Mentha arvensis × M. aquatica (M.
× verticillata L.), Whorled Mint

Native, occasional.

1806; Scott, Prof. R.; Co Fermanagh.

July to November.

Apart from M. arvensis (Corn Mint), the less common of its parent species, this extremely variable, usually sterile hybrid is the most commonly grown garden mint in Co Fermanagh, rather than M. spicata (Spear Mint). Whorled Mint grows to 30-90 cm tall and is very variable in its leaf size, shape, dentation and hairiness (Sell & Murrell 2009). When it flowers, from August to October, it can be distinguished from its parents by its tubular (not campanulate) calyx, 3.0-3.5 mm, the tube of which is twice as long as wide, and by the form of the inflorescence. The inflorescence is of distant whorls below or crowded ones in the upper part of the flowering stem. The whorls are often stalked. The bracts on the inflorescence are like the leaves, often decreasing in size, the upper ones sometimes shorter than the flowers. The pedicels of the flowers are eglandular hairy (ie simple hairs without glands) (Sell & Murrell 2009).

In common with both its parents, it is a rhizomatous perennial, spontaneous hybrids arising where the two species overlap, there being no genetic barrier between them (Stace et al. 2015). Although there will be a 'weeding out' at the seedling stage, hybrid plants often establish in the wild mint population and, with their characteristic vigour and well-developed rhizomatous growth, they can persist and even spread clonally to colonise new sites in the absence of their parents (Stace et al. 2015).

M. × verticillata is usually sterile, but occasionally highly fertile plants do occur, probably as a result of backcrossing. Interestingly, Whorled Mint, the most commonly occurring mint hybrid in B & I, is derived exclusively and spontaneously from native wild populations. Other mint hybrids also tend to originate from garden material, probably dumped in wayside or disturbed ground sites, but this does not occur in the case of Whorled Mint. However, since it is widely cultivated for culinary use, M. × verticillata can occasionally be found as a surviving relic of garden cultivation around old, abandoned homesteads (Stace et al. 2015).

M. × verticillata mainly grows in damp to wet ground in the presence of its parents and only very occasionally without them. It is found in grasslands, alongside tracks and near water margins in ditches, streams, slow rivers, lakes and ponds. Like both its parents, it is tolerant of winter or wet-weather flooding.