Horticulture Report: May 2013

Rhododendrons & Azaleas
What’s the difference between Rhododendrons and azaleas?

Azaleas are Rhododendrons. They belong to the subgenus pentanthera (deciduous) and the subgenus Tsustusti (mostly evergreen).
Azaleas are elepidotes, they never have scales.
Azaleas have five lobes to the flower.
Most azaleas have only one stamen for each lobe of the flower, while most other rhododendrons have two stamens for each lobe.
Azaleas tend to have appressed hairs (hairs that grow parallel to the surface of the leaf). This is particularly true along the midrib of the under-surface of the leaf and is easily seen in the so called “evergreen” azaleas.
Azaleas have tubular funnel or funnel shaped flowers. You need a microscope to see this, but the hair on a “standard” rhododendron will often branch, while the hair on azaleas never does.

Basic Needs:
Rhododendrons must have a constant supply of moisture.
They are sensitive to poorly drained conditions.
Hot wet conditions are more dangerous than cool, wet conditions.
They need an acid soil of pH 5-6.
Fertilizer: A 10-6-4 formula is optimal. For the best growth and flowers apply fertilizer after the plant goes dormant between late Nov and Jan, a second application in Feb or March, a third time in April or May, and a final time in June or July.

Always prune right after flowering. Even large plants can be cut back. Rhododendrons & azaleas flower on the prior year’s wood, therefore the buds for next spring’s flowers form during the previous summer or fall. If you prune them later in the summer, fall or winter you will be trimming off your flowers for the spring.
Prune larger leaved rhododendrons just above the growth joints.
Prune azaleas and small leafed rhododendrons anywhere along the stem. These plants have dormant growth buds nearly everywhere, though you may not be able to see them.

Deadheading and Pinching:
Light pruning and shaping should be done yearly when you deadhead. Snap off the old flower trusses. You may pinch out the single terminal growth buds just as they start to grow. It will make a shorter and bushier plant.

For more information:
American Rhododendron Society

American Azalea Society

Elepidotes are large leaved rhododendrons. They are the type of shrub that most individuals would associate as being a rhododendron. They do not have scales located on the underside of the leaves. Plants tend to be very large in their maturity.

Lepidote rhododendron have smaller leaves and are usually low growing or dwarfs. They usually bloom earlier in the spring than the larger leaved elepidotes. Tiny scales cover the undersides of the leaves of some rhododendron species.