Tuesday 3 January 2012

Poa annua - Annual Meadow-grass

Poa annua - Settle Railway Station in October  2007
Grass of the Month for Jan 2012

(For more grasses, see  Grasses Index )

Poa annua - Annual Meadow-grass was set to be my "joker" or reserve card in this series - It is one of the easiest grasses - because you can find it in flower ANY MONTH of the year!.. And in EVERY PART of the British Isles .. I was going to save it till there were no other grasses in flower to describe..
Well - we are now starting Year Three, and it is January, so I shall use it - You can look at the 24 grasses already described at Grasses Index

Poa annua is a good introduction to the Poas - I plan to  present Poas for the next six months (!!!) - that should be very useful for you because they are very common..  bookmark this blog!.

Go out and look in the paving stones where you live - any small grass in flower now is almost bound to be Poa annua.  Easy!

A walk round Settle or most towns and villages will reveal a head poking up between paving stones somewhere!

The walk from Ripon to Fountains Abbey on Boxing Day 2011 revealed - Groundsel, Ivy-leaved toadflax, Red Dead-nettle, White Dead-nettle, Ragwort, and the grasses Cock's-foot and Annual Meadow-grass in flower.

All Poas have blunt boat shaped-tips to their leaves - and the flower heads are like Christmas Tree panicles.

The blades have "tramlines" -
Definition of "tramlines":- Look at the blade from diagonally above - on either side of the midrib there is a groove which looks darker because of shading from the light. These are the tramlines - or maybe it refers to the midrib which sticks up from the centre of the grooves - as the metal bar does in street tramlines.

At the base of the 2 grooves are special cells which can contract under dry conditions (thus causing the blade to fold) and expand under moist or sunny conditions allowing the blade to open and collect more light for photosynthesis.  The majority of the stomata (holes where gases move in and out of the blade) are on the upper surface of the blade. Hence water loss can be reduced by the blade folding under dry conditions.

Since all common Poas have boat shaped tips to the blades and tram lines and Christmas-tree flower heads,  how do we differentiate Poa annua?

Poa annuaPoa trivialisPoa pratensis
blade tipboat-shapedboat-shaped boat-shaped
blades sidesvery slightly taperingtapering parallel sided
colour of plantwhitish greenbright shiny yellow greenbluish-green
ligule on flowering shootbig wide whitelongvery short
ligule on vegetative shootbig wide whitevery  shortvery  short
rhizomes/ stolons/nonetufted only; shoots grow
at diagonal angles
stolons (eventually)
vegetative shoots
lie flat along ground

1st Jan 2012 - Poa annua growing
in Allhallowgate Car Park, Ripon 
Poa annua does not have rhizomes (as do Poa pratensis, Poa humilis and Poa angustifolia)
Poa annua does not have stolons or shoots that lie flat against the ground as does Poa trivialis

Poa trivialis and Poa pratensis are the most common other Poas. Poa pratensis always has short ligules.
Poa trivialis has long ligules on its flowering stem but short ligules on the vegetative stems.
Poa annua  has big white, wide, ligules on all its stems - giving tiny plants a whitish green colour.

The blades of Poa annual can have undulations or crinkles in them which is very characteristic - but these can occasionally be found in other grasses too.

(For more grasses, see  Grasses Index )

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