Radiant Rhododendrons -June 2009

Radiant Rhododendrons

by: Hilary Newell

Bursting with color come late spring, rhododendrons

serve as the floral welcoming committee for sunny summer days and carefree island living.

There’s a path around a lake where I like to walk in summer. It’s called The Laurel Path, named for the plethora of mountain laurel found there, but it could just as easily be called the Rhododendron Path, as it seems there are just as many of those as there are mountain laurels.

The name rhododendron sounds a bit less sexy, but the flowers are lush and inviting just the same. In some places, they completely cover the path, creating a six- to eight-foot-high cave-like tunnel that serves as a passageway along the trail. It is only the occasional year when we get to be there when the rhododendrons are blooming, but even as the summer wears on, and the blossoms scatter themselves all over the path, the green, leathery foliage provides comforting protection from the sun and even from a light rain. A walk on that path is beautiful any time of the year, as both the mountain laurels and the rhododendrons are evergreen, and therefore provide something of interest to look at while walking.

Rhododendrons and azaleas are in the heath family and as ericaceous plants, they thrive in acid soil and hate lime. Other well-known ericaceous plants we find on Nantucket include blueberries, cranberries, heaths, heathers and mayflowers. Our acid soil (pH 5.5 or so) is the perfect host for all of these plants, and our woodland garden is on its way to becoming massed with them. But where do azaleas fit in here?

Taxonomists include all azaleas in the Rhododendron genus as their botanical characteristics are so similar. All azaleas are in the Rhododendron genus, but rhododendrons (with a small “r”) are not azaleas. Confused? You are not the only one. The taxonomy of these two shrubs has been debated and changed a number of times in the history of plant-naming. In a nutshell, the genus includes four major sub-genera: large leaf rhodies, small leaf rhodies, deciduous azaleas and evergreen azaleas.

How do you tell the difference? Azaleas can be evergreen or deciduous, (all the native New England varieties lose their foliage) and rhododendrons are evergreen. Azaleas have five stamens and rhododendrons have 10 stamens. Generally, mature specimens of azaleas are smaller than mature rhodies, though there are a few very small rhodies native to the American northwest that don’t fit these descriptions. The name comes from the Greek rhodon (meaning rose) and dendron (meaning tree.) Indeed, the blossoms of some varieties are somewhat like roses … if you stand back at a distance and squint. I don’t see the resemblance, but maybe the Greeks had something else in mind. “Azalea” is derived from azaleas, the Greek for “dry,” when, in fact, they need copious water because of their shallow root systems. Go figure.

Several species are native to New England, including R. maximum, also known as the rosebay rhododendron, though the number of native stands is shrinking with development. This magnificent shrub is also the denizen of the previously-mentioned Laurel Path. Accounts of explorers forging their way west from the Atlantic tell of masses of gnarled and tangled native rhododendrons that were nearly impossible to penetrate and reached for miles across mountainsides and valleys. Imagine springtime with acres of blooms. Another breathtaking native is R. viscosum, the swamp azalea. There are several walking paths on Nantucket where this can be found, or rather smelled. You will often notice the amazing scent of this plant and may never actually lay eyes on it. If you follow your nose, you may find a tall plant with white tubular flowers. There is one in our woods that I have never seen, but in late June or early July I can walk out the door on a sunny calm day and know that it is blooming somewhere nearby. The perfume is strong and pleasant and lasts for several days. The deciduous foliage turns orange or yellow in the fall so it makes for a nice, two-season shrub.

But one does not have to “go native” to get a really great rhododendron or azalea. Varieties that are native to other areas of the country can be successfully grown one or two zones away from where they are normally found. Cultivated varieties and hybrid crosses are often selected from native types and offer interesting colors, sizes and shapes that retain the vigor or other positive traits of the parent plant.

Rhododendron carolinianum is a stand-out variety that seldom reaches more than five or six feet. Pale rose-pink flowers show in profusion in mid-spring. The shady side of a house or the edge of some woods is a perfect location as they need to be protected from mid-day sun.

R. catawbiense is one of the finest native species available. The parent plant is native to the Carolinas and Georgia, and its hybrid children come in colors from white to light lavender to crimson, purple and red. There’s a pink one at the edge of our yard that serves as the all-important backdrop for photos of family events. It grows to about six feet tall, is about five feet wide and is covered with extravagant rose-pink blooms in June.

