Sunday, April 27

A rhododendron by any other name

Taking the mystery out of azaleas

The best way to compose a large collection of azaleas is to group them by color and the ultimate height of each variety.

Many gardeners in our area have given up on rhododendrons and azaleas entirely. Others, after years of disappointment, have reluctantly decided to grow them as "seasonal annuals" only—despite their potential status as long-lived shrubs. Still others think these plants belong only in the highly-maintained collections of dedicated specialists.

Besides rodents, be on the watch for the redheaded azalea caterpillar (Datana major). It feeds on new growth and immature flower buds and is nearly invisible at this stage, but is very easy to control organically with a spray or dust of Bacillus thuringiensis (trade name Thuricide), a biological larvacide.

That's unfortunate because there are special selections of these plants, all classified botanically within the genus Rhododendron, that deserve even the average gardener's attention. With minimal care, and just a little bit of luck, they can even persist—as some of mine have—for decades. And when in bloom, there are few plants that can rival their beauty.

Varieties add spice
The first group, and the one closest to my heart, is the deciduous (or leaf-losing) azalea. With several species native to wide areas of North America, and additional relatives native to the oddly-divergent geographies of both Eastern Asia and the Balkans, there is a great natural diversity among these plants. The species alone are beautiful, but breeders have also produced spectacular hybrids in a range of colors, habits and bloom seasons. Some, such as the Lights series introduced by the University of Minnesota Landscape Arboretum, are among the hardiest large-flowering shrubs.

So why are these plants such a mystery? They have populated independent garden centers and even home improvement center nursery yards for years, but often even the garden center staff has little personal experience growing them beyond the sale. Large collections are infrequently encountered in public gardens and the collection at the Morton Arboretum has periodically declined due to overgrowth from larger evergreens mistakenly planted among them for shade. Under such conditions, azaleas may grow and bloom poorly. These are plants which require full sun, good air circulation and a moderately rich, only slightly acidic, soil.

One of the most common myths is that azaleas are prone to mildew and among the common misconceptions about growing them is the belief that they should only be sited in dense shade, mulched deeply and watered constantly. Now tell me, if somebody kept you in the dark all day, piled shredded bark around your feet and sprayed you with a garden hose at whim, wouldn't you be mildew-prone? (Not to mention, more than a little irritated!) This amounts to pure azalea torture and, unfortunately, may well be the beginning of your collection's downfall. Not only do these plants bloom best in full sun, or light afternoon shade, but it is only there that their exquisite burgundy, gold and orange fall colors will fully develop.

One of a series introduced by the University of Minnesota Landscape Arboretum, the 'White Lights' deciduous azalea is one of the hardiest large-flowering shrubs available. Photo by Bob Gabella

Growing conditions
Is acid soil really necessary? I have several specimens growing in pH 6.5-7, which is neutral, and this speaks very well of their origins. Many of the wild species grow in the organic debris that settles in between craggy limestone outcroppings, at varying degrees of incline. This makes them very tolerant of both variations in soil pH and seasonal fluctuations in moisture.

To plant azaleas, begin by preparing the entire soil bed uniformly, digging at least 25 percent organic or commercial compost into the soil to a depth of at least a foot. Do not spot-treat individual planting holes; this discourages a root run throughout the bed. Use soil sulfur sparingly for acidification at planting time. Incorporate it at no greater than four pounds per 100 square feet. Too much can kill the plants.

Azaleas need room, so plant most varieties no closer than four feet on center. The first year in the ground it is the summer, not the winter, which will kill them. Heat and drought conspire against the slowly-establishing root system and often-rapid top growth. Be patient. Yes, a few inches of loose mulch are important, but by all means don't bury the plants. My established specimens are mostly un-mulched. If you host a marauding gang of rodents, a 12-inch-high chicken wire collar for the first two winters will allow the plants to attain varmint-proof stature.

Do not crowd azaleas with too many garden buddies. Among bulbs, Hyacinthoides hispanica 'Excelsior,' in brilliant blue, blooms at the same time as the most common varieties; and for perennials, Baptisia australis—blue false indigo—clumped or massed strategically, is hard to beat.

Its bronze-colored foliaged is a perfect foil for 'Klondyke's' golden-yellow blooms. Photo by Bob Gabella

Proper siting
Site golden, yellow and bright orange azalea varieties at a distance because the eye will advance these colors. Place the pastels and deep reds closer to the main view or they can be upstaged by the brighter colors.

The best way to compose a large planting or collection of azaleas is to group them by color family and by the ultimate height of each variety. In my garden, the brilliant orange 'Gibraltar' tops out at less than five feet tall, but is wider, while 'Klondyke' is nearly eight feet tall and two-thirds as wide. The rose-pink and orange bicolor 'Mt. St. Helens,' at six feet, falls nicely in between. 'Viscosepala,' 'White Lights,' 'Golden Lights' and 'Daviesii' usually stay at a tidy four feet tall, with 'Daviesii' (which I purchased years ago at my local Jewel-Osco) so dense and round it looks hedge-clipped. This defies the sometimes-informal habit of many of the cultivars. 'Weston's Pink Lollipop' is about the same height, but its lateral stems snake and meander to almost six feet across.

With varieties in bloom from early May to mid July or even later, a fragrance like cloves and honey, and fall color to boot, a bed of these fine shrubs will give you well-earned bragging rights and will grow in beauty with each passing spring.

(EDITOR'S NOTE: Next week Bob will introduce some ironclad evergreen rhododendrons that are perfect for northern gardens.)

A lifelong gardener, Robert F. Gabella has been involved in the horticulture industry since 1983. With an AAS in ornamental horticulture and a BA in business management, Bob is an Illinois Certified Nursery Professional, an independent consultant, hybridist, writer and photographer. He also tends prize-winning gardens at his Villa Park home.