Sonoma Horticultural Nursery

Rhododendron & Azalea Care & Culture

Planting Environment

Rhododendrons and azaleas will do well in most locations if they are protected from the hot, mid day sun, grown in well-draining areas, and regularly irrigated and fertilized. Areas with morning or late afternoon sun, dappled sun through overhead branches, or against north facing walls or fences with open sky above, are all good locations. They bloom best with some sun, and insufficient light is often the cause of poor blooming.

Planting Mix

The most important factor in successfully growing rhododendrons is the planting mix. Rhododendrons have a fibrous, wide-spreading root system that needs moisture and oxygen to flourish. The greatest enemy of these plants is phytophthora (root-rot fungus), making good drainage essential. In poorly draining areas, or in clay soils, we recommend building raised beds or mounding on top of the existing ground level. In areas like ours where the summers are warm and dry, 100% organic material is necessary. We use a mixture of ground redwood bark composed of large and small particles. Recent research seems to indicate that there is a natural suppressant of phytophthora fungus in tree bark. Some rhododendrons are alpine plants, and therefore demand even better drainage. With these plants, we use a mix of 25% 3/8” crushed lava rock and 75% redwood bark. The crushed lava rock is beneficial because it never breaks down, is relatively light weight, and is porous. It is always best to plant the root-ball above the existing ground level, building up to the sides with bark. By keeping this mound of bark under the plant evenly moist in the warm months, the evaporating moisture helps to humidify the shrub. The addition of 3 to 4 inches of new bark or humus each year will insure that the plants will continue to thrive and will also help with weed control.


Rhododendrons require an acidic soil to grow properly. The use of a specially formulated fertilizer for azaleas and rhododendrons is usually sufficient to both feed the plant and lower the soil pH. After years of experimentation, we now fertilize once a year, in spring, with Osmocote 15-9-12 plus minors. The ratio between the three major elements (nitrogen, phosphorous, and potassium) is very important. Excess phosphate build-up can burn the leaves, while too much nitrogen can result in poor flowering. It is also important to have the trace elements (minors) in a soilless culture such as bark. Osmocote is encapsulated in plastic, releasing a small amount of fertilizer each time you water, over a period of 8 to 9 months. Since we started using Osmocote, we have experienced less burning, better growth, and increased sets of flower buds.

Deadheading, Pinching, and Pruning

Removing the spent flowers (deadheading) is important for rhododendrons and deciduous azaleas, otherwise plant energy is wasted on seed production. At the same time, pinching out the largest terminal growth buds just as they start to grow will produce a bushier, compact plant. Pruning into older wood, if needed, should also be undertaken in late spring, but will result in less flowers the following year. Evergreen azaleas can be sheared (even quite severely) as hedges, but do not need deadheading.