In direct height contrast, R. roseum elegans is a classic lavender-pink cultivar that tops out at over 12 feet. It is hardy to -25 degrees, and tends toward a less formal and more open look. It’s a good choice for naturalizing in a wild area, away from the house and any formal gardens you may have.

R. Yaku Prince is a compact, reliably heavy bloomer that has a really nice shape. It is extremely cold tolerant and with no maintenance, it keeps its shape. The blossoms open as deep rose and transform to lighter pink and finally fade to white when they are spent. Cool May temperatures help keep the flowers fresh longer. They only need about three to four hours of direct sun. Originally bred in Northern Japan and called R. yakushima, they have extremely handsome foliage and the dense mounds top out after only four to five years at a height of three to four feet tall, making them perfect for a formal border. Yaku Prince has earned the distinction of being a “Proven Performer” by the Massachusetts chapter of the American Rhododendron Society. These are varieties that have consistently performed well throughout the diverse New England landscape of USDA zones 5 and 6. Certain varieties of R. catawbiense, R. Carolinian and R. viscosum are also on the list, clearly all good choices for our landscapes.

Deciduous azaleas have been hybridized since the 1800s. As early bloomers, they generally put out their flowers before the foliage shows, so these are quite valuable for areas that have more wind, as the foliage won’t get tattered before the plants can show their glory. R. viscosum (swamp azalea) is the parent of many hybrid azaleas, passing along some of its best attributes to its children. They come in a wide variety of vivid colors and also in white.

The Exbury hybrids are among the better-known deciduous azaleas. These became the standard for deciduous azaleas into the 20th century. Oxydol is a beautiful white with a distinctive yellow blotch in the center. Gibraltar is a vivid orange. Because they bloom early, the vivid, glowing colors are a welcome sight in your spring garden. You can enjoy the flash of color early, and then let the rest of your garden palette take over for the summer. Plant some squill (Scilla) or sweet woodruff (Galium odoratum) underneath, and you will enjoy a carpet of blue or white as contrast to the bright azalea flowers.

In the early 1980s the University of Minnesota introduced the Northern Lights hybrids. Hardy to -35 (well, they are from Minnesota) they perform very well in our climate as well. The simple names of the varieties Pink Lights, White Lights and Rosy Lights don’t do justice to these notably beautiful and hardy plants.

Most rhododendrons and azaleas prefer an area to grow where they get light shade. As a native of the forest, R. maximum can tolerate deep shade, and R. yakushima can tolerate more sun. Protection from the afternoon sun is important for any rhodie or azalea and shelter from the wind is key to long-term success. A northeastern exposure fits the sun-exposure criteria, but be mindful of the wind coming from that direction.

Protection from buildings or larger trees and shrubs will help your plants thrive for years. Too much sun can cause your plants to appear stunted, lose foliage and look burned during our summer dry season. And if they are exposed to too much wind, the foliage can get yellow and crispy around the edges. If you decide to plant rhododendrons or azaleas, there are some other things to consider. Most of our Nantucket soil is too sandy in its virgin state to support these loam-loving plants. The acidity level is OK, but soil amendments are needed to provide a suitable basis for planting. Azaleas require soil that is rich with humus that holds moisture, but is not soggy. The liberal addition of peat moss, humus, organic matter and compost will go a long way in ensuring healthy plants. The importance of digging a good hole and providing a good beginning cannot be understated. An azalea or rhododendron will last for many, many years, and can provide beauty for generations. Two annual applications of fertilizer are part of the recipe for long-term success. The first should be applied in early spring, as the plant is emerging from its winter slumber. The second should be applied in early summer, between mid-June and mid-July. Acid fertilizers like Espoma Holly Tone or cottonseed meal fit the bill for both feedings. Fortunately, this group of plants is very hardy and not susceptible to a lot of diseases. Most problems can be mitigated by proper planting and good growing techniques.

Choose a good location, prepare the soil well, don’t plant too deep and fertilize twice a year to keep your plants growing well. If you get the bug to really get into these magnificent plants, check out the American Rhododendron Society which has chapters all over the United States.

There are few shrubs that are as dramatic as rhododendrons and azaleas when they are in full flower. Planting a few around your home can provide joy and beauty for future generations, and if you happen to purchase a home with mature specimens, you should silently thank the forward-thinking person who planted them there.

Hilary Newell is the greenhouse production manager at Bartlett’s Farm and compiles “The Farm Dirt” each week.

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