The Shropshire Botanical Society newsletters contain excellent guides to some of the more difficult plants such as grasses which can be found here .Poa are distinctive because
‘The spikelets have a very characteristic sharp edged oval outline because the backs of the lemmas are sharply keeled and there is a complete lack of awns, but in most species the spikelet is small (often less than 0.5 cm long) and there may not be many flowers in the spikelet, so that, especially early in flowering, you might need to look with a lens to be sure it is a Poa rather than an Agrostis. ‘(p23)
The ends of the leaves are often keeled in Poa species though beware Dactylis and Seslaria .
I am concerned here with two of the more common meadow grasses found in meadows, namely Poa trivialis Rough Meadow grass and Poa pratensis Smooth Meadow Grass. Traditionally the rough and smooth epiphets refer to how the stem sheaths feel and this distinction is used by Hubbard (1984. p185), along with the long pointed membranous ligule to determine from other Poa species.
According to Cope and Gray (2009. p212) Poa trivialis is extremely variable varying in growth form, panicle shape and colour as well sometimes having quite smooth stems. The pointed ligule being the most important character although this is similar to Agrostis canina, which however lacks the keel shaped tips to its leaves.
Poa pratensis is however more complicated. Cope and Gray (2009) treats it as three subspecies whereas Streeter (2009) for example names them as separate species.
Poa pratensis subsp. irrigata = Poa humilis (= Poa subcaerula[Hubbard])
Poa pratensis subsp. pratensis = Poa pratensis
Poa pratensis subsp. angustifolia = Poa angustifolia
Poa pratensis displays extremely variable and difficult taxonomy and Cope and Gray (2009) say that the key to the subspecies is a compromise (p223). For my purposes I am happy to use the broader description, though to take it further the number of nerves on the glumes and the leaf width are critical.
Differences between Poa trivialis and Poa pratensis
|Poa trivialis||Poa pratensis|
|Ligule||Long pointed up to 3-9mm||Blunt up to 3mm|
|Stem||Rough to smooth||Smooth|
|Panicle||Ovate to oblong||Pyramidal, open|