deadheading

Q:  Should I deadhead my rhododendrons?  If so, how do I do it?

Dr. Glen Jamieson says…
Short answer:  You don’t have to deadhead rhododendrons, but there are several reasons why you may want to deadhead them.

1. Deadheading can increase flower quantity in rhododendrons

Deadheading, or removing old spent flowers, is the simplest form of pruning.  Deadheading is beneficial for lots of plants including rhododendrons and azaleas. This puts more energy into new bud production and leaf growth rather than seeding.  Remove spent flowers by snapping off or cutting away the flower head. Do so as soon as they are finished blooming.

Left: A rhododendron truss after flowering. Right: Deadheading the truss.

You can prune a rhododendron almost any time of year without harming it, but the best time is within a few weeks after it has finished blooming, to give it the maximum time to produce new growth and to set flower buds for next year. Pruning to late in the year may result in growth that has not yet matured and hardened off before winter cold weather arrives.

If you don’t deadhead, your rhodo will likely pump out about the same amount of flowers next spring as it did this year. However, if your goal is to produce more flowers, deadheading will encourage increased branching, and that usually results in more blooms.  (Some rhodo species simply don’t bloom every year, or will bloom heavily one year and need another to rest before doing it again.)

2. Deadheading rhododendrons improves appearance

Dead flowers and dried seed pods may remain on the plant for several years, so deadheading can improve the plant’s appearance.

Left: Opened seed pods. Right: A rhododendron that was not deadheaded.

3. Dead flower bud removal can improve plant vigour and hence flowering:  If the flower buds formed in the autumn, but turned brown during the winter, it suggests that you may be growing a too tender variety as they may have been frosted, and that some freezing weather protection may be required (e.g., if potted, move to a sheltered location).  If frosted, the buds will be brown or black and smooth-looking.  Another possible cause of black rhododendron buds is bud blast. If the flower buds have turned black, and are covered with spikey black fungal growths, they are probably suffering from bud blast from the fungus Seifertia azalea.

Removing damaged buds won’t increase flowering that year, but improves the vigour and appearance of the plant.  If the dead flower buds are left on, this inhibits vegetative growth from the petiole buds and may reduce flower quantity the next year.

Left: A rhododendron bud suffering from bud blast. Right: Rhododendron Leafhopper Graphocephala fennahi.

Bud blast may be spread by the rhododendron leafhopper (Graphocephala fennahi), a pretty species of leafhopper native to North America. Its common name derives from it feeding on the sap of rhododendrons. This insect lays its eggs on the developing flower buds of rhodos.

Leafhoppers can jump or fly short distances and most do not feed on or cause noticeable damage to garden plants.  The Rhododendron leafhopper only feeds on rhododendron. It is active from late spring to autumn but is most noticeable when the brightly coloured bluish-green adults are present in late July-October.

Control of Rhododendron Leafhoppers: There are no simple control measures for rhododendron leafhopper. However, flower buds affected by bud blast can be picked off and disposed of away from the garden to reduce the quantity of fungal spores being released in the vicinity of the plants. This may only be feasible on smaller plants.

4. Deadheading can improve plant structure and flower quantity

If your plants are too leggy and you want to “thicken” the plant (make it “fuller”), simply snap out the leading growth bud at each whorl of leaves as they start to expand. Instead of one lanky shoot, you should gain three to four bushy shoots from the buds above the leaf petioles, each of which can support flowers the next season. This may thus also increase flower quantity.

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Please remember:  Gardening is part science, part art, part skill, and a large part luck.  Sometimes even if you do all the “right” things, a plant still won’t thrive.  We offer the best advice we can, but we can’t guarantee that your plant(s) will thrive… or even survive.  MARS assumes no liability for any damage that may occur to your plants, person, or property as a result of or coincidental to our advice.

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