A common site this time of year is the blooming of the Rhododendrons and the Azaleas. Did you know that all Azaleas are actually Rhododendrons taxonomically, but that not all Rhododendrons are Azaleas? The botanical genus Rhododendron is in the Ericaceae family which includes blueberries, cranberries, and mountain laurels. The genus Rhododendron is made up of almost one thousand species of flowering shrubs and each species is separated into subgenera and subsections. Most of these woody plants on the market are native to Asia, or are hybrids of Asian species, however the popularity of native ones is growing.
Rhododendrons 101
Rhododendrons and azaleas are available in many shapes and sizes. Ranging from small groundcover-like shrubs to large shrubs that rival the size of some trees. The most important thing to consider when selecting and planting a rhododendron is the mature size and the environmental conditions of the site.
With very few exceptions, rhododendrons require well-drained soil that is high in organic content. Continually wet soils can cause root problems and weaken the plant. Soil that is high in organic content helps with drainage as well as soil aeration that helps root growth.
Rhododendrons require acidic soil (soil with a pH between 4.5 and 6.0), but do not use aluminum sulfate to change the acidity of the soil as it is toxic to the root systems of rhododendrons. Instead use wettable sulfur or ferrous sulfate. Due to the acidic requirements, you should avoid planting them near concrete driveways, sidewalks, or patios because of the possibility of lime leaching out to these areas. Rhododendrons are also not tolerant of road salt because it changes the pH of the soil and can make the soil to alkaline.
They generally bloom on old wood, meaning the flower buds in spring were set during the previous summer. Since the blooms are set so early it is not advisable to prune a rhododendron until right after flowering, and the window of opportunity exists for about three weeks after flowering.
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