The lone voice of horticultural reason

Growing Rhododendrons & Azaleas

They appear in late spring every year, in unkempt rows, sometimes short but often long. They vary in height and form but are all members of the same genus: homeowners, in line at the service desk of our local garden centers, waiting to return the stone-dead rhododendrons they each now clutch, having planted them, poorly, the spring before.

Every gardener should grow rhododendrons. Use of shrubs in the landscape, particularly within the perennial bed, has recently achieved an elevated status in my mind, and I’m glad that “rhodies” (and their cousins, the azaleas,) were among the first shrubs I planted when I began gardening.

Travelling in May through England and Scotland years ago was all it took for me. In the U.K.’s more temperate climate, rhododendrons grow upwards to 12 feet tall with equal width, blooming thick with great blasts of white, pink, red or purple flowers. In Scotland they flow down from the peaks of the Highlands like rivers of color, fanning out as they descend to form rolling clouds of intense hues, visible for miles.

Gardeners here in the north began buying cultivars developed by the University of Minnesota and other researchers 20 years ago, and since then both rhodies and azaleas have been improved to where they are extremely winter-hardy. Additionally, landscapers have discovered that “rhododendron” and “azalea” are impressive-sounding, correct Latin names for plants that most of their ilk can remember and pronounce, greatly increasing these shrubs’ use in the residential landscape. (This same factor was responsible for the great potentilla scare of the mid-1980s, which claimed vast numbers of west-suburban town-home developments.)

Growing rhododendrons is not difficult; if there is one suggestion I might make to the local nursery industry, however, it’s that they attach a bold tag to each plant that reads, “ATTENTION: GROWING THIS THING INVOLVES GARDENING.” Here, then, are the simple gardening steps to successfully growing these magnificent shrubs:

Rhododendrons and azaleas both thrive in light shade created by an overhead tree canopy. Rhodies can take a full morning or afternoon of direct sun but will perform best if this exposure doesn’t exceed four hours. Azaleas may be planted in part shade to full sun. In any and all cases, these plants must be mulched heavily throughout the growing season to keep the roots cool and the soil from drying out.

Soil is crucial. Rhodies and azaleas will die if planted in clay or any soil that does not drain efficiently, and will grow poorly without blooming in soil too high on the pH scale. They need ample moisture but rot if the soil stays too damp. They wish to develop a thick mass of very fine, shallow roots that can spread with ease horizontally from the main stem. These roots will rarely go more than eight inches deep but can travel as far as eight feet out. These roots need acidic soil, lower on the pH scale than the soil in your yard. All these seemingly difficult conditions are extremely easy to create.

Prior to planting, dig a large, bowl-shaped hole at least three feet in diameter and 18 inches deep. If you can dig it wider, do it. When finished, the hole should resemble the impression made by a sporty little flying saucer, or giant birdbath. Do not dig a bathtub with abrupt sides. In the wheelbarrow, make soil that is one-third peat moss and/or compost, one-third pulverized back dirt, and one-third coarse sand and pea gravel. Fill your hole with this mixture then plant your rhodie or azalea in the middle, being careful not to compact the soil. Scatter handfuls of soil sulfur across the circle, amount according to the directions on the bag, and rake it in. Water the entire “bowl” thoroughly, then mulch with a two- to three-inch layer of straw, wood chips or cocoa bean husks. That’s it. You’ve just planted the shrub perfectly, and it will thrive.

During hot periods with no rain, water moderately twice a week. Each spring I remove the mulch and add a layer of fresh peat moss, then reapply the mulch. I work in a bit more soil sulfur about every two years, to keep the soil acidic.

Fertilize as you would any shrub. Miracid applications three or four times during the spring and summer are a good idea, but stop all fertilizing after August 10th. These plants set their flower buds for next year in mid-summer, so they mustn’t be pruned in the fall or winter. Depending on the cultivars, the plants will reach heights of six feet in our area, and in the spring will provide your yard with a display of bloom that is a wonder to behold.

Don Engebretson
The Renegade Gardener