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Keeping Azaleas and Rhododendrons Happy

Q. I planted several azaleas about eight years ago; they are large but kind of scraggly, having grown tall instead of becoming round and full. A lady in our community cuts hers back to small stubs every year and the next season they are large, full, beautiful bushes. After reading a great deal on this subject I'm confused as to whether it’s a good idea or not. Some say ‘do’, some say ‘don't’. Right now the plants are covered in blooms so I know not to do anything. But when is the best time? And what should I do?  Thanks,
            ---Jennifer in McComb, Mississippi

I’m glad you asked, Jennifer—improper pruning is one of the biggest mistakes people make with azaleas and other rhododendrons (all azaleas are in the rhododendron family, and both types of plants require the same basic care).

It’s never wrong to prune them right after the flowers fade. A month later is still technically okay, but sooner is always better—especially down South, where your heat can come on fast and stress recently pruned plants. No matter where you are, never prune after June or you risk removing the following year’s flower buds.

(Note: This advice applies only to the typical early-Spring blooming types, which comprise over 90% of the rhododendron family plants grown in American landscapes. Some rare and beautiful varieties grown by enthusiasts and botanic gardens can bloom very late in the season. Consult an expert if you have a non-typical azalea or rhododendron. (Heck—if you have one, you probably ARE an expert…)

Now, your neighbor sounds like one of those ‘green thumbs’ who can do pretty much do anything and the plants still thrive. It is generally not wise for normal people to imitate such behavior. And your gentle winters—much more to these plants’ liking than my brutal Pennsylvania ones—may make her technique a bit safer. But I’d still stick with the always-safe advice of removing no more than a quarter of the plant in any one season. In your case, just take it off the top (if disease were an issue, I’d suggest removing some inner branches for better airflow instead); if your plants are eight feet tall, remove no more than the top two feet of growth. Six feet tall, a foot and a half, etc…

Such a pruning will begin to improve their look dramatically without the possibility of you entering the fall with the delightful sight of bare, dead sticks outside your house. Repeat this every season, and in a few years the shape of your plants should be exactly what you’re looking for. Remember—you’ve been letting them get leggy for eight years; give yourself at least a few seasons to correct that condition.

After pruning, feed and mulch as I describe below and keep them well watered; these thirsty plants should be the first ones in our landscapes to see the hose during dry times.     

Q. Over the past years I’ve had a problem with my azaleas and rhododendrons that the extension service at Penn State has diagnosed as Botryosphaeria. I have a lot of clay in my soil and thought making a raised bed would help. A friend who owns a small nursery has offered me a mix that is 50% three-year-old, ground-up decayed leaves and 50% soil from the bottom of a lake. What do you think?
            ---Allen in North Wales, PA

A. Botryosphaeria is a canker caused by growing conditions so bad it should kill the plants, but they hang on long enough to die of an unpronounceable disease instead. As with most of these kinds of problems, the answer is to simply give the plants what they want and go home early.

And what these plants want is a naturally rich, acidic soil; they LOVE growing in a mix of half milled peat moss (available in those big bricks at garden centers) and half compost. If they’re available alone, those three-year-old leaves should BE compost by now (unless they still look like leaves, then find real compost). The lake bottom stuff might be good topsoil or it might be really polluted; either way, you don’t need it here.

I love raised beds for annual flowers and veggies, but prefer in-ground planting for these kinds of perennials in the North; their legendarily shallow roots could freeze if that bed kept them above ground during a really severe winter. Dig a VERY wide hole—don’t just plop them into a little island of good soil or their roots won’t travel outside of it—throw away all the nasty clay you encounter and break up the stuff at the bottom with a garden fork for good drainage. Although they are thirsty, these plants can’t stand wet feet, which unimproved clay soil virtually assures.

Fill the hole with a well-mixed batch of half compost, half peat moss and plant the bushes at least as high above the soil line as they were when you got them. After planting, spread two inches of peat moss around their base, then cover that with an inch of compost. You can substitute a couple inches of shredded oak leaves, pine straw, pine needles or other naturally acidic mulch for this peat/compost mix, but DO NOT use sawdust, wood chips, ‘triple-premium shredded bark’ or any other kind of wood mulch.

Mulch you must! And re-mulch every year. Spread an inch or two of peat moss around the base of the plants, and then cover that with an inch of compost in the North; two inches in the South. This will feed the plants, keep moisture in the soil and create a visual reminder to keep your big feet away from those very sensitive roots. If you want to use one of those naturally acidic mulches instead, go right ahead—just remember: No wood mulch of any kind. Or rubber. I hope you knew THAT already...

And finally, some basic rules:

• Don’t plant azaleas or rhododendrons in full sun or deep shade—understory plants in the wild, they thrive beneath the canopy of deciduous trees or thin evergreens (or a location that gives them morning sun and afternoon shade).
• Keep them far away from concrete, lime and other alkaline materials.
• Prune them at least a little bit every year to remove the faded flower heads.
• Water during dry spells. And keep their soil moist for a good week or three after planting or transplanting (which is best done right after the blooms are gone.)
• Do not overfeed. Stay away from chemical fertilizers; these plants just despise the concentrated chemical salts in products like Miracle-Gro and Osmocote.

Do all these easy things, my Spring-blooming friends, and you will never have to worry about pests or disease again.

Special thanks for reviewing this article to the folks at Tyler Arboretum in Media, Pa—home to a world-class collection of azaleas and rhododendrons so diverse that something is in bloom from late winter through August!  Details: http://www.tylerarboretum.org/wistercollections.htm

You Bet Your Garden   Question of the Week  ©2006 Mike McGrath

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