Rhododendron x hybridum Glenn Dale Group

Glenn Dale group azaleas are hybrid, woody, evergreen shrubs in the Ericaceae (blueberry) family that were developed for their large showy flowers and winter hardiness. The genus name derives from the Greek words rhodo, which means rose, and dendron, meaning tree. The common name is a reference to the place name where the azalea hybridizing program began in Glenn Dale, MD. 

These plants prefer moist, well-drained, acidic soils in dappled shade and good air circulation. Protect them from afternoon sun. They do not tolerate dense clay, so if you have clay soil, amend with organic material. The roots may rot if soil does not drain well. Plant the root ball high in the soil to help with drainage issues. Mulch with pine bark or oak leaves. Prune rhododendrons and azaleas after they bloom to preserve the current season's bloom, and allow time for the plant to produce flower buds for next year's bloom.  Rhododendrons and azaleas set next year's flower buds by mid to late summer. 

These are large-flowered landscape azaleas with a varied rounded habit, meaning some grow more erect and others are more spreading. Their size at maturity is 4 to 6 feet tall and 3 to 4 feet wide. There are many cultivars and colors, and some cultivars are winter hardy to zone 5. They bloom from April to early June. 

Plant Glenn Dale azaleas as understory shrubs in groups in woodland or naturalized areas, or as specimens in pollinator gardens where the showy flowers will attract bees.  They worlk well as foundation plantings in shaded locations.

Insects, Diseases, and Other Plant Problems:  Rhododendrons are susceptible to insect and disease problems.  Insect problems include aphids, borers, lace bugs, leafhoppers, mealybugs, mites, nematodes, scale, thrips and whitefly.  Diseases include canker, crown rot, root rot, leaf spot, rust, and powdery mildew. This plant is frequently damaged by deer.

While these shrubs remain very popular for landscape use, many cultivars are susceptible to Phytophthora root rot—this leads to leaf loss, reduced vigor, branch dieback, and wilting. Implement good cultural practices first, such as improving drainage with organic matter or berms and avoiding overwatering or overfertilization. However, if you have a site with a history of this disease, consider planting a root rot-resistant alternative species. 

For suitable alternatives, see this video created by Charlotte Glen as part of the Plants, Pests, and Pathogens series.

 VIDEO Created by Elizabeth Meyer for "Trees, Shrubs and Conifers" a plant identification course offered in partnership with Longwood Gardens.

More information on Rhododendron.