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.
