Mountain laurels and rhododendrons belong to the heath family, Ericaceae. Both grow best in moist, well-drained, organic-rich soils with an acidic pH. The plant tissues contain chemical toxins that can cause stomach pain or death if eaten in large amounts.
Mountain laurel (Kalmia latifolia) naturally grows only in the eastern United States' forests. Rhododendrons (Rhododendron spp.) include about 1,000 different species native to various habitats across the Northern Hemisphere. Roughly 28,000 different cultivars exist today -- the result of breeding and selection.
Mountain laurel is evergreen, with pointed oval leaves, while rhododendron's leathery round-tipped leaves curl up like cigars in sub-freezing temperatures. Small mountain laurel flower buds look crimped, while rhododendron buds are plump and pointed. A laurel flower resembles a tiny cup or bowl with five points or minuscule lobes. Rhododendron blossoms are larger, with obvious five-petal lobes.
Many rhododendron cultivars reveal various tolerances to other growing conditions, plus all azaleas are botanically rhododendrons, too. Mountain laurels include several varieties with different mature size or flower color. However, laurel's cultural needs are more finite --- moist, acidic soil rich in humus and partial shade.