Rhododendron (Azalea hybrid - Knapp Hill-Exbury)
Knapp Hill-Exbury Azalea
There are hundreds of species and thousands of varieties of rhododendrons. Most horticultural selections are vibrantly flowering shrubs, but the group includes trees and ground covers as well. Botanists have arranged species into series and subseries; one of these series includes the azaleas. With careful selection, gardeners in all climates—even the coldest, hottest, and driest ones—can find ways to grow certain members of this genus (in containers or as houseplants, if need be).
Rhododendrons and deciduous azaleas generally do best in Zones 4—6, 15—17, and 32—42, though there are many exceptions. Evergreen azaleas, however, are grown extensively much farther south—in the deep South, Southern California, and Hawaii, for example. But see variety listings for specifics.
Rhododendrons and azaleas need more air in the root zone than almost all other garden plants, but they also require a constant supply of moisture. Without excellent drainage, they can succumb to root rot. Soils rich in organic matter have the desired qualities; if your soil is deficient, amend it liberally with organic material.
In clay or alkaline soil, planting in a raised bed is the simplest way to give these plants the conditions they need. The finished bed should be 1–2 ft. above original soil level. Mix a generous amount of organic material into the top foot of native soil; then fill bed above it with a mixture of 50 percent organic matter, 30 percent native soil, 20 percent sand. This soil mix will be well aerated but moisture retentive and will permit alkaline salts to leach through. Plant azaleas and rhododendrons with top of root ball slightly above soil level.
Plants are surface rooters and benefit from a mulch such as pine needles, oak leaves, or wood by-products (for example, redwood or fir bark or chips). Never cultivate around these plants.
Sun tolerance of azaleas and rhododendrons differs by species and variety. Too much sun causes leaf centers to bleach or burn, though most of these plants can take full sun in cool-summer areas. Ideal location is in filtered shade beneath tall trees; east and north sides of house or fence are next best. Too dense shade results in lanky plants that bloom sparsely. In areas where dissolved salts accumulate, periodically leach the planting by watering heavily—enough to drain through the soil two or three times. If leaves turn yellow while veins remain green, plants have iron chlorosis; apply iron chelates to soil or spray foliage with iron solution.
Insects and diseases seldom cause well-grown plants much harm. Root weevils are the main problem, especially in the Pacific Northwest. Damage by root weevil adults, which notch leaves, is usually minor, but their larvae can girdle roots. A number of pesticides that range from acephate to pyrethrins can help in serious infestations.
During recent years, leaf mildew has become a problem on some varieties of rhododendron. If you want to grow a variety that is susceptible, plant it in a sunny spot that gets plenty of air circulation. The best prevention, however, is to plant resistant varieties. Both wind and soil salts burn leaf edges; windburn shows up most often on new foliage, salt burn on older leaves. Late frosts often cause deformed leaves.
Pruning rhododendrons is simple; follow these general guidelines. Tip-pinch young plants to make them bushy; prune older, leggy plants to restore shape by cutting back to a side branch, leaf whorl, or cluster of latent buds. Do any extensive pruning in late winter or early spring (wait until danger of frost is past in colder areas). Pruning at this time will sacrifice some flower buds, but the plant’s energies will be diverted to latent growth buds, which will then be ready to push out their new growth early in the growing season. (Several varieties will not produce new growth from latent buds.) You can do some shaping while plants are in bloom, using cut branches in arrangements. To prevent seed formation, which can reduce next year’s bloom, clip or break off spent flower trusses, taking care not to damage growth buds at base of each truss.
Evergreen azaleas are dense, usually shapely plants; heading back the occasional wayward branch restores symmetry. To keep bushes compact, tip-pinch frequently, starting after flowering ends and continuing until July.
Prune deciduous azaleas while they are dormant and leafless (in cold climates,wait until frost danger is past). You don’t have to prune azaleas as carefully as you do rhododendrons—the leaves are fairly evenly spaced along the branches,with a bud at base of each leaf, so new growth will sprout from almost anywhere you cut (in either bare or leafy wood). Azaleas can even be sheared into formal hedges.
Plants vary from upright to spreading, 4 to 6 ft. tall. They produce the largest flowers found on deciduous azaleas (up to 5 in. across), borne in clusters of 7–18. Blossoms are sometimes ruffled or fragrant, in colors ranging from white through pink and yellow to orange and red, often with contrasting blotches. Both Knap Hill and Exbury azaleas come from the same original crosses.'Cannon's Double'
Grows 4—6 ft. tall and wide, with pink flowers.'Klondyke'
Flowers come in a golden tangerine color on a 4 to 6 ft. plant.
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