Polemonium caeruleum L., Jacob's-ladder

Introduction, neophyte, a very rare casual garden escape. Eurosiberian boreo-temperate, but also widely naturalised including in both W Europe and N America.

1947; MCM & D; Carrickreagh Bay, Lower Lough Erne.

Only recorded once in Fermanagh, in a much-frequented rocky limestone lakeshore situation on Lower Lough Erne, in Ireland this decorative perennial is always a garden neophyte and, at this location, it was probably derived from dumped garden waste. Jacob's-ladder has not been seen again at Carrickreagh, which is a frequently visited site, so we can assume that it did not persist. Hackney et al. (FNEI 3) made a similar statement about this species' occurrence in NE Ireland.

Pollen grains of P. caeruleum have very characteristic surface sculpturing that makes them readily identifiable to species level. Fossil pollen has been identified in deposits in England from all zones of the last Ice Age (Weichselian) and from the early Flandian Post-glacial, proving the species is native (Pigott 1958; Godwin 1975). Thus, at least in a number of sites in N England, Jacob's-ladder is one of the special, rare and very beautiful native plants of constantly moist, flushed soils of the Pennine Carboniferous limestones, on cool, N-facing, steep but stabilised, upland calcareous scree slopes at the base of cliffs (Wiggington 1999). Jacob's-ladder also occurs in two sites further north in Northumberland, on andesite clay soil cliffs along river banks, where it is also considered native. Altogether there are a total of around 20 native P. caeruleum stands in England and it is a Red Data Book species of conservation concern (Swan 1993; Wigginton 1999).

The prime requirements of P. caeruleum are for a moist, fertile soil and freedom from competition. It can tolerate a certain degree of shade and grows in tall herb vegetation with associated species that typically include Arrhenatherum elatius (False Oat-grass), Dryopteris filix-mas (Male-fern), Epilobium montanum (Broad-leaved Willowherb), Festuca rubra (Red Fescue), Filipendula ulmaria (Meadowsweet), Heracleum sphondylium (Hogweed) and Urtica dioica (Common Nettle) plus a thick carpet of moisture-holding mosses and liverworts. Jacob's-ladder plants are intolerant of prolonged water loss, so must have a regular seepage of groundwater, or a frequent rainfall supply (Pigott 1958). Seedlings are particularly vulnerable to wilting and a light, open canopy of Fraxinus excelsior (Ash) helps to maintain a humid environment. Grazing is needed to keep the Ash in check, although this means flowering performance is seriously suppressed. A careful balanced management regime involving light, occasional grazing is essential to maintain the scattered, though sometimes considerable-sized native P. caeruleum populations (I. Taylor, in: Wiggington 1999).

However, apart from these English native sites, Jacob's-ladder is also a deservedly popular, clump-forming decorative perennial, widely grown by discerning gardeners across B & I. It has fern-like, elegantly pinnate, radical leaves with 17-27 leaflets that give it its English common name and in May and June it bears terminal and axillary cymes of clear blue, or rarely white, small, open, campanulate (bell-like) flowers with strongly contrasting orange stamens. It performs best in almost full shade and has stems 20-60 cm tall.

In the garden setting, it is described as short-lived, but it seeds itself prolifically (Hansen & Stahl 1993). The native plant, on the other hand, is considered a long-lived, polycarpic perennial, reproducing by seed. It flowers a month or so later than the garden form, in June and July, but exceptionally can continue into September. It is pollinated by bumble-bees, although it is also self-fertile. Seed is released from the dry capsules from early autumn onwards, and germination is mainly in spring. Mature plants die down in winter, but the dead flower stems remain erect, eventually shedding the seeds (I. Taylor, in: Wigginton 1999).

As demonstrated by the New Atlas hectad map, garden forms of Jacob's-ladder very commonly escape from cultivation and can become naturalised in Britain from E Cornwall to Shetland (VCs 2-112). This happens very much more rarely in Ireland: the Cat Alien Pl Ir lists P. caeruleum's occasional, non-persistent 19th century occurrences from a total of just six Irish VCs, mainly in the far south of the island. There have also been 20th century discoveries from Cavan (H30), where it is planted but spreading, and from the three NE VCs (Cos Down (H38), Antrim (H39) and Londonderry (H40)). Thus, the solitary Fermanagh record listed above is not an isolated occurrence, but it is previously unpublished.

The escaped or discarded garden material is morphologically different from the native species and the two are easily distinguished (Pigott 1958). Garden escapes usually have pale, purplish-blue, lavender flowers, yellow pollen and narrower leaflets (Sell & Murrell 2009).