It is easy enough to recognise that an Oat belongs to the genus Avena, but the key to the species is quite challenging, despite there being only 5 species to consider.
Avena are all annuals, with spikelets of 2-3 florets. The lemmas are bifid or have 2 bristles at the apex, and they may or may not have a long, bent dorsal awn. There is very great plasticity in the size of the parts of the flower, and you need fruits that are fully ripe for ID.
We'll begin with the 2 species that have long (3-9mm (left) not short 0.5-2mm (right)) apical points to the lemma. The question is very tricky for beginners: "Rachilla disarticulating between the florests at maturity" or not so.
Thousands of years of selection on crop plants have resulted in the fruits not falling until after the crop has been harvested. Wild plants often have fruits that fall one at a time as they ripen. This falling-off is called "disarticulation" (early breaking-up of the rhachilla)
The difficulty is that before any of the florets have fallen, you can't be sure whether any of them are destined to fall off early. The old 'absence of evidence' problem again. You need to break off a floret (forcibly). Is there are smooth scar (left) or a jagged one (right)
The other question is easier: are there dense long hairs on the lower half of the lemma (left) or not (right). So hairy and disarticulating is Avena brabata, and not disarticulating and glabrous to sparsely hairy is A. strigosa.
Now for the 3 species that have short apical points to the lemma. First, the crop plant Avena sativa. This has been selcted so as not to disarticulate, so that when you pull off a floret it leaves a jagged tear (not a smooth scar). The lemma is usually unawned (selection again)
The last two species have at least 1 smooth disarticulation scar. Avena sterilis (left) has bigger glumes (25-30mm) than A. fatua (18-25mm, right). The latter has smooth scars for all florets, the former only above the glumes (i.e. there are 2-3 fruits per dispersal unit, left)
To summarise, our crop Oat (Avena sativa) typically has no awns. The 2 common crop weeds (Wild Oats) have long awns. In winter crops you are most likely to see A. sterilis (A. ludoviciana as was, left) and spring-sown, A. fatua (right)
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Key to the common species of Agrostis. You should collect underground parts and dissect a spikelet before beginning the key to determine whether the palea is large or small (more or less than 2/5 of lemma length). We shall start with the species with larger paleas.
Select a spikelet from the very top of the inflorescence. If the back of the lemma is hairy (left) and there is a sticking-out awn, you have Agrostis castellana (much planted in commercial grass seed). Hairless (right) is something else.
Look at the ligule on the upper-most culm leaf. If it is small (0.5-1.5mm) you have the widespread and abundant Agrostis capillaris. Larger ligules (2-6mm) are something else.
Genera of Poaceae. Agrostis. When you see a grass with a big open panicle like this (left) with tiny spikelets that look as if they might contain just 1 floret (right), your mind should turn to the genus Agrostis.
Agrostis is fiddly because the flowers are so small and tricky because it is essential that you bring back the underground parts from the field. You need to dissect the spikelt and expose the lamma and, crucially, the palea
Then, with some carefully cleaned underground parts, you need to say whether the plant has rhizomes (left) or stolons (right).
Here is one of my candidates for ‘most elegant of all British grasses’. It’s Apera spica-venti, and has a droopy, shining golden inflorescence, made up of tiny spikelets with ridiculously long awns.
It stands about 1m tall, and its considerable height means that the individual spikelets (less than 3mm long) look even smaller than they are. The awns can be up to 4 times this length (they are as much as10mm long and are seldom less than 5mm).
Apera is one of the genera where there is just 1 floret per spikelet, hidden completely by the glumes (Key F in Stace, with companions like Agrostis and Polypogon)
Before we embark on the (admittedly daunting) key to the species of Festuca, it’s a good idea for you to get a thoroughly good idea of what a fescue looks like. The most sensible species to use is Festuca rubra: it’s in flower now, it’s very abundant, and it’s easy to identify.
Go out into your nearest grassland and ignore the conspicuous Holcus lanatus, Dactylis and Arrhenatherum. Look at ground level for the grass with the narrowest, most hair-like, bright green leaves. Pull up a handful big enough to guarantee having a flowering stem in it.
Shake off all the other grasses and you should be left with a flowering individual of Festuca rubra (left). It is a good idea to check that it’s not Deschampsia flexuosa (it should have straight, not wavy (right), panicle branches).
So how do you tell Phalaris (left) from Phleum (top right) from Alopecurus (bottom right) ?
These 3 foxtail grass look-alikes are easy to separate as long as you use your x10 on the outrside of the flower head. If it has got awns, it's Alopecurus.
Genera of Poaceae. Phalaris. This is a genus of big contrasts, from huge perennials like Reed Canary-grass to delicate little annuals like Canary-grass. What they have in common is that the spikelets have 3 florets but only the terminal one is bisexual (other 2 are mere scales)
As an indication of the complexities involved in differences between the species of Phalaris, the genus appears in no fewer than 3 of the generic keys in Stace. The main one is Key G, but Palaris also appears in Keys C & K as well.
Key G concerns grasses with all spikelets bisexual and similar, but within each spikelet only 1 of the florets is fully developed, with 1 or more sterile or scale-like florets. The first question is easy: is it a Setaria with stiff bristles at the base (left) or not (right)? Not.