BSBI News article

Below the article Oli and I have published in the BSBI News – a great journal for those who love field botany! – available to all subscribing members of the Botanical Society of Britain and Ireland (BSBI).

“How did Polypogon viridis (Water Bent) find itself on the streets of the British Isles?


We recently reported Polypogon viridis (Water Bent) new to Sheffield and the metropolitan county of South Yorkshire in the local natural history press (Pescott & Baker, In press). Because the Sheffield area is relatively well recorded (Wilmore et al., 2011), we were surprised to discover as many as nine colonies of P. viridis in seven different monads within the city of Sheffield.

The largest population numbered around several thousand plants and it can be speculated that it has been naturalised at that site for several years. On the sole basis of our 2013 observations, however, it is not possible to say whether P. viridis has rapidly colonised the city in recent years, or whether it was previously overlooked.

The situation in Sheffield appears to be in line with national trends (Pearman & Bennallick, 2009), but little is known about the environmental conditions favouring this species establishment, about those inducing local extinctions, about its vector(s) of colonisation, or about the speed at which it is spreading.

These factors are of great interest for understanding how new plants, or groups of plants, enter our flora. The present note discusses the potential reasons why this alien grass appears to be spreading nationally and ends with a call to members to take part in a brief questionnaire survey concerning P. viridis.

This questionnaire is the first step in an attempt to better understand why and how this alien grass is becoming established across the British Isles. We hope that the second step will be the implementation of a series of small field surveys. We are counting on your participation and on your knowledge of this plant in order to gather nationwide information regarding P. viridis.

Potential reasons for increase

There appear to be three main reasons why P. viridis is being increasingly reported from towns and cities; it is possible that these act in combination:

  • Introductions have increased in frequency and distribution;
  • environmental conditions have changed and new areas have become favourable for the establishment of P. viridis;
  • P. viridis has been under-recorded historically.

Increased introductions?

Two main sources of seed and plant introduction can be considered. First, the New Atlas (Preston et al., 2002) suggests that P. viridis may be spreading from plant nurseries to pavement cracks via gardens. Pearman & Bennallick (2009) noted that it occurred in every plant nursery they visited, and that it was becoming increasingly common in their area (Cornwall). This would not be a unique case of colonisation from imported plant containers (Clement 2010; Hoste et al., 2009).

Second, bird seed mixes are often grown abroad and may contain various contaminants (Hanson & Mason 1985). Hanson & Mason do not mention P. viridis in their exhaustive list of aliens recovered from pet food, but seed contaminants are likely to have changed since 1985. Thus, introduction as bird seed contaminants cannot be excluded.

In addition to these two main sources of plant material, transport on vehicle tyres should also be considered owing to the increased exchange of traffic with the continent. Van der Lippe & Kowarik (2007) demonstrated for the Berlin area that vehicles are an effective agent of long-distance seed dispersal, although they did not record P. viridis in their samples.

More favourable conditions?

Three important environmental factors can be considered. First, the fertilising effect of nitrogen deposition induced fundamental changes in the vegetation of numerous habitats in the UK (Phoenix et al., 2012). The spatial distribution of nitrogen deposition has been extensively modelled for the UK and as a result a comparison with accurate distribution maps of P. viridis might enable a test of whether nitrogen deposition has an effect on establishment.

Second, increased CO2 concentration in the atmosphere has a fundamental effect on plant physiology, water loss in particular. Higher CO2 concentration allows more rapid CO2 intake by plants, resulting in a shorter period of stomatal opening, and, in turn, reduced water loss. Following this logic, increased CO2 may render plants more drought resistant, potentially allowing them to colonise new habitats.

Third, climate warming is also likely to have influenced the spread of P. viridis during the last decades. The BSBI’s ‘Local Change’ project noted a tendency for species with a Southern-temperate or Mediterranean biogeography to have increased significantly in built-up areas and gardens between the two survey periods (Braithwaite et al., 2006).


Our Sheffield experience could suggest that P. viridis has been under-recorded to date. One of the authors of this note (AB) had a similar experience when he moved to Reading (Berkshire, vc. 22). Despite P. viridis not being mentioned in the detailed Flora of Berkshire (Crawley 2005), he encountered many large populations on pavements between 2007 and 2009.

P.viridis is also thought to be under-recorded in Switzerland, where the standard floras do not mention it, but where well-established populations have been observed recently (Hoffer-Massard 2012).

Two factors may have caused under-recording in the British Isles: First, in a scenario where P. viridis was rare in Britain several decades ago, and has gradually increased since, it can be hypothesised that many botanists would not consider it as a likely encounter.
Second, it is possible that one of the habitats favoured by P. viridis, pavement cracks and front gardens, is under-recorded in comparison to other urban habitats, such as brownfield sites, and very under-recorded in comparison to non-urban habitats. As a consequence, there may be a longer time-lag between initial colonisation and recording.

The Water Bent questionnaire

In order to find out which of these hypothetical reasons best explains the spread of P. viridis, we are undertaking the collection of new data. First, we would like to hear about your experiences, and get your views on the matter, and we have designed a brief questionnaire to that end.

We would be very grateful if all readers would complete a copy. Confirmation of the absence of P. viridis, or of it potentially being overlooked, is as important as confirmation of presence, therefore we would ask you to complete a questionnaire even if you have never encountered this plant in the field. You can either complete it at, or request an electronic or paper copy from either author.

As a second stage we are planning small field surveys as early as next summer (2014), and again we would be extremely grateful if any members would be willing to monitor their local patch of P. viridis for changes. Please get in touch!


  • Braithwaite, M.E., Ellis, R.W. & Preston, C.D. (2006). Change in the British flora 1987 – 2004. Botanical Society of the British Isles, London.
  • Clement, E.J. (2010). ‘Weeds of container plants’. BSBI News, 114: 42.
  • Crawley, M.J. (2005). The flora of Berkshire. Harpenden, Brambleby Books.
  • Hanson, C.G. & Mason, J.L. (1985). ‘Bird seed aliens in Britain’. Watsonia, 15: 237-252.
  • Hoffer-Massard, F. (2012). ‘Polypogon viridis (Gouan) Breitsr., encore une nouveauté en Suisse romande’. Bulletin du Cercle Vaudois de botanique, 41: 99-100.
  • Hoste, I., Verloove, F., Nagels, C., Andriessen, L. & Lambinon, J. (2009). ‘De adventievenflora van in België ingevoerde mediterrane containerplanten’. Dumortiera, 97: 1-16.
  • Pearman, D. & Bennallick, I. (2009). ‘Polypogon viridis’. BSBI News, 111: 39.

Leave a Reply

Your email address will not be published. Required fields are marked *