Wednesday, May 24, 2017

At a glance: Smooth and Rough Meadow-grass

















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: Rough Meadow-grass has rough stems and Smooth Meadow-grass 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.

P. pratensis prefers dry grassland, whereas P. trivialis likes normal-to-damp grassland and is prepared to grow in the hollows between taller species. 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 variety, P. trivialis is the more abundant 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.

Yes, P. pratensis is usually around too, but in smaller quantities. For instance, in my own unmown patch of lawn there's one little group of P. pratensis by a paving slab, the rest of the Meadow-grass is P. trivialis and Annual Meadow-grass (P. annua).


Anyway, here's some suggestions for picking out P. pratensis from a distance.

1. Drier places.
2. P. pratensis is very upright, the stem often seeming to grow 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), stopping a long way short of the inflorescence. Stem leaves usually very erect, close to stem. Blades parallel-edged before suddenly narrowing to hooded tip. (P. trivialis leaves are not hooded.)
4. Spikelets are broader and stubbier than P. trivialis, with pointed glumes.
5. P. pratensis comes into flower a week or so earlier than P. trivialis. Those precious few days in late May are the best time to get acquainted with the two species (if you are not otherwise distracted by buttercups, cotoneaster, beaked hawksbeard, or young oak leaves...).
6. In general, P. pratensis tends to be a smaller plant than P. trivialis (though individual exceptions are quite common).


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Smooth Meadow-grass (Poa pratensis) (25th May 2017, Swindon)

Smooth Meadow-grass (Poa pratensis) (25th May 2017, Swindon)

Rough Meadow-grass (Poa trivialis) (25th May 2017, Swindon)

Rough Meadow-grass (Poa trivialis) (25th May 2017, Swindon)
















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Rough Meadow-grass on the left, Smooth Meadow-grass on the right


(The plant in the middle is a kind of sedge.)








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Poa pratensis - inflorescence









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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:

https://www.flickr.com/photos/plant_diversity/albums/72157627934466822

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

https://www.flickr.com/photos/plant_diversity/albums/with/72157627934466822

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


[Long-term readers may recall that this post appeared in a more primitive form last year.]


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