Below, the article Oli and I have published in Sorby Record, Sheffield. Sorby Record is published by Sorby Natural History Society, Sheffield, THE society if you are interested in biological recording and natural history from the Sheffield area.
“Water Bent (Polypogon viridis) new to Sheffield and South Yorkshire
Oli Pescott and Ambroise Baker
Several studies of urban areas have found that the composition of the urban flora can change considerably between survey periods. For example, a survey of the street flora of Aberystwyth between the 1970s and late 1990s found a large turnover of species (Chater et al., 2000), with species associated with drier, warmer conditions becoming more prevalent.
The BSBI’s Local Change project, a systematic survey of tetrads across Britain conducted between 1987 and 2004, also found that, for built-up areas and gardens, species associated with low rainfall and warm summers had done ‘especially well’ (Braithwaite et al., 2006).
In this way it can be of particular interest to monitor the urban flora, because the rapid changes in the plants that make their homes there can provide an insight into how our environment is changing.
In this spirit, we were excited to independently discover the alien grass Water Bent (Polypogon viridis) new to the streets of Sheffield and South Yorkshire in 2013. Whilst some historical records of this plant do exist for vice-county 63 (e.g., from wool shoddy, Halifax, 1960, F. Houseman) (Wilmore, 2000), this appears to be the first find of Water Bent for the modern county of South Yorkshire (GTD Wilmore, BSBI vice-country recorder, pers. comm).
Surprisingly, not just one new colony has been discovered, but 9, in 8 different 1 km squares (monads) in Sheffield. One of the populations is of considerable size, suggesting that this species has been present in Sheffield for several years.
Nationally, this is a species that appears to be spreading; its principal habitat is pavement edges and waste ground, although some records from canals have also been made. It is not clear whether this species negatively affects any of our native plants, although, with a warming climate, it will be interesting to monitor for any habitat changes that might occur in the future.
Our current records of this alien grass for Sheffield are given in Table 1 and mapped in Figure 1.
Table 1. Records of Water Bent in Sheffied made by the authors up to the end of September 2013.
| Site name | Grid reference | Date | Population size | Recorder |
| Eastwood Rd, Sharrow | SK335 858 | 11.06.2013 | a few plants | AB |
| Robertson Rd, Walkley | SK324 884 | 16.06.2013 | one large plant | OP |
| Stewart Rd, Sharrow | SK333 857 | 19.06.2013 | thousands of plants | AB |
| Truswell Rd, Crookes | SK324 874 | 12.07.2013 | over 20 plants | OP |
| Armthrop Rd, Nether Green | SK315 855 | 07.2013 | one large plant | AB |
| Clumber Rd, Ranmoor | SK313 864 | 07.2013 | over 20 plants | AB |
| Crookes Road, Broomhill | SK333 870 | 18.08.2013 | over 20 plants | AB |
| Netherthorpe Rd underpass | SK345 876 | 26.08.2013 | two plants | OP |
| Providence Rd, Walkley | SK326 887 | 08.09.2013 | one plant | OP |
We strongly suspect that Water Bent is lurking in other parts of Sheffield’s suburbia, and, given that it has also just been recorded for the first time in Derbyshire (Willmot & Moyes, 2012), this is a rare opportunity to monitor the spread of an alien plant in an urban area, especially one from a Mediterranean biome that is likely to do well under a warming climate.
We therefore encourage botanical recorders in the Sheffield area to search for this species in other locations, particularly the east of the city where we have not searched, and to periodically assess its populations over the coming years.
We have included a photo demonstrating a range of typical inflorescences to promote the recording of this grass (Figure 2). Hubbard (1954) (as Agrostis semiverticillata) and Cope & Gray (2009) provide excellent line drawings of the species. Water Bent is, unsurprisingly, most similar to a bent grass (Agrostis spp.); indeed, it has been previously classified in that genus. However, one key difference is that the glumes (very small modified leaves at the bases of the spikelets) fall with the seeds.
In our native bents, the glumes remain on the pedicels as the inflorescence withers. The fact that the glumes fall with the seeds in Water Bent gradually creates a skeletal flower-head. This is demonstrated as a progression from the flower-head on the right to the one on the left in Figure 2. Perhaps the only other grass that Water Bent could be confused with at this stage is Yorkshire Fog (Holcus lanatus); this grass is also an occasional pavement weed, and also drops its spikelets whole. However, in Yorkshire Fog the pedicels have a small horizontal bar at the apex, which can be seen quite clearly once the spikelets have fallen.
Given that Water Bent is now conspicuously abundant in parts of London (R. Blades, London Natural History Society, pers. comm.), it seems likely that it is on its way to becoming a common component of Sheffield’s pavement flora; how quickly it achieves increased coverage of Sheffield at the monad scale, and at what abundance, will be questions that we hope this note will allow future botanists to at least partially address.
References
Braithwaite, M.E., Ellis, R.W., Preston, C.D. (2006) Change in the British Flora 1987 – 2004, Botanical Society of the British Isles.
Chater, A.O., Oswald, P.H., Preston, C.D. (2000) Street floras in Cambridge and Aberystwyth. Nature in Cambridgeshire 42: 3-26.
Cope, T. & Gray, A. (2009) Grasses of the British Isles, BSBI Handbook no. 13, Botanical Society of the British Isles.
Hubbard, C.E. (1954) Grasses, Penguin: Middlesex.
Willmot, A. & Moyes, N. (2012) Derbyshire Flora Group Newsletter no. 22.
Wilmore, G.T.D. (2000) Alien Plants of Yorkshire, Yorkshire Naturalists Union.