Horticulture Report: Rhododendrons & Azaleas


Rhododendrons are native to Asia, North America, Europe and Australia, with the highest


Black Magic Rhododendron

species diversity in the Himalayas. There are over 1,000 species of rhododendron, woody plants in the Heath family that may be evergreen or deciduous. Azaleas make up 2 subgenera of the rhododendron family.

Some species are poisonous, both the pollen and nectar, yet we have observed honey being sold specifically from bees feeding on rhododendrons.

Growing Conditions:
After over 48 years experience growing both rhododendrons and azaleas in diverse conditions in the U.S., I believe these plants are much tougher than most people appreciate.

Among the false assumptions:
Rhododendrons and azaleas are best grown where summers are cool and moist—not so!
Azaleas are mostly deciduous—not so!

There are two conditions that are critical to success in growing these acid loving plants whose fine roots are primarily at the soil surface:

>>Good Drainage
>> Cool Roots

Whether your soil is clay or decomposed granite, it can be amended to meet the plants needs.

Clay Soil:
With clay there are two approaches, the easiest is to amend the existing soil with good acid compost to loosen it up and allow for drainage. As long as the planting site is elevated from the surrounding soil, you can plant directly into the soil. Another approach is to build a raised area about one foot above the existing soil level using river rock or another material and fill with good soil mulched with acid compost.

Decomposed Granite:
Typically decomposed granite drains rapidly so the addition of a good amount of acid compost will help it retain sufficient moisture to support plant growth.

Keeping Roots Cool:  In Southern Oregon it is very easy to keep plant roots cool as we have easy access to conifer forest detritus. Although fir trees are the dominate locally, we have pine trees which serve as the ideal mulch for azaleas and rhododendrons. Unlike compost, which can get soggy and compacted, pine needles piled around the base of the plants provide a cool, moist environment with good air circulation. As the pine needles breakdown, they help maintain soil acidity.

Selecting Plants



A factor to consider in purchasing rhododendrons and azaleas, is how the plant will fit into your landscape scheme. If you desire the tall lacy feel of rhododendrons reaching for the sky, then look for plants that exhibit an 8 to 10 inch or greater annual growth rate. In contrast, there are many species which grow in a dense, bush-like growth habit or , with the deciduous species, they retain a shrub like height, but exhibit a lacy open growth habit.

Spring is the ideal time to purchase these plants as they are in full bloom, giving you good color selection and an appreciation of some of the perfumed varieties. The disadvantage is often nursery suppliers push the plants causing the roots of some of them to be so compacted that they cannot be pulled a part. Under these conditions the plant will never perform well and I would suggest returning the plant to the nursery.

Submitted by:
Donna Rhee, AGC President