Potentilla erecta × P. anglica (P.
× suberecta Zimmeter)

Rare, certainly under-recorded and probably occasional with its parents.

1849; Mathew, Dr; Newtownbutler.

There are only three records of this partially fertile hybrid in the Fermanagh Flora

Database and they are dated 101 years apart. The hybrid has not been found in the VC by anyone since the survey by Meikle and co-workers recorded it twice in 1950. The reason P. × suberecta is rarely recognised is that it is intermediate in character between its parents to a very remarkable extent – sometimes even with respect to the number of carpels formed – and, when it is found, it is almost always amongst mixed populations of its parents. In addition, there is great seasonal and environmentally induced variation in both P. anglica (Trailing Tormentil) and P. × suberecta, so that these two taxa are often extremely difficult to distinguish. The fact that the F₁ hybrid, P. × suberecta, is only partially sterile means that some degree of backcrossing is feasible. The extent of this is unknown due to the amount of variation within the parent species and also in the hybrid itself (B. Matfield & S.M. Walters, in: Stace 1975).

The habitats where the parent species overlap include low-growing vegetation on heathy pastures (including those near the coast), inland heaths, dry roadside-, riverside- and railway-banks, tracksides, hummocks in damp fields, woodland rides, paths and margins, field edges, quarries and shingly waste ground near the sea (Stace et al. 2015). Most of these sites feature well-drained, acidic soils. The occurrence of P. × suberecta is probably mainly lowland, but the upper altitudinal limit and the true frequency level and the distribution remain unknown (D.J. McCosh, in: Preston et al. 2002).

The possible presence in these habitats of another pair of very similar Potentilla hybrids, those between P. anglica and P. reptans (Creeping Cinquefoil) (P. × mixta Nolte ex Rchb.), and the extremely rare hybrid between P. erecta (Tormentil) and P. reptans (P. × italica Lehm.), compound an already difficult identification problem (B. Harold (née Matfield), in: Rich & Jermy 1998). These latter two hybrids are so alike that effectively it is impossible to distinguish them in the field. Secure determination requires microscopic examination of their pollen (D.J. McCosh, in: Preston et al. 2002). P. × italica is so elusive, it has only been recognised once in the wild in Britain (Stace et al. 2015).

The only certain feature enabling field identification in hybrids involving P. anglica is in the degree of fertility displayed. P. anglica is seed- and pollen-fertile, but the hybrids it makes are sterile, or almost so. However, it is necessary to recognise that the level of seed set cannot be assessed early in the flowering season. A very useful indication of maturity is the presence of at least three withered flowers on the same stem distal to an open flower or a flower bud. The oldest of these withered flower heads should contain several conspicuously swollen carpels if the plant is fertile. P. × suberecta may sometimes manage to set a few seed, but undeveloped carpels will always be in the majority. In comparison, P. × mixta rarely sets any seed, or only around one per flower at most (B. Harold (née Matfield), in: Rich & Jermy 1998).

In addition to this, regarding fertility, it is important to realise that P. reptans is fully fertile but self-incompatible. Since the species can form large clonal patches by vegetative reproduction, some P. reptans colonies may consist of just one genome and, therefore, the flowers will fail to set seed unless a suitable cross-pollinating partner is available nearby.

Apart from sterility or near-sterility, it is essential for the supposed hybrid to also display a mixture of 3-, 4- and 5-nate, petiolate stem leaves and the mixture of 4- or 5-merous flowers typical of the two most probable hybrids (P. × suberecta and P. × mixta) that distinguish them from P. erecta and P. reptans (Harold 1994; B. Harold (née Matfield), in: Rich & Jermy 1998). As its name suggests, P. × suberecta is much more like P. erecta, with a predominance of ternate stem leaves on short petioles, generally only one leaf per node, and the leaf size decreases from plant base to the tip of the stem. Also, like P. erecta, this hybrid does not spread vegetatively and P. × suberecta usually sets just a few seeds per flower head. On account of these properties, P. × suberecta is only found in close proximity to both its parents. On the other hand, if P. erecta is not growing nearby, the plant in question could be either P. anglica or P. × suberecta, and a closer look for P. erecta should be made (Harold 1994).

Apart from the first record of P. × suberecta given above, the details of the other two Fermanagh MCM & D sites are: roadside at Drumbad House and near Inishmore Viaduct, Upper Lough Erne, both dated 1950.

P. × suberecta is said to be fairly frequent, or even common in some parts of B & I in comparison with other natural hybrids in these islands (New Atlas). Robert Northridge and the current author believe it is certainly under-recorded in Fermanagh, as indeed is probably often the case elsewhere. The hectad maps in the New Atlas and in The Hybrid Flora of the British Isles (Stace et al. 2015) display a remarkably patchy distribution for P. × suberecta, which suggests that relatively few recorders can distinguish this hybrid and also that they work their local areas thoroughly for it.

For a more local NI comparison with respect to this hybrid, the FNEI 3 lists just six records for the three most north-eastern Irish VCs (Down (H38), Antrim (H39) & Londonderry (H40)), all but one of them dating from the 1849-1935 period. The recording situation has greatly improved in the last 20 years since a careful examination of the BSBI Database, made by the current author on 30 November 2020, now provides a list of 40 records (plus a few duplicates) for the same three Irish VCs. No less than 27 of these records were made by the late John Harron between 1991 and 2003 and a further seven by the current BSBI VC Recorder (H38), Graham Day, between 2001 and 2018.

The status of this and other Potentilla hybrids clearly merits further detailed investigation whenever time permits.