Circaea lutetiana × C. alpina L. (C.
× intermedia Ehrh.), Upland Enchanter's-nightshade
Account Summary
Native, occasional, perhaps in decline. European temperate, but also in Asia and N America.
1839; Ball, J.; shore of Lower Lough Erne near Enniskillen.
June to August.
Growth form and preferred habitats
Upland Enchanter's-nightshade is a delicate, sterile hybrid herb with a more slender perennating rhizome than its more common parent, C. lutetiana (Enchanter's-nightshade). It also produces over-ground spreading stolons from the lower leaf axils of its aerial stem that are usually absent in C. lutetiana. It occurs either with or without its parent species in moist woods and other shady and rocky places, including streamsides and block scree in mountain gorges, on river shingle, lakeside boulders and in hedgerows, gardens and waste ground. Like its more common parent, and despite its English common name, Upland Enchanter's-nightshade is most frequent in lowland situations and it again prefers to grow in moist, mildly basic, fertile soils. C. × intermedia has also inherited the rather weedy, colonising tendencies of C. lutetiana and, although it is totally sterile, producing no seed, it can spread into sufficiently damp and shady disturbed wayside habitats by means of the vegetative growth of its rhizomes and stolons and their transport (Stace et al. 2015).
Identification
This inter-specific cross between C. lutetiana (Enchanter's-nightshade) and the misleadingly named C. alpina (Alpine Enchanter's-nightshade) is distinctly intermediate in appearance between its parent species and is easily recognised. Since C. alpina is extinct in Ireland, botanists here have only to distinguish the hybrid from the common woodland Enchanter's-nightshade, C. lutetiana, with which it ecologically overlaps in damp, shaded, disturbed habitats, often near water. While the hybrid often occurs along with C. lutetiana, it can also appear on its own.
The hybrid has a less robust habit than C. lutetiana, its leaves are thinner and more definitely toothed, more cordate at the base and more shiny (less hairy) than those of C. lutetiana. The hybrid inflorescence is also much shorter and fewer flowered than C. lutetiana, although it still bears the flowers rather remote from one another. The hybrid flowers are absolutely sterile; to quote from Raven (1963), "The anthers of C. × intermedia often fall undehisced, and I have never seen a single well-filled, morphologically normal, pollen grain in a plant of this taxon. Likewise, plants of C. × intermedia fail to produce mature fruit."
Fermanagh occurrence
C. × intermedia has been occasionally, almost quite frequently recorded in 31 Fermanagh tetrads, 5.9% of those in the VC. As the tetrad map indicates, all but two of the twelve Fermanagh tetrads that have post-1974 records are from the woods and islands of Lower Lough Erne. In view of the 14 widely scattered older records that were made by Meikle and his co-workers, RHN and the current author (RSF) reckon the current distribution of C. × intermedia in the rest of Fermanagh requires further investigation before it can be determined if it really has declined locally or not.
Irish and British occurrences
This hybrid is quite scarce, widely scattered, yet locally abundant in the northern counties of Ireland (FNEI 3; NI Vascular Plant Database; New Atlas). In comparison, it has just three southern outliers in Cos Wicklow (H20) and Co Dublin (H21) (Brunker 1950; Flora of Co Dublin). The records from both these VCs are pre-1975 – the Wicklow record, discovered by Praeger, dates from 1894 and the two Dublin ones from 1967 and 1973, and they all are or were associated with garden cultivation or disturbed ground.
The occurrence of C. × intermedia is much better recorded now than in the early 1960s when Raven was studying it, yet it still displays a frequent and wide-ranging distribution in the W & N of Britain, becoming rarer eastwards and southwards, where it reaches the mouth of the River Severn in S Wales (Raven 1963; Preston et al. 2002).
Population maintenance and vegetative spread
While easily recognised, C. × intermedia is still variable in many characters, a fact which strongly suggests it arose locally on many occasions in the past when C. alpina was more widespread than at present, and the parent species much more frequently overlapped. The alternative explanation is that the hybrid consists of a single or a few genets, which perhaps arose in a somewhat drier than the present day post-glacial period 5,000 years ago, and which then gradually spread to the current B & I distribution through the vigorous vegetative reproduction this hybrid possesses (Raven 1963).
However, this latter hypothesis fails to explain the observed variability of the hybrid and, in the view of the current author (RSF), it greatly overestimates the dispersal ability of the sterile hybrid into new ground. While the vegetative reproductive capacity of the hybrid allows it to maintain populations in existing sites, it fails to offer a feasible mechanism of jump dispersal permitting anything more than a very rare or occasional fortuitous colonisation of fresh habitat at any appreciable distance. However, it is necessary to consider the possible role of man in the dispersal of the hybrid, and there is no doubt that C. × intermedia can and does at least match C. lutetiana in its ability to reproduce vegetatively. Prof. Webb pointed out to Peter Raven that the Dublin and Wickow records of the hybrid represented garden weeds, and these occurrences are or were so remote from the other Irish records in the north of the island, that their origin very probably derived from horticultural activity. Thus it is a sensible assumption that these remote southern Irish outliers were introductions, perhaps the result of rhizome fragments that might have been transferred with soil on the roots of imported garden trees or shrubs (Raven 1963).
Nowadays the C. alpina L. (Alpine Enchanter's-nightshade) parent of this hybrid is totally absent from Ireland and is a rare or scarce species in N & W Britain, relatively common only in the English Lake District and on the Scottish isle of Arran. While the scattered populations of C. alpina may well be stable at present, there can be little or no doubt it is a relict species of cooler climatic conditions, representing a truncated, probably previously much wider distribution across both B & I in the cooler, early post-glacial period.
C. × intermedia thus appears to be both a relict native and a garden weed and this is very probably a situation not just confined to its Irish occurrence, since Raven (1963) noted, "Also in Merioneth [in Wales], for example, it [the hybrid] is mainly restricted to disturbed areas and man-made habitats, particularly in gardens and along roadsides, and is probably extending its range in connection with cultivation (Benoit & Richards 1961)."
Threats
None.