| Helianthemum nummularium, Helianthemum scoparium, Helianthemum vesicarium, Helianthemum greenei, Helianthemum guerrae|
Helianthemum /ˌhiːliˈænθᵻməm/, known as rock rose, sunrose, rushrose, or frostweed, is a genus of about 110 species of flowering plants in the family Cistaceae. They are widely distributed throughout the Northern Hemisphere, especially in the Mediterranean. There are New World species, but it has been proposed that these be transferred to genus Crocanthemum.
These are usually shrubs or subshrubs, and some are annual or perennial herbs. The leaves are oppositely arranged, but some plants may have alternately arranged leaves along the upper stems. The flowers are solitary or borne in an array of inflorescence types, such as panicles, racemes, or headlike clusters. The flower has three inner sepals and two smaller outer sepals. It has five petals usually in shades of yellow, orange, or pink. The style at the center is tipped with a large stigma. The fruit is a capsule containing many seeds.
Helianthemum are known to form symbioses with mycorrhizal fungi. In the Mediterranean they are associated with Terfeziaceae, the desert truffles. Together, plant and fungus may have a beneficial effect on the arid local landscapes, preventing soil erosion and desertification. Some symbiotic pairs include Helianthemum salicifolium and the truffle Tirmania nivea, and H. guttatum and T. pinoyi.
One of the most commonly observed mycorrhizae on Helianthemum is a member of a different family, Cenococcum geophilum. This fungus is not host-specific, and it often associates with oaks, as well. Some studies suggest that Helianthemum and oaks growing together in a habitat may "share" their mycorrhizae.
Helianthemum are food plants for the larvae of some Lepidoptera species, such as the large grizzled skipper. The leaf miners Bucculatrix helianthemi and B. regaella both feed exclusively on Helianthemum sessiliflorum, as does Coleophora eupreta. C. ochrea is limited to Helianthemum, and C. bilineella and C. potentillae have been observed on the genus.
Several Helianthemum species, and the numerous hybrids and cultivars derived from them, are widely grown as ornamental plants, popular in rockeries. A broader range of colours is available among the cultivars, including bright salmon-pink to dark red. They are best grown in well-drained soil in full sun, and have a long flowering period from spring to summer. Numerous cultivars have gained the Royal Horticultural Society's Award of Garden Merit:
Species include:Helianthemum aegyptiacum
Helianthemum apenninum – white rockrose
Helianthemum canum – hoary rockrose
Helianthemum greenei (syn. Crocanthemum grrenei) – island rushrose
Helianthemum nummularium – common rockrose
Helianthemum oelandicum – alpine rockrose (syn. Helianthemum montanum)
Helianthemum oelandicum subsp. alpestre (syn. H. alpestre)
Helianthemum oelandicum subsp. italicum (syn. H. italicum)
Helianthemum oelandicum subsp. oelandicum
Helianthemum oelandicum subsp. orientale (syn. H. orientale)
Helianthemum oelandicum subsp. rupifragum (syn. H. rupifragum)
Helianthemum scoparium (syn. Crocanthemum scoparium) – rushrose