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Rhododendrons and Azaleas

View the file below to learn about shrub care specific to rhododendrons and azaleas! Click here to view more information about general gardening care.

Rhododendrons and azaleas are long lived, woody evergreens that are both in the genus Rhododendron but in different plant families. These shade tolerant shrubs have extremely showy spring flowers, which are uncommon in shade loving plants, and attractive foliage that makes them ornamental even when they are not in bloom. Generally, rhododendrons have long, broad, leathery leaves, although some do have small ‘azalea type’ leaves. Their large bellshaped flowers grow in clusters on branch ends and because of their massive upright growth, rhododendrons grow best when given lots of room in an informal setting. In contrast, azaleas usually have small, narrow pointed leaves, small flowers that grow in large numbers to cover entire branches, and a short, spreading growth habit. They look especially beautiful massed together to create an ocean of color. Rhododendrons and azaleas both can be used to create beautiful hedges and colorful accents in flower beds and borders, and they are indispensable in woodland settings!

LOCATION: With few exceptions, rhododendrons and azaleas should be planted in lightly shaded locations or areas with morning sun and afternoon shade. A few varieties will tolerate full sun. Avoid sites where hot sun is reflecting from light colored houses and paved areas.
Avoid sites exposed to extreme winter cold or wind – such as the corner of your house. DO plant where there is good air circulation to reduce potential for fungus diseases. Avoid dry sites under roof overhangs or under trees, and very dense shade. If your landscape is sloping, do not plant in low-lying frost pockets.

SOIL: Rhododendrons and azaleas require moist, acidic, well-drained soil that is rich with organic
matter. They both have shallow fibrous roots that spread close to the soil surface. These roots have an unusually high requirement for air in the soil. Because of these root growth characteristics, soil must be moist because roots are incapable of deep growth to find water.
But soil must also be well drained, for standing water in the soil will displace the oxygen that rhododendrons and azaleas crave so badly. Planting in the proper soil is extremely important! Clay soil or soil compacted by new construction has poor drainage and is not suitable for rhododendrons and azaleas. Plant only in soil rich with organic matter. If you need to improve very poor soil, be sure to amend a nice large area around each planting spot – NOT just the planting hole. Roots must be encouraged to grow wide spreading – not just pampered in a small spot of rich soil. Also, soil texture differences between poor clay soil and rich organic soil will cause drainage and water flow problems that are disastrous to plant roots. It is best to leave average soil unamended. Poor soil can be improved with 30-50% organic matter such as decayed leaves, aged manure or well-rotted compost. A small amount of peat moss can be used as part of the organic matter – but be aware that soil with higher amounts of peat does not drain well, and does not re-wet readily when it becomes very dry. Remember, if you choose to improve soil, dig up a nice wide area around each planting spot.

SOIL pH: Soil pH is important because it determines which nutrients a plant can take up from the soil. Rhododendrons and azaleas prefer acidic soil with pH 5-6. Use a pH test kit or take a soil test to determine your soil pH. To modify an alkaline soil (pH above 7), add lots of organic matter to the soil before planting and mulch with composted pine bark, leaf mold or soil builders like “Bumper Crop”. You may also use acidic fertilizers but only on well-established plants.

If your soil is very alkaline, it is better to choose plants other than rhododendrons or azaleas that don’t mind the higher pH, because trying to drop soil pH by too much is impractical.

A very acidic pH (below 4 or 5) can be modified by adding pulverized limestone to the soil. Follow soil test recommendations. Be aware that minerals leaching from concrete foundations, patios, and walkways make soil alkaline, so avoid these planting sites if possible.

PLANTING: Rhododendrons and azaleas are easily damaged by planting too deeply. When planted, the top of the root ball should be at the surface of the ground or above it – never deeper. In naturally rich organic soil, dig a hole at least 2 times as wide as and only as deep as the root ball. In clay soils that are less porous and have poorer oxygen and water flow, plant in a raised mound made up of a mixture of soil, compost, aged manure, peat and bark mulch.

Before planting, soak roots very thoroughly then pull apart pot-bound roots so that they will fan out into the soil. As you plant, firm the soil around root ball and be sure to leave no air pockets behind. Water deeply, 2 times during planting to settle soil. Add a 2-inch layer of well-aged mulch to keep roots cool and moist. Be sure not to mound mulch around base of shrub for this, as well as planting too deeply, will kill your plant.

WATERING: Although rhododendrons and azaleas don’t like soggy soil, they do need plenty of water. This is especially important for recently planted ones, because their roots are slow to grow into the surrounding soil. Try watering more often, but not as deeply. Drip irrigation is ideal because it provides just the right amount of water that can be steadily taken up by the roots.

