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Article from Bean's Trees and Shrubs Hardy in the British Isles
'Helianthemum oelandicum' from the website Trees and Shrubs Online (treesandshrubsonline.
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As interpreted in Flora Europaea, H. oelandicum is a polymorphic species, subdivided into five subspecies, of which the typical one (subsp. oelandicum) is confined to Oland, off the south-east coast of Sweden, and Spitzbergen. The first of the two subspecies described here is certainly in cultivation and the second may be:
subsp. alpestre (Jacq.) Breistr. Cistus alpestris Jacq.; H. italicum subsp. alpestre (Jacq.) Beger – A dainty little shrub 3 to 5 in. high, forming a tuft of dense spreading branches covered thickly with pale, minute hairs. Leaves without stipules, green on both sides, oblong-elliptic, elliptic-lanceolate, or narrow-oblong, tapered at the base, 1⁄4 to 1⁄2 in. long, up to 1⁄8 or slightly more wide, furnished with a few comparatively long hairs, especially at the margins. Flowers produced in June and July, three to six (rarely more) in a terminal raceme-like cyme, each flower 1⁄2 to 3⁄4 in. wide, bright yellow, unblotched, borne on a slender downy stalk. Sepals five, hairy, the three inner ones oval and about half as long as the petals.
Native of the mountains of C. and S. Europe, usually on limestone, extending high into the alpine zone (to 8,000 ft or more); introduced in 1818. It is quite hardy and admirable for the rock garden.
subsp. italicum (L.) Font Quer & Rothm. Cistus italicus L.; H. italicum (L.) Pers.; H. penicillatum Thibaut ex Dun. – This subspecies is mainly confined to the Mediterranean region, but is also found in the S. Tyrol. From subsp. alpestre it differs mainly in its laxer habit, its inflorescences with up to twenty flowers and sometimes branched, and in the flowers being much smaller, up to 1⁄2 in. wide. The stems are often reddish, though this may not be a constant character. It is sometimes of procumbent habit.
The typical subspecies of H. oelandicum has almost glabrous leaves and very small flowers, with petals scarcely exceeding the sepals in length. The type came from the limestone island of Öland, where it was collected by Linnaeus in 1741.
The genus Helianthemum is mainly represented in cultivation by plants of garden origin, deriving for the most part from intercrossing between H. appeninum, H. croceum, and H. nummularium, though some other species may have been involved. Many of these are near to JL nummularium, which they resemble in their flat leaves, green and sparsely hairy above; and it is possible that some of these may be the result of sporting and intercrossing within that species. But many, perhaps the majority, clearly cannot belong to H. nummularium since their leaves are covered more or less densely on the upper surface with whitish hairs, rendering them grey-green or even silvery.
A number of hybrids between the above-mentioned species were already in cultivation when Sweet published his Cistineae (1825-30), but the present garden stock is of more recent origin. The famous ‘Ben’ group, named after Scottish mountains, was raised by John Nicollof Monifieth near Dundee, who died in 1926.
The following is only a selection from the large number of hybrids now in commerce. All flower from late May until end June, with a few flowers later. For cultivation and propagation see the introductory note to Helianthemum. The plants, especially those that are of open habit, should be trimmed after flowering.