According to the Rhododendron Species Botanical Garden in Washington State, there are more than 850 different species of the flowering shrub in the wild. They are native to regions in Asia, North America, Europe and Australia, the Botanical Garden notes. While rhododendrons generally bloom for just a few weeks at a time, there are species that bloom as early as January.
Rhododendron species vary greatly in size and shape, with some like ground cover, just a few inches high, and others that are tree-like, as as tall as 100 feet. Flower colors range from yellow to pink to rose to red, purple and lavender. Foliage varies in size and shape, from 1/4-inch round leaves to pointed, oval leaves as long as 3 feet. Most rhododendrons in home landscapes are hybrids, and early varieties normally range from 2 to 10 feet tall.
Rhododendron mucronulatum is one of the earliest varieties to bloom. Bright purple flowers adorn this deciduous shrub in January or February before leaves emerge. It takes about 10 years for this variety to grow about 10 feet. Other February-blooming varieties include Christmas Cheer and Rosamundi. Both flower white or pink and have large leaves. They grow slightly slower than mucronulatum, about 3 feet over 10 years. The slowest-growing of the February rhododendrons is perhaps Cilpinense; this bushy shrub reaches just 30 inches over a 10-year span.
Several rhododendron hybrids bloom in late winter or very early spring. Bric-a-brac is a slow-growing variety with white flowers that appear in March. Conemaugh features bell-shaped lavender-pink blooms in late March. It reaches an eventual height of about 3 feet. 'PJM' is a widely popular March-blooming rhododendron. A faster-growing dwarf hybrid, It also features lavender-pink flowers, but is well-known for its small, 2-inch-long leaves. Several later-blooming varieties of 'PJM' are also available for landscape use.
Most rhododendrons prefer part shade and acidic, well-drained soil with plenty of organic material. The best time to prune or fertilize is just after bloom. Most early rhododendrons, however, are dwarf varieties and will rarely need pruning to control plant height. Use an acid-based fertilizer specifically labeled for rhododendrons. Early-blooming rhododendrons require protection from late-winter frosts.
Rhododendrons are susceptible to root problems if planted too deeply. Make sure the top of the root ball is planted even with the soil level to ensure healthy growth and prevent root rot.
Lace bugs are widely common pests that attack rhododendrons and azaleas. They cause yellow spotting as they suck juices from the underside of plant leaves. Insecticide sprays are the best treatment for such pests. Root weevils are problems as well. Adults chew leaves at night and lay eggs at the base of the shrub. The eggs hatch and the larvae feed on plant roots, eventually girdling the plant. This blocks the transfer of nutrients between the roots and leaves. Keep debris away from the base of the plant and use an insecticide spray during evening hours to control root weevils.