Lythrum hyssopifolium

Lythrum hyssopifolium L. (Lythraceae)

Grass-poly, Gwyarllys Isopddail

Status in Britain: VULNERABLE.

Status in Europe: Not threatened.

This is a plant of disturbed ground which is flooded during the winter months. It is usually found in hollows, ruts and other low-lying areas in arable fields, in sites where autumn-sown crops have been killed by flooding or where the ground was too wet for crops to be sown in spring. In these areas, L. hyssopifolium is often accompanied by other spring-germinating annuals including Juncus bufonius, Persicaria maculosa, Plantago major ssp. intermedia, Polygonum aviculare, and ephemeral bryophytes such as Physcomitrella patens and Riccia cavernosa (Preston & Whitehouse 1986). However, the largest British population of L. hyssopifolium does not occur on cultivated land, but on winter-flooded ground at Slimbridge which is disturbed in summer by waterfowl. In addition to its occurrence in winter-flooded sites, this species is sometimes found as a casual in a range of other sites to which seeds have been introduced as a contaminant of imported grain or in other ways.

L. hyssopifolium is an annual which germinates after the water recedes in spring. The flowers are slightly protogynous, but if cross-pollination fails they are automatically self-pollinated and almost always set seed. Like many annuals, plants may differ greatly in size and fecundity. Populations may also vary greatly in abundance from year to year. In hollows in arable fields near Cambridge L. hyssopifolium and its associated species may fail to appear after dry winters, but are present in abundance in favourable years (Preston 1989). If the flood water disappears but the hollows remain damp the plants may grow luxuriantly; if the hollows dry out rapidly they flower and fruit as small plants. The seed may remain viable in the soil for many years.

L. hyssopifolium has been recorded in seasonally flooded habitats from scattered sites in southern England. There are particular concentrations of records in the London, Oxford and Cambridge areas, perhaps because these were well recorded areas in the 17th and 18th century when the species may have been more frequent than it is today. It is impossible to obtain a clear picture of its former abundance, as it is an inconspicuous plant which may appear and reappear at a site at irregular intervals and is also able to colonise new sites. The species was certainly rare by the time that botanical recording increased in intensity in the middle of the 19th century. It is currently known in five British sites (Callaghan 1996). The large population of an estimated several hundred-thousand plants at Slimbridge, Gloucestershire, is continuing to spread, and is colonising newly excavated hollows made for wildfowl. Substantial populations of up to 10,000 plants occur in hollows in arable fields south of Cambridge and smaller populations are known in Dorset, Oxfordshire [but v.c. 22] and Sussex. It also occurs in Jersey.

The ecological requirements of L. hyssopifolium are specialised, and it relies for its continued occurrence on the flooding and disturbance of its sites. If the arable sites are not cultivated they rapidly become overgrown by perennial species and, eventually, by Salix scrub. The absence of disturbance is currently a threat to the small Oxfordshire population (although the long viability of the seeds may allow the plant to withstand a period of unfavourable management). 

L. hyssopifolium is widespread in central and southern Europe, North Africa and West Asia. The northern limit of its native distribution is unclear: in Britain it may be a native species or a long-established introduction. It has been introduced in many areas outside its native range, including South Africa, North and South America and Australia (Meusel, et al. 1978).

C. D. Preston