As with all plants the roots are designed to absorb water and nutrients and to anchor the plant in the surrounding soil.
Rhododendrons usually grow on rocky ground or mountain slopes in their countries of origin; some tropical species are even epiphytes (which means that they grow on other plants but can nourish themselves independently by drawing moisture from the air).
In the wild, even when rooted in the ground, these shrubs usually have only a thin layer of humus at their disposal. For this reason, over the course of time, they have developed a widely branching, very flat and spreading root system through which they can absorb vital nutrients from the surface layers of the soil.
This has proved an advantage as rhododendrons are thus easy to transplant because of their shallow rooting system. On the other hand, they may also dry out rather easily because their roots cannot penetrate into the deeper levels of the soil, so they need a regular supply of water and mulch.