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A genus of about twenty species, nearly allied to Helianthemum, but differing in having the seed-vessels five- or ten-celled, whilst in Helianthemum they are three-celled. Leaves opposite, evergreen. Flowers of a rose-like appearance, having five broadly wedge-shaped petals and very numerous stamens; sepals three to five. Seeds numerous. In a wild state the cistuses are found in the Mediterranean region, and are especially abundant in Spain and Portugal. The flowers usually last only a few hours in the morning, never more than a day, but a constant succession of them is maintained during sunny weather, making a fine display in June and July. None of the cistuses is yellow-flowered, but they very frequently have a patch of that colour at the base of each petal.
Although the number of true species is comparatively limited, they have hybridised freely. Some of the best are hybrids, such as × cyprius, × purpureus, and × corbariensis. These, and some others, will be found in their alphabetical position. Hybrids of lesser account are treated under one or other of the parents (see the Index to this volume). It must be emphasised that hybrid names in Latin such as C. × skanbergii, C. × aguilari, etc. are group-names that cover any form of the cross, whether it arose in the wild or in cultivation. It is purely accidental if such hybrids are represented in cultivation by a single clone. Some are, and all could be, grown in many slightly differing forms. Hybrids are likely to occur when plants are raised from seed in gardens where many species grow together. Numerous hybrids were raised artificially by Bornet at the Villa Thuret, Antibes, during the years 1860-75, and thanks to his researches it was established that many cistuses previously thought to be species were in fact hybrids. In the natural state, crossing between species is less likely to occur than in gardens since the various species tend to occupy different habitats or different geographical areas. However, some crosses are common in the wild: for example, Dansereau reports that, where C. monspeliensis and C. salvifolius are in contact, their hybrid (C. × florentinus) may make up ten per cent of the stand.
Unfortunately the rock roses with few exceptions are not genuinely hardy. They survive our mild winters, but many succumb in severe or even moderately hard ones. The great frosts of February 1895 killed all the cistuses at Kew except C. laurifolius and C. × corbariensis. The cold winters of 1961-3 confirmed the hardiness of these two, though the latter appears to be represented in cultivation by several clones, some less hardy than others. C. parviflorus, although not the most decorative of cistuses, has proved remarkably hardy. Others that survive well at Kew are C. × cyprius and ‘Silver Pink’ and to these might be added C. ladanifer, although in other gardens it is often reckoned as tender. They like a light, well-drained soil, and more than anything a position exposed to full sun, but otherwise sheltered, and something above the surrounding level. They never suffer from drought, and any dry, sunny bank will suit them. A covering of bracken or leafy branches in severe weather is a help, and will often save plants that would otherwise perish.
Propagation may be effected by seed or by cuttings, the latter being necessary for some of the hybrids which do not perfect seed. They are best taken in late summer, and struck in mild heat. Until planted out permanently, rock roses should be grown in pots, as they suffer badly from transplanting. Many of the species exude a fragrant gum, known as labdanum or ladanum, from the young stems and leaves. The most prolific source of this gum, which is used in perfumery and, at least at one time, in medicine, is C. creticus – a rather tender shrub. It is also got largely from C. ladanifer.
In the wild state, species of Cistus are hosts of the remarkable parasitic plant Cytinus hypocistus, which is the only European representative of the tropical and subtropical family Rafflesiaceae.
The genus Cistus was popular in gardens in the early nineteenth century: in Sweet’s Cistineae, published 1825-30, 112 species, varieties, and hybrids are figured. Interest dwindled and its revival is largely due to Sir Oscar Warburg and his son, the late Dr Edmund Warburg. They travelled extensively in the cistus regions of Europe and N. Africa, and horticulture is indebted to them for a number of new varieties and hybrids which they introduced and propagated. Their note on the genus (Journ. R.H.S., Vol. 55, pp. 1-52) is still of great value. The species of Cistus are reviewed by Dansereau in Boissiera, 1939, pp. 1-90.