Several studies of urban areas have found that the composition of the flora can change considerably between survey periods. For example, a survey of the street flora of Aberystwyth between the 1970s and 1998-9 found a large turnover of species (Chater et al. 2000), with species associated with drier, warmer conditions becoming more prevalent. In this way it can be of interest to monitor the urban flora, because the rapid changes in the plants that make their homes there may provide an insight into how our urban environment is changing. In this spirit, Ambroise Baker and myself were excited to independently discover the alien grass Water Bent (Polypogon viridis) new to the streets of Sheffield and South Yorkshire. Whilst some records of this plant do exist for vice-county 63 (e.g., from wool shoddy, Halifax, 1960, F. Houseman), this appears to be the first find of Water Bent for the modern county of South Yorkshire (GTD Wilmore, pers. comm., 2013).
Surprisingly, Ambroise and myself have not just discovered one new colony, but 6, in 5 different 1 km squares (monads) in Sheffield, with some populations of considerable size, suggesting that this species has been amongst us for at least a few years. This is a species that appears to be spreading throughout the British Isles; its principal habitat is pavement edges and waste ground, although some records from canals have also been made. It is not clear that this species negatively affects any of our native flora, although, with a warming climate, it will be interesting to monitor for any habitat changes that might occur in the future. So far, our records of this alien grass are:
Eastwood Rd, Sharrow, SK335858, 11.06.2013, a few plants (AB)
Robertson Rd, Walkley, SK324884, 16.06.2013, one large plant (OP)
Stewart Rd, Sharrow, SK333857, 19.06.2013, thousands of plants (AB)
Truswell Rd, Crookes, SK324874, 12.07.2013, over 20 plants (OP)
Armthrop Rd, Nether Green, SK315855, July 2013, one large plant (AB)
Clumber Rd, SK313864, July 2013, over 20 plants (AB).
It seems likely 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, 2012/13, Derbyshire Flora Group Newsletter No. 22), this is a great opportunity to get some good baseline data to monitor the spread of an alien plant in an urban area. The above photo demonstrates how it is most likely to look at this time of year. Water Bent is, unsurprisingly, most similar to a bent grass (Agrostis spp.); indeed, it was previously classified in that genus; however, one key difference is that the glumes (very small leaves at the base of the flower spikes) fall with the flowers. In our other common bents, and in meadow grasses (Poa spp.), parts of the flower remain on the stalks as the plant dies. 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 the photo.