Glyceria grandis S. Watson (American Manna- grass) in Surrey George Hounsome

Glyceria grandis S. Watson (American Manna-grass) in Surrey

GEORGE HOUNSOME

In August 2019 I was exploring Squires Great Wood, an area of mixed Forestry England woodland north of Coldharbour, Surrey (v.c.17). I came across a small pond (at grid reference TQ147461) with evidence of planting in the dim and distant past, and at one end was a small patch of a fruiting grass that I didn’t recognise. The tallest culms were approaching a metre in height and the panicles, although occupying a lot of space, were extremely diffuse with tiny spikelets, more so than any grass of that size that I’d seen before. Most of the spikelets were empty, with only the glumes remaining, but the lemmas of the few remaining ones were just over 2mm long with seven prominent veins. A few florets were occupied by small black bodies which I incorrectly took to be caryopses, but more of that later. I assumed it was an established planting, took a few photographs and a piece to take home to try

Plate 1. Glyceria grandis (American Manna-grass) en masse at Great Squires Wood, Surrey, June 2020. Photographs by the author. to name it and moved on. A few hundred metres to the north, by a narrow ditch in woodland, where it certainly would not have been planted, I found another patch, so I revised my assessment of its status to ‘naturalised’.

I still had no idea even which genus it might be, but after several phone calls to Eric Clement, sending him a specimen and following up likely suggestions we arrived at Glyceria grandis S. Watson (American Manna-grass), native to North America and Canada. Indeed, the wet habitat and the shape of the leaves, parallel-sided narrowing towards the tip, was characteristic of the genus.

It was once listed in the RHS Plant Finder but not since 1997, so as it was probably introduced

before that date it has been there for at least twenty years. The BSBI Distribution Database (DDb) has a single record for the UK, by M.J. Lucas on marshy ground near a supermarket in Huddersfield in 2009.

In November 2019 Eric and I returned to the site and were squelching through alder carr about a mile and a half north-east of Squires Great Wood. We came across much more G. grandis in wet woodland, so the plant seems to be more widely naturalised than I had first thought. It was associated with square metres of Cardamine amara (Large Bitter-cress). The common factor in all three of the Great Squires Wood locations is that they are by or near the Pipp Brook, a stream arising from a spring at Magg’s Well on Coldharbour Common, which then flows north to the River Mole at Dorking. The plant may have become established in other sites downstream so, although much of it is inaccessible, over the course of the winter I looked for it in the few of the parts I could reach, but fruitlessly. A summer search might have more success. On the planned return visit to the Squires Great Wood pond site in June 2020 it was in full flower in several places round the pond (Plate 1), showing the airiness of the panicles (Plate 2). The spikelets are about 5mm long with up to six florets.

Ryves et al. (1996) lists both G. grandis and G. striata (Lam.) Hitchc. (Fowl Manna-grass), both with the comment that they are potential aliens, established and spreading in North-west Europe and Scandinavia. They are very similar, distinguished by the length of the lemmas, 2–2.7mm in the former and 1.25–2.0mm in the latter. The seven prominent veins of the lemmas are present in both species. I have never seen G. striata and as far as I can tell it has never been available commercially in these islands. I can’t imagine why anyone would want to plant either of them as they have no obvious beauty. G.canadensis (Michx.) Trin., another North American species, listed in the DDb in one site in Dunbarton (v.c.99) in 2006, has a similar habit but is a larger plant with longer, obscurely-veined lemmas.

The black objects in the autumn florets: Eric informs me that they were caused by a smut, possibly Ustilago glyceriae, which infects the ovaries, filling them

Plate 2. Glyceria grandis panicle.

with powdery black spores. My thanks go to him for guiding me through unfamiliar areas of botany.

References

Ryves, T.B., Clement, E.J. & Foster, M.C. 1996. Alien Grasses of the British Isles. Botanical Society of the British Isles,

London.

George Hounsome

14 St John’s Rise, Woking GU21 7PW george.hounsome@btinternet.com