Also, keep in mind that rhododendrons and azaleas root balls are very dense. Once they dry out, they are difficult to re-wet; in fact, they are even capable of shedding water. As a result, the soil around the root ball may be wet, but the actual root area will remain dry. If you suspect that the roots are not absorbing water – often evidenced by leaf wilt or curl, or flowers that droop, drop immediately, or don’t open at all – try gouging a small hole into the center of the root mass to direct water to the roots. Use a dibble, a large spike, or a screwdriver, inserting it 2 to 4 inches away from the trunk. In extreme cases, if a dried-up plant is still of manageable size, dig it up and soak the entire root ball overnight in a container or water.

MULCHING: Mulch to maintain moisture. Mulching helps prevent rhododendron and azalea roots from drying out, keeps plants cool in summer, protects roots from sudden shifts in winter soil temperature, and prevents weeds from competing for moisture. In their natural environment, these shrubs provide their own leaves as mulch and take advantage of the leaves of surrounding trees. In the garden, mulch with coarse bark because it allows air and water to reach the roots. Never use black plastic, or fine peat moss, which will tend to dry out and shed water. Never pile mulch up on the stem because it might rot. Rather, arrange it away from the trunk, creating a ring on top of the periphery of the root ball that will catch the water and direct it to the roots of the plant.

FERTILIZING: In Zone 6 where the ground regularly freezes in the winter, fertilize established plants once in early spring and again in early summer as needed. Avoid applying fertilizer in the fall; this can delay dormancy and cause the plants to be less cold hardy in the first freezes. Use only root stimulating fertilizers on new plantings, save stronger granular and water-soluble formulations for the second or third growing seasons. Select a fertilizer prepared for acid-loving plants. There are many good products on the market, organic or otherwise. Rhododendrons and azaleas need an extra dose of nitrogen to grow and bud well. Try a formula that is higher in nitrogen such as 10-6-4, or one that releases nitrogen over a long period of time.

PRUNING: Deadheading and pinching keep rhododendrons and azaleas compact and in good shape. Deadhead annually after blooming to speed the plant into flower production for the coming year and make the plant look tidier.

After deadheading, the plant sends up growth buds where the bloom was. On the part of the shrub that did not bloom, there is usually only a single growth bud, which can be pinched out. After the single bud has been removed, the plant immediately produces multiple growth buds there. These result in additional branches and a bushier plant. It is possible to prune rhododendrons and azaleas if they get too big for your site. But if you cut back heavily, you won’t have flowers for a year or two because they flower on the previous year’s growth.

Good grooming also includes removing weak interior limbs and any crossed or ground touching branches. Each year, after flowering, when these ornamentals send out new growth, there is a visible point where this growth starts. This is the growth joint, where dormant growth buds are located. Pruning should be done just above a growth joint for optimum rebound. The plants will take severe pruning, though, and reward you with good growth and luxuriant foliage.

TROUBLESHOOTING PESTS: Lace bugs are the most common rhododendron and azalea problem. These tiny pests suck the juice out of the foliage, causing speckled yellowing. Look for damage beginning in April and treat as needed with horticultural oil, or an insecticide in more serious cases. Follow label directions. Other insect pests include root weevils – which attack by chewing notches in leaf margins and laying eggs that hatch into root girdling larva – and borers which tunnel into, and subsequently, kill stems.

Treat adult weevils with an insecticide from mid-June on, as needed. Control borers by cutting out and destroying dead and dying branches and spraying in late spring with borer and leaf miner spray. Repeat as needed according to label directions.

DISEASES: Fungal disease problems on rhododendrons and azaleas include rust and leaf spot. Rust is spread by moist conditions in moderate 55–75-degree temperatures. It cannot be controlled with chemicals. When practical, pick off the yellow or orange-red blistered and speckled leaves (characteristic of rust). Leaf spot is spread by splashing water and wind. Tan, gray, or brown spotting that results from this disease is unsightly but rarely seriously harmful.

Once leaves are spotted, they remain so. Pick off and collect and destroy fallen leaves. Prevent new infections with a fungicide when new growth begins in spring. Repeat as needed following label directions.

CULTURAL PROBLEMS: Acid loving rhododendrons and azaleas will become iron deficient and anemic in soil that is too alkaline (pH is above 6.0). Leaf veins will remain green while other leaf parts turn yellow. To prevent, test your soil and correct soil pH before planting. To treat, add organic matter to planting site and carefully spray chelated iron fertilizer on plants and adjacent soil according to directions.

Clay soil, poor drainage and wet soil conditions cause root rot in rhododendrons and azaleas. Young leaves and stems turn yellow and wilt. Eventually the entire plant dies. Symptoms may develop in just a few weeks or may take months. No chemical control is available. Attempt to save young plants by digging them up and drying them out, then improve drainage or replant in a raised bed or a better site. If applicable, amend watering practices.

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