Green Thumbs Up: Know and grow the showy rhododendrons

Suzanne Mahler

The sweet scents of the springtime flowers drift through the garden carried by gentle breezes on a perfect morning in mid-May. Birds sing their spirited springtime melodies as they perform their annual courtship rituals, gather materials to build their nests, and feed their young. A hummingbird zips through the garden, pausing to sip nectar at the delicate, dangling red and yellow blossoms of columbines. My gardens are a tapestry of lush, multicolored flowers and foliages as plentiful sunshine and mild temperatures have produced a truly delightful spring season.

While the fleeting blooms of many of our spring-flowering trees have come and gone, several weeks in advance of typical years, a diversity of shrubs will extend the floral parade in the weeks to come. Bridal wreath spireas with their arching stems smothered in button-shaped snow white blooms are consistent performers from year to year while their cousins offer rose, pink, or white blooms sporadically throughout the summer months. Close relatives, the ninebarks, produce similar blooms in addition to attractively tinted foliage and peeling stems, for multi-season interest. The family of Viburnums, many of which are delightfully fragrant like the Korean Spice Viburnum, come into flower during May and June followed by dazzling fall fruits in shades of yellow, blue and red.

Perhaps the showiest and most popular flowering shrubs are members of the Rhododendron family. The deciduous Korean rhododendron (R. mucronulatum) is one of the first shrubs to bloom in my garden, its delicate rosy-purple blossoms providing a welcome splash of color beside my back walkway during the month of April. A selected variety of this deciduous shrub, Cornell pink, exhibits bright pink blossoms. The flowering of these Korean rhododendrons marks the beginning of a long progression of bloom offered by this diverse family of attractive and often flamboyant flowering shrubs which derive their name from Greek words meaning “rose tree.” There are more than 1,000 species and thousands of hybrids of these ornamental plants and by selecting different varieties, colorful blossoms are possible from April through July and even into August.

This large family of plants is typically divided into four garden groups: the classic large-leaf rhododendrons, small-leaf rhododendrons, evergreen azaleas and deciduous azaleas. Once considered a separate genus, all azaleas have been classified as members of the Rhododendron genus since the mid-1800s, although the general public still most often uses the common name of azalea.

A visit to your local nursery will reveal a large selection of these tantalizing shrubs. As you browse through the fresh, healthy nursery stock, be sure to consider the ultimate size of your choices as they relate to their intended use in addition to the exposure and soil conditions they require.

The popular large-leaf rhododendrons prefer to be sheltered from strong, desiccating winds, and shaded from continuous hot sun in both summer and winter. During the month of June, these evergreen rhododendrons take center stage with giant trusses of white, pink, lavender, purple, red, and even soft, creamy yellow. Rhododendrons become magnificent specimen plants over time, but many cultivars quickly outgrow their allotted spaces requiring drastic pruning or complete removal. Consequently, many of the large-leaf varieties are best used as background plants beneath the high shade of deep-rooted trees rather than as part of a foundation planting. Consider the Yakushimanum rhododendrons for gardens and foundation plantings as they offer slow, dense, compact growth in addition to fuzzy new growth and soft felt on the underside of the leaves.

Small-leaf rhododendrons can tolerate sunnier, more-exposed environments than the large-leaf rhododendrons. Perhaps the best-known members of this group are the popular PJM hybrids. These early-blooming rosy-purple PJM rhodies offer homeowners hardy, early-flowering, drought-tolerant shrubs; Olga Mezzitt and Aglo are bright-pink relatives that bloom slightly later. Reaching a maximum size of 3 to 6 feet, these shrubs display elegant, mahogany-colored foliage during the fall and winter months if grown in full sun.

From mid-May into June, azaleas flaunt their finery in sizzling shades of salmon, red, pink, white and lavender. From compact, low-growing cultivars to taller upright varieties, the possibilities are endless with evergreen, semi-evergreen and deciduous types. The deciduous flame azaleas, which include the Exbury hybrids, expand the color range offering large clusters of fiery orange, yellow, hot pink, and gold flowers against a backdrop of soft green foliage. Evergreen azaleas tend to prefer sheltered sites in partial shade, while the deciduous varieties are suitable for sun or partial shade.

To extend the bloom season, look for our native swamp azalea (R. viscosum) and its cousins Weston’s Lemon Drop, Lollipop, Golden Showers and others. With charming fragrant flowers in July and August in addition to good fall color, these extremely hardy deciduous shrubs thrive in damp soils.

All rhododendrons require a moist, but well-drained, fertile soil, rich in organic matter. Because they are shallow-rooted, they resent competition from other aggressive shallow-rooted trees such as hemlocks, maples and beeches. Our typically acidic soils are usually ideal, but heavy, wet, clay soils should be avoided. Amend all soils with organic material in the form of peat moss or compost. Do not plant too deeply. Bright light is essential for optimum growth and blooming. Cover the spreading root system with a 2- to 3-inch layer of pine bark mulch to help conserve moisture and to maintain cool soil temperatures. Avoid letting the mulch come in contact with the woody trunk or branches as this may encourage disease or insect infestations. Pruning is usually best performed immediately after blossoms fade. If given suitable conditions, rhododendrons are easy, carefree and long-lived shrubs.

Suzanne Mahler is an avid gardener, photographer and lecturer who has been developing the 1.5-acre property surrounding her home in Hanover, Mass., for more than 30 years. She is a member of two local garden clubs, past President of the New England Daylily Society, an overseer for the Massachusetts Horticultural Society and is employed at two garden centers.