About Rhododendron L.
Rhododendron maximum, commonly called rosebay rhododendron, is an evergreen shrub. It usually grows 4 m (13 ft) tall, and rarely reaches 10 m (33 ft) tall. Its leaves are sclerophyllous, simple, alternate, and oblong. Most sources note leaves measure 9–19 cm (3.5–7.5 in) long and 2–4 cm (0.79–1.57 in) broad, while another range given for leaf size is 10 to 30 cm long and 5 to 8 cm wide. This species retains its waxy, deep-green leaves for up to 8 years, and shed leaves decompose very slowly. The leaves can be poisonous. Its flowers measure 2.5–3 cm (0.98–1.18 in) in diameter, and are white, pink, or pale purple, often marked with small greenish-yellow spots. It produces large, showy flowers ranging from white to purple each June and July, though flowering can occur from March through August. The fruit is a dry oblong capsule 15–20 mm (0.59–0.79 in) long, which contains numerous small seeds. The capsule ripens in fall and splits along its sides shortly after ripening to release its seeds, with roughly 400 minute seeds per capsule.
Rosebay rhododendron is the most common and dominant Rhododendron species in the southern Appalachian region, and occurs occasionally on mesic hill-slopes across the upper Piedmont Crescent of the Southeastern United States. Approximately 12,000 square miles of habitat in the southern Appalachians are occupied by this species, where it dominates the forest understory. Historically, this species was confined to riparian areas and other mesic sites, but where it is already present, it takes advantage of disturbed areas to advance onto sub-mesic sites. It prefers deep, well-drained, acid soils high in organic matter, and forms a thick, peat-like humus in these sites. It grows best in low to medium light conditions for optimal carbon gain, and has a strong ability to avoid cavitation during freeze-thaw cycles. Where extensive overstory mortality has removed most of the overstory, this species forms a thick, continuous subcanopy that is locally called 'laurel slicks' or 'laurel hells'. Rosebay rhododendron is an important structural and functional part of southern Appalachian forest ecosystems. It remains unclear whether this species is currently undergoing a period of advancement or retreat. For example, large areas of rosebay rhododendron — particularly at high elevations, on poorly drained ridge or upper slope sites — have died out in recent times, likely due to infection by the Phytophthora fungus, or to recent prolonged periods of below-average precipitation. However, rosebay rhododendron now grows on sites that were historically free of evergreen understory. Important questions remain to answer to fully understand this species' role in forest understories. In North Carolina, United States, a genus of ascomycete fungi called Pestalotiopsis causes grey-brown spots to form on the species' living leaves.
Rosebay rhododendron is clonal, and can reproduce both vegetatively and sexually. Vegetative reproduction occurs through layering: it produces roots from above-ground woody parts when these parts come into contact with the forest floor. Requirements for the seed germination microsite are relatively specific, for example needing substrates high in organic matter like rotting logs. Because of this, the majority of reproduction is vegetative, leading to a clonal distribution pattern.
In cultivation, rosebay rhododendron seeds are extremely small; it is estimated that 1 kilogram of seeds contains approximately 11 million individual seeds. Most commercial seed production comes from cultivated hybrids, and seeds from wild rosebay rhododendron are not commonly sold commercially. Rosebay rhododendron is slow-growing, and has very high sprouting potential. If mechanical removal is attempted for forest management, this species regrows to extremely high densities within just a few years. Prescribed fire has also been used to control this species, but with only limited success.
Rosebay rhododendron forms a striking, aesthetically pleasing feature in mesic southern Appalachian forests. It is one of the largest and hardiest rhododendrons grown commercially, and several cultivars with flowers ranging from white to purple have been selected for the horticultural trade. It maintains deep-green foliage year round. In its natural range, it provides protection for steep watersheds and shelter for wildlife. Its wood is very hard, and is occasionally used to make specialty wood products.
