Your success at growing rhododendrons and azaleas depends upon a few key factors.
BEFORE PLANTING: Both location and soil improvement should be considered before planting. The location should be in part shade. In the Northwest a full sun location might be acceptable if certain conditions are met. Many rhododendrons are sun tolerant. Generally these are red or purple. Also tolerant are those with small leaves. Soil Improvement is crucial. These plants require soil that is both well drained and yet moisture retentive. Sandy soil is not able to hold the moisture well enough and clay soil is to wet in winter and too dry in summer. Rhododendron roots are not fibrous enough to penetrate heavy soil. This means that unless you are planting in existing areas that support existing rhododendrons, it is imperative that you add organic matter to the existing soil or import a planting mix and plant in raised beds or berms. A soil amendment such as Blended Mint Compost from Lane Forest Products mixed into the top 18 t0 24 inches of existing soil will create a mix that rhododendrons grow in naturally in their native habitat. If you are going to plant in a raised bed use Lane Forest Products organic planting mix called Nature’s Best Planting Mix. This material contains a beautiful fertilizer mix that will slowly feed the root zone. It also has the texture that rhododendrons enjoy.
AT PLANTING: Rhododendrons and azaleas are shallow rooted so you must be careful not to plant too deep. Ideally 2 to 3 inches above grade in a wide mound. It is best to take the plant out of the container and lightly loosen the roots so that some of the roots are surrounded by the new soil. As the hole is filled you should water as you go. If you have mixed a soil amendment with your soil, it is a good idea to apply water with pressure to thoroughly mix the two types. This action is like what a river does to produce some of the best soil possible. Your goal here is to make mud. You will not need to make the soil this wet ever again but this helps over come the planting “shock” that new planting sometimes suffer especially if future irrigation depends upon automatic systems. Check the root zone moisture in a few days and adjust if needed.
AFTER PLANTING: If you follow the steps above, the rest is easy. Even experts disagree about fertilizer timing but rhododendrons like food, just not all at once. Lane Forest Products sells a slow release lawn fertilizer called Pro-Spring Plus. This fertilizer will add the Iron that these plants need to help avoid clorosis, a condition often seen in which the leaf is green but the veins of the leaf is yellow. Pruning, if necessary is best done as the flowers fade or soon thereafter. Growth takes place at the small bud just above a leaf or a leaf scar (where a leaf once was attached). Summer pruning if too late, will remove next year’s potential buds.
Properly planted and cared for, rhododendrons and azaleas experience few major problems from insects and disease. However, in the Northwest root weevil can become a major pest. Observe the NEW leaves as they emerge each spring and watch for notches (not holes) on the edge. This is caused by the adult weevil and is only unsightly but this creature will lay eggs that will hatch underground and the resulting larvae will feed on the roots during late winter and early spring. The best way to control this problem is to eliminate the adult before it is old enough to lay eggs. The adult feeds at night and hides under the mulch during the day so a systemic insecticide such as Orthene works best. Follow direction carefully. A once a month application may be necessary in severe cases. Once new damage is not seen, the spraying can be stopped. For organic control, make sure leaves and branches do not touch the ground and apply double stick tape to all stems that reach the ground. Weevils do not fly and only reach a new leaf by crawling up the plant to reach its favorite nighttime snack.