Orobanche hederae Duby, Ivy Broomrape
Account Summary
Native, very rare. Submediterranean-subatlantic.
1872; Brenan, Rev S.A.; walls at Old Crom Castle.
May to September.
Growth form and preferred habitats
Medium-tall, leafless and devoid of chlorophyll, O. hederae is a total parasite, stems 10-50 cm tall, or very exceptionally up to 100 cm (Rumsey & Jury 1991). The colour of the plant is very variable, frequently deep purplish-brown, occasionally yellowish or yellowish-brown, sometimes reddish-brown below and yellowish above, rarely yellowish over its entire length. It can vary from slender to stout, usually strongly swollen at the base of the aerial stem, and be densely covered with a mixture of short and medium glandular hairs (Sell & Murrell 2009).
O. hederae sometimes behaves as an annual but is generally perennial (see below) (Jones 1987). In the wild, it grows mainly, but not exclusively on the roots of well-established Ivy (Hedera helix) which is diploid, but very probably it also occurs on the tetraploid Irish Ivy (H. hibernica). Due to the difficulty of differentiating between H. hibernica and H. helix, many more recent O. hederae records still cite H. helix as the host than mention H. hibernica (Jones 1987).
O. hederae has an underground creeping rhizome covered in fleshy scales. Where the rhizome meets the roots of its Ivy host plant, a haustorium, in appearance like a scaly bulb, is formed. The scales continue up the flower stem, thinning out to look like tiny leaf-bracts among the creamy-white, lilac-veined flowers (Rutherford 1985). O. hederae has a longer flowering period than any other broomrape in B & I, flowering spikes appearing between June and October.
Ivy Broomrape is differentiated from the rather similar and closely related O. minor (Common- or Lesser-Broomrape), by its characteristically distally pinched corollas, the flowers extending over most of the stem, the long, acuminate floral bracts, and its large terminal 'bud' of unopened flowers that give the spike a pointed top. Both these Orobanche species can parasitize members of the Araliaceae, the Ivy Family, so care is needed to properly distinguish them (Rumsey & Jury 1991).
The predominantly Atlantic coastal distribution of O. hederae in the S & W of B & I suggests that it is primarily a parasite of Hedera hibernica rather than of the more eastern form of Ivy, H. helix (Rumsey & Jury 1991). It is seldom found in areas where the mean minimum February temperature is under 2oC and clearly it must be fairly sensitive to frost (Jones 1987). Another indication of low temperature sensitivity is that many inland records, like those in Fermanagh that lie close the NW extremity of its world distribution, tend to be of plants living close to walls, or under trees or bushes, both situations that afford the plants a real level of shelter and frost protection. In maritime situations, O. hederae is a plant of coastal cliffs, undercliff woodlands and hedgebanks (F.J. Rumsey, in: Stewart et al. 1994)
Ivy Broomrape is also associated with alkaline soils, particularly in sheltered gorges in chalk and limestone areas of B & I that have a mild climate (Lousley 1969; Rutherford 1985; F.J. Rumsey, in: Stewart et al. 1994).
Flowering reproduction
The species has a prolonged flowering period, from June to October. The inflorescence is a spike, at first cylindrical and dense above, many-flowered. Later, it becomes lax below, the lowest flowers set far apart and very low on the stem. The flowers are bisexual (ie hermaphrodite, perfect), glandular pubescent, the spike, 8-40 cm tall, clothed with lanceolate-acuminate bracts, reddish-brown to brown or black in colour. The calyx is tubular, 6-16 mm, segments entire or bifid, dark red or dark brown, densely glandular-hairy. The corolla, 10-22 mm, is pale yellow or yellowish (dull cream), rarely yellow-brown, its back tinged violet or red, especially near the upper lip.
The corolla is sparsely glandular and veined purple, the tube slightly inflated below, gradually narrowed or constricted towards the mouth. The upper lip of the corolla is entire or notched; the lower lip is 3-lobed, the middle lobe just the largest, all three sub-acute, crisped and denticulate. The stamens are four, filaments two long and two short, inserted 3-4 mm above the base of the corolla tube. The filaments are hairy below, glabrous above; anthers brown, protruding from the corolla mouth. The stigma is variously coloured, pale yellow to reddish-yellow, turning brown, of two elongated lobes (Jones 1987; Rumsey & Jury 1991; Sell & Murrell 2009).
Field experiments have shown that a solitary, isolated inflorescence can produce viable seed, confirming that O. hederae is self-fertile. The flowers are strongly protandrous (ie male parts mature first) and they may readily self-pollinate. Observations of other related Orobanche species suggest they are visited by bees that probably pollinate them and it is possible that O. hederae may also be insect pollinated. Similar experimental evidence of regular recurring inflorescence appearance at marked sites in successive years, together with the fact that the period between seed being sown and inflorescence production is at least 20 months, strongly suggests the species is perennial rather than annual (Jones 1987).
