Thread by @crawley_mick

Many finger-grasses were wool-aliens from Australia and South Africa. They were imported as seeds tangled up in the wool or embedded in the dags that clung to the tail and rear-end of the animal. Once importation of dirty fleeces stopped (mid 20th C) they were seldom recorded.

The digitate grass genera in each of the 4 categories are separated as follows. First the paired inflorescences. There are just two genera, both rare. Paspalum distichum (left) has glumes thinner than upper lemmas. Andropogon distachyos (right) has glumes thicker than lemmas.

Digitate genera: 1 common (Digitaria), 1 occasional (Cynodon), 3 rare (Eleusine, Chloris & Dactyloctenium). Telling Digitaria from Cynodon is tricky (x20 ideally). Count the scales beneath the fertile floret (glumes + lemmas + paleas). Cynodon = 2 (left); Digitaria = 3 or 4 (R).

Dactyloctenium is easy because it has a bare length of rachis at the finger-tip with no spikelets (illustrated is D. aegypticum). The others have sprikelets right up to the tip.

You tell Eleusine from Chloris very simply by the lemmas: they are awned in Chloris (left) but not in Eleusine (right). Both are big genera, but only Eleusine indica is seen at all frequently in the UK these days (illustrated on next tweet)

Eleusine indica (left) and Chloris divaricata (right)

There are 3 genera with semi-digitate inflorescences: 1 common (Miscanthus) and 2 rare (Bothriochloa and Dichanthium). Miscanthus is the easiest because it has spikelets in pairs on pedicels of different lengths (L), each spikelet with 1 bisexual and 1 (lower) infertile floret.

You tell Bothriochloa from Dicanthelium by a comparison of the glumes and the lemmas. Bothriochloa has glumes thicker than the lemmas, Dicanthelium has membranous glumes that are thinner than the lemmas. B. ischaemum (left); D. sericeum (right), both very rare as UK aliens.

Finally, we have the finger-grasses with spaced, step-like inflorescences. There is 1 common and widespread genus (Echinochloa) and 2 rare genera (Urochloa and Brachiaria). In Echinochloa (left) at least some of the lower lemmas are awned; not so in the other two genera.

Urochloa is told from Brachiaria by the surface of the upper lemma: rugose and verrucose in Urochloa panicoides (left), smooth in Brachiaria platyphylla (right), the only species you are likely to see in Britain.