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RHODODENDRON


How to Grow Rhododendrons and Azaleas


EXPOSURE Though sun toler- ance differs by species and variety, most fare best in fi l- tered sunlight beneath tall trees; east and north sides of house or fence are next best. Where summers are mild or foggy, they can take more sun, but the sunnier or hotter the climate, the more protection plants will need. Too much sun causes leaf centers to bleach or burn, while too much shade results in lanky plants with sparse bloom.


R


SOIL & WATER Rhododen- drons and azaleas need more air in the root zone than almost all other garden plants, but they also require a con- stant supply of moisture. Reg- ular watering is one part of the equation, and excellent drainage is the other. If water is allowed to stand, plants can succumb to root rot, which makes them turn yellow, wilt, or collapse. Soils rich in organic matter have the desired quali- ties; if your soil is defi cient, amend it liberally with organic material before you plant. In clay or alkaline soil, plant in bed raised 1–2 ft. above original soil level. Mix a gener- ous amount of organic mate- rial into the top foot of native soil; then fi ll 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 mois- ture retentive and will permit alkaline salts to leach through. Plant azaleas and rhododen- drons with top of rootball slightly above soil level. Plants are surface rooters that bene- fi t from a mulch such as pine needles, oak leaves, or bark or wood chips. Never cultivate around these plants. 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 an iron solution.


FERTILIZER Some experts dig fertilizer into the soil to get new plantings off to a good start. Feed established plants with commercial acid fertilizer well before bloom, as buds swell (in early spring, except for the earliest bloomers). Feed again when new leaves begin to grow, just as blooms are fading. Top-dress with compost in early fall.


PINCHING & PRUNING These are essential for shaping plants and encouraging thick, bushy growth.


Rhododendrons. Tip-pinch young plants to shape them and 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 extensive pruning in late winter or early spring (wait until danger of frost is past in colder areas). Pruning at this time will sacri- fi ce some fl ower 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. (There are several varieties that 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 fl ower trusses, taking care not to damage growth buds at base of each truss. Azaleas. Evergreen azaleas are dense, usually shapely plants; heading back the occa- sional wayward branch restores symmetry. To keep bushes compact, tip-pinch frequently, starting after fl owering ends and continuing until July. Prune deciduous azaleas while they are dormant and leafl ess (in cold climates, wait until frost danger is past). You don’t have to prune azaleas as carefully as you do rhodo- dendrons—the leaves are fairly evenly spaced along the branches, with a bud at base of each leaf, so new growth


will sprout from almost any- where you cut (in bare or leafy wood). Azaleas can even be sheared into formal hedges.


CHALLENGES If you choose the right plant for your region and your garden’s conditions, you’ll fi nd rhododendrons mostly trouble-free. Still, there are problems to watch for. Root weevils. Especially


in the Pacifi c Northwest, root weevils are the main insect problem. Root weevil adults cause cosmetic damage by notching leaves, and their lar- vae can do major damage by girdling roots. A number of pesticides that range from acephate to pyrethrins can help in serious infestations, as can natural controls such as parasitic nematodes. To keep root weevils from climb- ing into the rhododendron’s canopy at night to chew leaves, girdle the trunk with a sticky insect barrier. Mildew. 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 pre- vention, however, is to plant resistant varieties.


Sudden oak death (Phy- tophthora ramorum). Most susceptible of those we list here are R. augustinii, R. catawbiense hybrids, R. macro- phyllum, and R. mucronula- tum. Least susceptible are R. keiskei and R. degronianum yakushimanum, and any rho- dodendron with substantial indumentum (a covering of fi ne hairs) on the leaves. Symptoms appear as dieback along midribs of leaves, start- ing either at the leaf tip or at the petiole. Infected plants and the leaves and mulch beneath them should be removed and burned or sent out with the trash. Leaf burn. 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.


with blossoms in a combination of light pink, deep pink, and white.


Rutherfordiana hybrids.


Zones 14–24. Greenhouse plants; also good in garden where temperatures don’t go below 20°F (–7°C). Bushy, 2–4- ft. plants with handsome foli- age. Medium-size blossoms may be single, semidouble, or double. Available varieties include ‘Alaska’, white with chartreuse blotch; brick red ‘Dorothy Gish’; orchid-pink ‘L. J. Bobbink’; and ‘Red Ruffl es’.


Satsuki hybrids. Zones 5– 9, 14–24. Includes plants sometimes referred to as Gumpo and Macrantha azaleas. Hardy to 5°F (–15°C). Low- growing plants, some of them true dwarfs; many are pendent enough for hanging baskets. Large fl owers come late. Popu- lar varieties include orange-red ‘Flame Creeper’; white ‘Gumpo’; rose-pink ‘Gumpo Pink’; bright pink ‘Hi Gasa’; red ‘Nuccio’s Wild Cherry’; and ‘Nuccio’s Wild Moon’, white with a bright purple border.


Southern Indica hybrids.


Zones 8, 9, 14–24. Most take temperatures of 10°F to 20°F (–12°C to –7°C), but some are damaged even at the upper end of that range. These include varieties selected from Belgian Indica hybrids for vigor and sun tolerance, and hybrids devel- oped by California’s Monrovia Nursery, using Belgian Indica and Southern Indica azaleas. They generally grow fast into vigorous 4–6-ft. mounds, thrive in warm, dry areas. Often sold in California under the name “sun azaleas.” Used for mass- ing and as specimens—as shrubs, standards, espaliers. Popular choices include carmine- red ‘Brilliant’; salmon-pink ‘Duc de Rohan’; ‘Fielder’s White’; brilliant rose-purple ‘Formosa’ (also sold as ‘Coccinea’, ‘Phoe- nicia’, ‘Vanessa’); light pink ‘George Lindley Taber’; ‘Ivery- ana’, white with orchid streaks; ‘Little John’, a dense, rounded bush up to 6 ft. tall (despite the name) bearing burgundy foliage and a few deep red fl owers; brilliant red ‘Pride of Dorking’; and watermelon pink ‘Southern Charm’ (sometimes sold as


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