Wednesday 21 March 2012

Poa pratensis - Smooth Meadow-grass

Grass of the Month for March 2012

Photo of Poa pratensis taken on 28 January 2012 outside Church Hall

Poa pratenis between two paving stones at St John's Church Hall, Settle
This year I am "Wading through the Poas". Whilst I like to maintain that I can recognise most grasses any month, March proved hard. I drove around to favourite sites looking for Poa compressa, and expecting to see Poa trivialis and Poa pratensis everywhere. The Poa compressa was rather dead and undistinctive. The Poa trivialis - normally so common around here - was alsmost absent. I suppose all last year's growth had decomposed. However here are two photos I took at the end of January of Poa pratensis.

All the Poas have tramlines and boat-shaped tops.

I can tell it is not Poa annua (which is flowering now) because there are a lot of thick dead sheath bases and old leaves - implying this plant is a perennial, and the blades are thick textured and slightly bluish green, and it has short ligules..  Poa annua would not have many dead remains, the blades would be thin and whitish or mid green, and it would have bigger white ligules. Poa annua does not have rhizomes.

Poa pratensis has rhizomes - and it is relatively easy to guess that this plant between the paving stones may have rhizomes - the way the shoots are spaced along the crack with gaps between the tufts. If you dig it up you should see the rhizomes.

Poa pratensis has parallel sided leaves (unlike trivialis which has tapering leaves),
Poa pratensis has bluish green leaves (unlike trivialis which has shiny bright green or yellow green leaves)

The shoots of Poa pratensis  rise vertically from the ground (or almost so) and then the blades stick out at a wide angle to catch the sunlight

Poa pratensis and Poa humilis
Poa pratensis is extremely similar to Poa humilis  (Spreading Meadow-grass) formerly called Poa subcaerulea. Poa pratensis has tufts of shoots joined by rhizomes whereas  Poa humilis has single shoots.  Some books say that Poa humilis has tiny hairs at the base of the leaf blade - but I have found that both species have this.
Poa humilis can be found in mole hills in damp peaty soil. Poa pratensis is more likely to be found in cracks in dry walls. but there is a big overlap.
For the critical feature you need to have a flower:
Poa pratensis has one vein on the lower glume
Poa humilis has three veins on the lower glume

I will be teaching a grass identification course at Juniper Hall in ten days time.

(See other months' grasses)

No comments: