The Glenn Dale azalea breeding program, launched at the USDA research station in Glenn Dale, Maryland during the 1930s and 1940s, had a specific ambition: combine the large-flowered quality of tender Asian cultivars with the cold tolerance needed to survive mid-Atlantic winters. The result was more than 400 named cultivars spanning nearly every color in the azalea range. At 4 to 6 feet tall and 3 to 4 feet wide, the plants offer a substantial presence in the landscape, and the blooms, arriving from April into early June, are among the largest produced by any garden azalea.

They are best treated as understory shrubs, positioned beneath high branching trees where dappled light and protection from desiccating afternoon sun keep the foliage healthy through summer. Mulching with pine bark or oak leaves maintains the soil acidity and moisture that these plants need. Good air circulation reduces the likelihood of fungal issues. Some cultivars extend winter hardiness into zone 5, though checking the individual selection's ratings before planting in marginal climates is worth the extra step.