Rhododendron is a genus of a shrub with about 800 species worldwide. Its ovate evergreen or deciduous leaves are alternate, 1/2 - 8 inches in length depending on variety, with smooth untoothed margins. They are dark green with a glossy upper surface and a dull underside. Large trusses of bell-shaped flowers bloom from spring to early summer. Plants are available with flowers in colors such as white, purple, deep rose, red, yellow, and orange. Rhododendron and its closely related azalea have been hybridized for many uses in gardens and rarely reach above 3-5 feet tall in northern states including Illinois.
In Illinois, most species found are ornamental types that usually thrive in protected areas of gardens.
Tall, wild varieties can reach over 35 feet high, and are found throughout the coastal mountain ranges from New York to Georgia. Designated as West Virginia's state flower, rhododendrons are particularly abundant in the Great Smoky and the Blue Ridge mountains. Species in the Pacific northwest from northern California to British Columbia vary in heights.
Conditions of Poisoning
All parts of the plant, but especially the foliage, contain the poison, and two or three leaves may produce severe toxicosis. Sucking flowers free of nectar may produce serious illness. Rhododendrons are more likely to retain green leaves year round than are most other plants, and therefore most toxicoses occur in the winter and early spring, when other forage is unavailable.
All parts of this plant contain toxic resins (andromedotoxins, now commonly referred to as grayanotoxin) with the leaves being the most potent. Grayanotoxin produces gastrointestinal irritation with some hemorrhage, secondary aspiration pneumonia, and sometimes renal tubular damage and mild liver degeneration.
Clinical signs usually appear within 6 hours of ingestion. Affected animals may experience anorexia, depression, acute digestive upset, hypersalivation, nasal discharge, epiphora, projectile vomiting, frequent defecation, and repeated attempts to swallow. There also may be weakness, incoordination, paralysis of the limbs, stupor, and depression. Aspiration of vomitus is common in ruminants and results in dyspnea and often death. Pupillary reflexes may be absent. Coma precedes death. Animals may remain sick for more than 2 days and gradually recover.