Thursday, June 23, 2016

Poa trivialis and Poa pratensis

Of the 100 or so grasses in the UK, only about a quarter are at all common, but that doesn't mean that identifying them is a breeze.

Even among the commoner grasses, there are quite a few serious hurdles to get over, and this post is about one of them: distinguishing Rough Meadow-grass (Poa trivialis) from Smooth Meadow-grass (Poa pratensis).

Now in one sense this is a no-brainer: one has rough stems and the other has smooth stems. A more reliable feature (because P. trivialis isn't always noticeably rough) is the totally different ligules: That of P. trivialis is  quite long (4-8mm) while that of P. pratensis is a short neat collar (1-2mm).

That's not the challenge. The challenge is to distinguish the two species without manual inspection, from a distance. To know which of the two species you're walking past.

If you look at the distribution maps in Fitter et al, you'll see that both species are absolutely ubiquitous in N. Europe. What you mustn't infer from this (as I did for years) is that both species are equally abundant and that any patch of grassland is likely to contain both.

Worldwide, P. pratensis is the more widespread species (native to the USA, for example). Even in Sweden  P. pratensis (Ängsgröe) is regarded as more common than P. trivialis (Kärrgröe). But in the Atlantic climate of the UK it's P. trivialis that dominates.

In most typical UK grassland of the semi-urban type P. trivialis is vastly the more common species. This is the bog-standard meadow-grass species that you find on verges and fields, inter-growing with Perennial Rye-grass, Wall Barley, Lop-grass, Barren Brome, Cock's-foot, Yorkshire fog, False Oat-grass, etc.  P. pratensis doesn't overlap all that much because it prefers distinctly dry grassland, whereas P. trivialis likes normal-to-damp grassland and is prepared to grow in the hollows between taller species.

(A plant that you will find alongside P. trivialis is P. annua, especially on paths and anywhere the ground is trampled.)

Here's some suggestions for picking out P. pratensis from a distance.

1. Drier places.
2. P. pratensis is very upright, the stem growing straight up from the rhizome. (P. trivialis has stolons and the stems are initially procumbent.)
3. Blade of uppermost stem leaf is very short (shorter than sheath). Stem leaves very erect, close to stem. Blades parallel-edged before suddenly narrowing to hooded tip. (P. trivialis leaves are not hooded.)
4. Spikelets are slightly larger than P. trivialis, with pointed glumes.


When it comes to a genus like Poa, images that you pick up from Google searches should be treated with grave suspicion. Many are palpably misidentified: either by the contributors themselves, or by Google being led astray by the surrounding text.

Poa links.

Matt Lavin's Albums on Flickr are a treasure-house. Here's his Poa album:

Matt's numerous other grass albums can be reached from here:

Unfortunately Matt is based in Montana, USA and his Poa album has no images of P. trivialis , which is not native to America.

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