British and Irish occurrence
O. hederae is one of the commonest British broomrape species and certainly is the most abundant in Ireland. The species is thinly but widely scattered in Ireland, including much more in the way of inland native sites than is the case in England and Wales. In view of the small, lightweight seed produced, Rumsey & Jury (1991) suggested that inland stations [in Britain] may have arisen from wind borne seed, but they concluded that in most cases deliberate introduction is likely, since they noticed that many of the reported sites were in or near botanic-, public amenity- and private-gardens (Jones 1987; M.J.Y. Foley, in: Preston et al. 2002).
Ivy Broomrape has been found at least once in 31 of the 40 Irish VCs, although in many of these stations it has not been seen recently (BSBI Atlas 2; Cen Cat Fl Ir 2). Despite this, in the Irish Red Data Book, Curtis & McGough (1988) did not regard O. hederae as either rare or threatened in the RoI. It should be said, in fairness, that the Irish Red Data Book on vascular plants is widely recognised by all concerned to be seriously out of date and in urgent need of revision.
While it remains a scarce species in both B & I, O. hederae is not under any great threat at present and, indeed, may be increasing its presence slowly as it is capable of exploiting its cultivated host Hedera beyond its natural range (Rumsey & Jury 1991). In NI, O. hederae is listed under Schedule 8 of the Wildlife (NI) Order, 1985, and is given special conservation protection.
Fermanagh occurrence
Ivy Broomrape reaches its most northerly sites in Europe in W Donegal (H35). In NI, however, Ivy Broomrape is considered a very rare species. Currently there are seven known stations, one first discovered in 1999 at the base of ivy-covered sea cliffs at Garron Point, Co Antrim (H39) (NI Vascular Plant Database) and all the remaining six sites are confined to Fermanagh. As can be seen in the accompanying tetrad distribution map, the post-1975 sites occur in eight such squares.
The species remains well established at several positions on or near Old Crom Castle where it was first discovered in 1872. The second very vaguely described site was found by MCM & D in 1952 listed without a grid reference as, "Enniskillen road E of Belcoo". A station corresponding to this description was refound in 2006 by Mr John Wann and communicated to RHN. On 30 September 2006, there were 14 flowering spikes here at Drumharriff Td, lying E of the Abocurragh Road and N of the A4 Belcoo to Enniskillen road. In June 2007, there were between 300-400 spikes observed at this roadside site, and at another related station, further along the road towards Enniskillen near Cloonaveel, another 50 spikes were found.
The third (or rather, now the fourth) site was discovered by the NI Lakes Survey in 1991, in woodland on Sally Islands, off Rossinnan Td shore, Lower Lough Erne. The two remaining sites were discovered by RHN in 1992 and 2002, and they are under trees on Isle Namanfin, Lower Lough Erne, and at Lisgoole, on the outskirts of Enniskillen, at Old Jones Memorial School. Plant numbers vary greatly from year-to-year in the Fermanagh sites, from six spikes to approximately 350 across the six sites.
European and world occurrence
Ivy Broomrape belongs to the Submediterranean-subatlantic phytogeographical element. It occurs in W, S & SC Europe and reaches its most northerly extension in NW Donegal on the Inishowen Peninsula (H34). It extends eastwards across the Mediterranean basin to Turkey, Iran, the Caucasus and is present in NW Africa and the Canary Isles (Sell & Murrell 2009). In Majorca, and perhaps elsewhere, it is now largely restricted to man-made habitats. O. hederae has also been introduced to N America and S Africa (F.J. Rumsey, in: Stewart et al. 1994).
Names
The genus name 'Orobanche' is from the Greek 'orobos', meaning 'a kind of vetch' and 'anche' meaning 'to strangle' (Stearn 1992). Some Orobanche species do parasitize legumes, so the 'Legume strangler' name is appropriate. O. crenata (Bean Broomrape), for example, is a weed of bean fields in the Mediterranean (Gilbert-Carter 1964), O. minor parasitizes clover, and it is especially appropriate to O. rapum-genistae, (Greater Broomrape) which is parasitic on the roots of Cytisus scoparius (Broom), Ulex spp. (Gorse) and occasionally Genista tinctoria (Dyer's Greenweed). The Latin 'rapum genistae' translates as 'root-knob of the broom', on which the species is parasitic (Grigson 1974). This idea also gives rise to the English common names 'Broomrape', the English equivalent of 'rapum genistae', and 'Strangleweed' (Grigson 1955, 1987).