British distribution (genus): Nine native species, mostly southern, several very local and rare.
Biology and Ecology
Most species are highly host-specific, sometimes restricted to a single host species or genus. Others are capable of parasitising a number of unrelated plants, but usually still show strong regional preferences. The seeds germinate when in contact with host roots, triggered by chemical recognition. The fine root of the broomrape grows into the host root, reaching and entering the vascular tissue. An underground tuber develops, from which, eventually, the flowering stems may develop.
Broomrapes are thermophilic (warmth-loving) and often highly demanding in their habitat prefences. Frequently they require dry, open, often nutrient-poor grasslands but they are vulnerable to agricultural 'improvement', scrub development or other types of habitat loss. Some also appear to be sensitive to minor climatic changes and may vanish from sites that still appear suitable. Many are extremely local throughout their entire geographical ranges and are now endangered, requiring conservation measures and legislative protection.
A review of European species (excluding those which are exclusively Mediterranean) is provided by Kreutz (1995) – illustrated by numerous, stunningly superb, colour photographs.
Broomrapes as weeds
Important weed species are O. crenata (a number of different hosts but causing especially severe losses to fields of peas, beans or other legumes), O. minor (numerous hosts but especially on clover (Trifolium) crops), O. ramosa (especially on tobacco, tomatoes, maize and hemp), O. aegyptiaca (on tomatoes and other Solanaceae) and O. cumana (more or less restricted to sunflowers (Helianthus annuus) but causing major economic losses where this is an important crop).
In Britain, Orobanche species cannot be regarded as a significant agricultural problem. O. minor has been a problem on clover in the past but is now rarely seen in crops, though it is widespread in England on a number of wild hosts. O. ramosa was once frequent on hemp (Cannabis sativa), when this was cultivated in East Anglia, but it is now considered extinct here (Rumsey & Jury, 1991). O. crenata has been sporadically established, especially in a small area of S.Essex, but it is more a candidate for conservation than for control. O. aegyptiaca occurred transiently in a tomato nursery in Sussex in 1952; O. cumana has never occurred here (nor has O. cernua, in which species O. cumana is included by some authorities).
Identification: British species
The photograph is of var. minor, the commonest variant in Britain, though it may well be an ancient introduction from Central Europe. It grows on a wide range of hosts, including, though now rarely, clover crops. It favours roadside banks and other ruderal habitats, where it is often transient or sporadic.
Another distinctive and probably native colour variant, differing also slightly in corolla shape, occurs on dunes in east Kent, especially on Sea Holly (Eryngium maritimum). It has been claimed as the Mediterranean O. amethystea, but Rumsey & Jury (1991) give reasons why it does not match that species. They consider that it falls within the range of variation of O. minor subsp. (as var.) minor. Similar plants are frequent there too on Restharrow (Ononis repens) and are scattered on other hosts, though looking less distinctive and grading into more typical O. minor, as Rumsey & Jury themselves point out. The hosts themselves may be influencing phenotype.
O. minor is a substantially self-fertilising species and so it would be expected to form genetically distinct local populations. Races showing physiological adaptation to particular hosts are known to occur (Musselmann &, Parker, cited from Rumsey & Jury, op. cit.). The Kentish plant on Sea Holly seems best regarded as one such biological entity within the species, conspicuous and attractive though it is. True O. amethystea is reported in Britain as an introduced weed in Cambridge Botanic Garden (Sell & Murrell, 2009).
Orobanche minor Sm. subsp. maritima (Pugsl.) Rumsey (= O. maritima Pugsl.) (Carrot Broomrape)
It occurs on slopes on sea-cliffs and on dry, coastal banks, and almost exclusively parasitises the southern coastal subspecies of Wild Carrot (Daucus carota subsp. gummifer), which has much the same geographic range (but which extends up the Welsh coast). The occurrence of subsp. maritima outside Britain is uncertain, though it is likely to be on the coasts of France and Spain.