Rhus L.

At one time the largest genus in the Anacardiaceae, Rhus has been much reduced over the years, as a number of segregate genera have gradually been split from it, beginning with the well-known and long-accepted Cotinus Mill. (Smoke-bush), the process culminating with the more recent removals of the sister genera Toxicodendron Mill. (Poison-ivy) and Searsia F.A. Barkley. As a consequence, the formerly capacious Rhus has been reduced from about 200 species to just over fifty.

Rhus is probably most familiar to gardeners in the form of the sumacs, R. typhina and its allies. Taken as a whole, however, Rhus provides many useful ornamental trees with bold pinnate leaves, some of which – such as R. chinensis – are reasonably well known in cultivation, others less so. The name Rhus javanica regularly appears in horticulture, but the taxonomy associated with it is extremely confusing. The name was first published by Linnaeus, but his species has since been transferred into the genus Brucea J. Mill. (Simaroubaceae). The same name was also published by Thunberg for a species of Rhus that has now been placed in synonymy with R. chinensis Mill. (Chandra & Mukherjee 2000). The name ‘Rhus japonica’ was never validly published, but appears on some cultivated plants introduced into the United Kingdom that seem also to be referable to R. chinensis. The larger, tree-forming species should be given sites in good soil with plenty of sun, and will do best in areas with hot summers.

Many species of Rhus are shrubby. Among those that are commercially available is Rhus integrifolia (Nutt.) Brewer & S. Watson, a chaparral shrub from southern California, where it is known as Lemonade Berry as the fruits are said to produce a lemonade-flavoured beverage if steeped in water. Although tolerant of dry conditions, it is frost-tender (Wikipedia 2008a). Rhus integrifolia and other Californian or Mexican species should be given sunny, well-drained sites in mild areas if they are to have much chance of surviving. Quite different, and coming from damp forests, R. ambigua Lavallée ex Dippel is a climbing species from eastern Asia, that turns brilliant red in autumn (Crûg Farm Plants 2007–2008), but this should be regarded with caution as it appears to be an Asian poison-ivy, as its alternative name Toxicodendron orientale Greene suggests. None of the poison-ivies should be sold without a clear health warning, and care should be taken with any unfamiliar species of Rhus, as they may have inflammatory sap or allergenic pollen. Rhus can be rooted from cuttings, or grown from seed. The shrubby African rhus have been split off as the segregate genus Searsia.

In contrast to the notorious Toxicodendron, the sap of Rhus s.s. is generally considered not to be poisonous, though all Anacardiaceae sap should be treated with caution, and there have been reports of injury in susceptible individuals even from the sap of reputedly innocuous Rhus species (sensitivity may vary between individuals).

[Bean’s Trees and Shrubs]

A large genus of shrubs, small trees, or climbers, with ternate or pinnate leaves, found in most temperate regions of the globe, and occasionally in the tropics. About a dozen species are grown in the open air in the British Isles, but several others can be cultivated in Corn­wall and similar places. Individually the flowers of the sumachs are small and of little beauty, being greenish, yellowish, or dull white, but in a few species the panicles are sufficiently large and the flowers white enough to give a pleasing effect. In some species the fruits are handsome, but, on the whole, their value in gardens is in the size and autumn colouring of the foliage. The leading characters of the genus are the alternate leaves and usually dioecious flowers, the five-lobed calyx (which adheres to the fruit), the five petals, the one-celled ovary with three styles, and the usually globose fruit, either glabrous or hairy, containing one bony seed.

The cultivation of all the sumachs is simple. They do not require a very rich soil except when they are grown purely for size of foliage as R. typhina (q.v.) and R. glabra sometimes are. Where autumn colour is desired, ordinary garden soil without added manure is sufficient. Like many other trees with soft wood and a large pith, they are subject to the attacks of the ‘coral-spot’ fungus (Nectria cinnabarina). Branches so attacked should be cleanly cut off and burnt, the wound coated with tar. Most can be propagated by root-cuttings, and seed is often available.