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Apart from any other characters, the large size and one sided panicle (well shown in this photograph) are immediate features to recognise in the field.
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Sorry, but are you sure this Dactylis glomerata?
The form of the spike is too bilateral, Dactylis glomerata is not symetrical. Also the awns are not long enough.
Not sure what this is from the close photo but could it be a Lolium species?
No doubt in my mind (he said with possibly misplaced confidence). Of course the photo gives one no chance of looking at the key diagnostic characters, but this has short awns and very hairy keels on the lemmas. All Lolium sp. have glabrous lemmas or at least the UK species do - I haven't checked the international database. I can't think of any other sp. which has hairs this long on the lemmas. And the one sided panicle is an absolute giveaway. But D. glomerata can be deceptive early in the season (when this photo was taken) when the panicle is unexpanded and the flowers closed. Sorry Bill, this sounds a bit pompous. Unintentional.
Lat/Lng: 52.1, -3.4
OS grid ref: SO0451