O. pinnatus is a decumbent annual, germinating in spring and flowering generally in April and May. Although the flowers are small, their brilliant colour can be quite eye-catching. In wet seasons, plants may persist until October, but in dry years plants can be difficult to find and disappear early. Such variable longevity makes any population change difficult to assess.
Although it can be found at its traditional sites every year, it can also appear sporadically in suitable habitats elsewhere. It does not appear to be under any major threat at present, though isolated populations may be vulnerable to uncontrolled burning of its heathland habitat. Other populations can apparently be lost, especially where fields have been abandoned and have reverted to grass and tall vegetation. However, it is possible that buried seed remains viable for some time, since plants have been known to reappear after cultivation.
An annual found on dry heaths and heathy grassland, around granite cairns, in consolidated dune turf and in disturbed areas such as gardens and bulb-fields with sandy soil. Inland it is a rare casual of docks and waste ground. Lowland.
In Britain, O. pinnatus is found only in the Isles of Scilly. It has been recorded from all the larger islands, except Samson, though now appears to be extinct on St Mary's. It is a plant of dry heaths, grassland and disturbed sandy areas, growing on granite carns and dry, heathy slopes, and also on disturbed sandy soil as, for instance, in neglected bulb fields and gardens.
On the slopes of the granite carns it typically grows with Erica cinerea, Lotus corniculatus, Ornithopus perpusillus, Plantago coronopus and Sedum anglicum. In some places it is found in association with the scarce Lotus subbiflorus, Ophioglossum azoricum and at one site with O. lusitanicum. On dune heath and other sandy sites, it may grow amongst Aira caryophyllea, Calluna vulgaris, Carex arenaria, Erodium cicutarium, Lotus corniculatus, Viola riviniana and colonising dune plants. In some localities it can form extensive patches, trailing across the other vegetation in long skeins over 30 cm in length.
This plant might be poisonous
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