Mendocino Coast Botanical Gardens, 47 acres of botanical bliss fronting the Pacific Ocean

Growing and Caring for Rhododendrons

Before choosing the site, consider the plant growth rate and how it will fit as it matures. Avoid planting in dry soils, too close to your foundation, under roof overhangs, or under trees with aggressive root systems or dense shade.

In their native habitat, rhododendrons grow in acid soils rich in organic matter. They enjoy loose soil that is cool and shaded, and their shallow surface roots can be disturbed by cultivation. Plant rhododendrons on slightly raised mounds, then pack forest duff and native soil around the root balls, and water them carefully until they are established.

Rhododendrons and azaleas are best planted in early fall or spring, but can be planted nearly year round with extra care. Large-leaf rhododendrons and evergreen azaleas generally prefer a site protected from drying winds and hot summer/winter sun. Small-leaf rhododendrons and deciduous azaleas tend to be tolerant of more exposed planting sites.

Rhododendrons and azaleas grow best in light, well-drained soils with good soil aeration and an ample supply of soil moisture during the summer. They can be grown in heavy soils if special precautions are taken. A soil high in organic matter or humus such as decayed oak leaves, pine needles or other acid-type compost is desirable for growing rhododendrons and azaleas. It is not suitable for mixing with heavy clay soils. In hot or alkaline soil areas where rhododendrons are frequently grown in straight pine bark with little or no soil added, care should be used in obtaining a fine grade or small particle pine bark. Rhododendrons and azaleas, in general, require an acid soil with pH about 5.5. Soils with pH higher than 5.5 should be acidified.

In the coastal climate, rhododendrons and azaleas are fairly drought resistant, but because of their broad leaves that constantly lose water through transpiration, water is a priority in their maintenance. Old, settled plants can survive long periods of drought in late summer when not in active growth, but if foliage shows wilting in the morning, the plant needs a deep soaking. Daytime wilting over long periods will stress plants with consequences of diseases such as twig-blight or die-back.

Newly transplanted plants require close attention to watering and should not wilt for any extended time. Misting new growth during the heat of the afternoon is beneficial (in high temperature areas mist in the morning or after the sun goes down to avoid burning). Check the soil beneath the mulch weekly to be sure it is moist but not wet. Forming a saucer or berm near the outer edge of the hole of newly planted rhododendrons will hold water to irrigate the roots.

Extra attention to water needs of newly transplanted plants is necessary for up to three years, depending on how far out into the soil the roots extend. Each year, the intensity of necessary care declines as the root system develops, making rhododendrons easier to care for as they grow older in your garden. Big leaf rhododendrons, which are from very wet areas, need to be watered more frequently, while most other rhododendrons need to be watered every few weeks.

In fertile soils rhododendrons and azaleas can be grown well without receiving further fertilization. However, if plants are mulched with something like fresh sawdust or wood chips, there will be a nitrogen demand caused by the decomposition of these materials, and unless nitrogen is added, the plants are likely to show yellowish foliage and poor growth. In this case a fertilizer such as ammonium sulfate should be added. It is safer to use mulches other than those containing fresh sawdust or wood chips, then you don't have to be concerned with exactly how much extra fertilizer to add, as excess fertilizer can harm your plants by "burning" the roots.

For rhododendrons planted in less fertile soils, a complete fertilizer designed for acid-loving plants may be applied in late winter or early spring. Be careful to use only the amounts recommended for rhododendrons and azaleas, which normally require less fertilizer than plants such as grasses and vigorous shrubs. In cold climates, nitrogen fertilizer should not be applied after late June as it may promote new lush growth susceptible to winter damage. Recent research indicates that plants reasonably well supplied with nutrients, including nitrogen, are more resistant to low temperatures than those that are starved.

Phosphorus in fertilizer tends to favor early production of flower buds. If your soil is deficient in phosphorus and since phosphorus does not readily move through the soil, phosphorus should be incorporated into the soil at planting time if needed. Magnesium in the form of Epsom salts is sometimes recommended for rhododendrons. Magnesium is an essential element and lack of it will cause yellowish areas between the leaf veins on older leaves. If the leaves are a solid green the addition of Epsom salts would not be useful.

Lack of iron causes much the same symptoms as lack of magnesium, but with the younger leaves showing yellowing between the veins. Iron deficiency is frequently caused by too high a soil pH, often the result of mortar or mortar building debris in the soil near the roots. A soil test should be performed to see whether high pH is a problem and if it is the soil should be acidified. For a quick but temporary solution, ferrous sulfate can be added to the soil or chelated iron can be sprayed on the foliage, but the pH should be corrected for long term good growth.

Calcium is also essential to good rhododendron growth. Calcium can be obtained either from gypsum or from agricultural lime. Gypsum will not raise soil pH, while lime will, therefore, lime is not generally recommended in areas with naturally alkaline soil or water.

Pests and diseases
A widespread insect pest is the root weevil. This may be one of several different species. The damage done by the adults shows up as notches chewed in the edges of the leaves. Greater damage, however, is done after the adult weevil lays eggs on the ground under the plant. The eggs hatch into larvae which feed on the roots and stem, often completely girdling (removing a ring of bark) and killing the plant. Control is difficult because the adults move around freely, often laying eggs under plants on which they have not chewed. By the time root damage is noticed, it is too late to save the plant. Symptoms of a girdled plant are much the same a a dry plant, since the plant can no longer supply moisture from its roots to its top. The insecticide Orthene offers some control. In certain areas a stem borer may cause damage to azaleas and rhododendrons. The adult beetle, which usually appears in June, makes two girdles at the tip of the growing shoot about ½ inch apart and inserts an egg between the girdles. The larva bores downward, expelling frass from holes cut through the stem and pupates in the crown of the plant, just below the soil surface. The weakened stems are easily broken off and die. The girdled tips should be cut or broken off, as soon as observed, and destroyed. Be sure to cut low enough to eliminate the tiny borer whose threadlike brownish tunnel can be seen just below the girdles. In some areas aphids, mites, scale insects, thrips or leaf eating caterpillars may cause some damage and may be controlled with a spray if they appear.

There are several important diseases of azaleas and rhododendrons. They can usually be avoided by purchasing and planting healthy plants, planting so that the plants have excellent drainage, mulching to conserve water, providing cold protection if needed and moisture when needed as well as nutrients based on a soil test, and pruning out dead and dying stems and remove from the vicinity of the plant. Root rots that occur in some landscapes cannot be controlled once active in rhododendrons. Every effort should be made to prevent them from occurring. Powdery mildew caused by two different fungi occurs primarily on the leaves of certain hybrid deciduous azaleas. Usually the disease occurs late in the summer. Rust diseases that attack the leaves of rhododendrons and deciduous azaleas appear as yellow pustules primarily on the lower side of the leaves. Usually, rust appears late in the summer and chemical control is probably not practical.

Pruning and Maintenance
Properly sited rhododendrons and azaleas need minimal pruning maintenance. If branches must be trimmed, this is best done right after flowering finishes to enable the best re-growth and flowering the following year. Pinching back of new growth can create a busier growth habit. Deadheading, the twisting off faded flower heads soon after blooming, enhances bud-set the following year by preventing energy from being expended in seed production and tidies up the plant. Be careful not to damage vegetative growth shoots.

For more information on rhododendrons and azaleas, see the website for the American Rhododendron Society at, or contact the local Rhododendron Society Chapter.