The Encyclopedia of Psychoactive Plants: Ethnopharmacology and Its Applications details an account of some psychoactive species of Rhodedendrons:
The Osset live in the mountains of the northern Caucasus and are thought to be later descendants of the ancient Scythians. The Oriental scholar Julius Klaproth visited the Osset during the nineteenth century and returned with a description of a divination ritual in which the Caucasus rhododendron (whether the botanical identification is correct remains open) was used as a psychoactive incense (Klaproth 1823,2:223 f.): He described their ardent devotion to the prophet Elias, who was regarded as their greatest protector. In caves consecrated to him, they [the Osset] offered goats and consumed their flesh; after which they spread the skins out under a large tree and honored these in a special fashion on the prophet's feast day so that he would keep away the hail and grant them a bountiful harvest. The Osset would often go to these caves to inebriate themselves on the smoke of Rhododendron caucasicum, which would cause them to sleep deeply. The dreams that appeared to them under these circumstances were interpreted as prophecies. (Ginzburg 1990, 165) The Caucasus rhododendron (section Pontica) is a broad bush that grows to only about 1 meter in height. The flowers are creamy or pale yellow, sometimes with pink spots. The plant typically blooms from April to May and is found primarily at an altitude between 1,800 and 2,700 meters. It is found across northeastern Turkey and the Caucasus Mountains (Cox 1985, 175). Its evergreen leaves are weakly aromatic. Rhododendron caucasicum is only rarely encountered in rhododendron gardens, for it is much more difficult to cultivate than are other species.
In Nepal, the closely related Rhododendron lepidotum is still used today as a ritual and shamanic incense, the effects of which are very subtle. In Tibet and China, other rhododendron species are also used as incense. The yellow-flowered Rhododendron cinnabarinum is found in the high mountains of Sikkim. Its smoke is said to have a profound effect on yaks, producing a strong inebriation and altering their behavior. It is possible that it also has psychoactive effects upon humans. In Nepal, the leaves of a Rhododendron species are mixed with tobacco (Nicotiana tabacum) for smoking. A snuff is made from the bark of a Rhododendron species and tobacco leaves.
Other rhododendron species, e.g., the rustyleaved alpine rose (Rhododendron ferrugineum 1.) and the Pontic rhododendron (Rhododendron ponticum 1.), yield psychoactive/toxic honey.
The Tartars made a tea from the leaves (ten or more) of the gold-yellow alpine rose (Rhododendron chrysanthum) that is said to have produced a state of inebriation (Roth et al 1994, 612*). There is also a cultivar, Rhododendron x sochadzeae, resulting from a cross between R. ponticum and R. caucasicum (Cox 1985, 175 f.). This rare ornamental variety may have potent psychoactive effects.
The aromatic species of rhododendron contain relatively high concentrations of essential oil. Mongolian species contain primarily limonene, aromadendrene, caryophyllene, d-candinene, r3selinene, and gurjunene (Satar 1985).
It is would be an interesting task to investigate a possible cultural link between rhododendron forests and psilocybin mushrooms. Rhododendron groves are a preferred habitat of some psychoactive mushrooms, e.g., Psilocybe cyanescens.