Rhododendrons are healthiest when the acid levels in the soil are optimum (pH 4.5-6.0). They also have an affinity for shade and precipitation. But why do they have an affinity for acid and moist shade? Let's have a look in depth.
"Rodon" means Rose and "dendron" means Tree, referring to rose tree. If you have seen Rhododendrons on high elevations, you might know what I am referring too. My first memory of Rhododendrons is seeing them growing in the wild in a tree form on high elevations in northern India as well as in tree forms on slopes at Mendocino Coast Botanical Gardens (MCBG). (check out: https://www.gardenbythesea.org/…/photo-galle…/rhododendrons/).
Dexter rhododendrons in woodland gardens
Rhododendrons originated on high elevations. Slopes, elevations and rocky sites couldn't be more prefect for drainage and for all of the Ericaceae family. "Their native habitat indicates that Rhododendron generally inhabit mountains. The climatic and soil conditions or mountains areas appear most congenial to their growth and can be characterized by cooler temperatures and more precipitation. These same climatic conditions readily evolve acidic soils and a buildup of organic matter on top of the soil. Furthermore mountain soils are formed by erosion of bed rock, resulting in a coarser, well drained soil" (reference book: Reiley H. E. (1992), Success with Rhododendrons and Azaleas, Timber Press Inc.)
In terms of companion plants, for upper canopy: any trees that can provide filtered light and those with deep root systems, that won't compete with the shallow rooted Rhododendrons like Hollies, Dogwood, Firs and Pines will add texture and filtered light. At London Town, the lower canopy layers features many ground covers that need the same habit: Solomon' seal, Lily of the Valley, Snowdrops and Ferns.
Azalea with sport (genetic mutation) resulting in multiple colors
Companions when not compatible, but competitive are grown together, stress is apparent. But companions don't always refer to plants, think about soil, think also wildlife. All of that makes a cohesive habitat. Speaking of wildlife, Butterflies are probably one of the most compatible wildlife for Rhododendrons (we will explore this soon). Also, all of the above applies to Azaleas as well, since 'all Azaleas are Rhododendrons, but not all Rhododendrons are Azaleas'.
Butterfly as companion