Sedge with yellowish leaves showing both male and female part.
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The trick with starting to ID the C. viridula subsps is to compare the length of flowering stem with that of the leaves. If the leaves are not more than half as long as the stems then it's subsp brachyrrncha, if they're (or some of them) more than half as long as stems then it's one of the other two. These look longer to me, and so the next character is to look at the female spikelets. In general, all clustered together = subsp viridula, separated = subsp oedocarpa. So it looks like the latter, but not too easy too see the leaf/stem relationship from the photograph.
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Sorry it has taken so long to respond. You are right that the leaves are longer.The plants in question were escapes from a re-introduction program of Long stalked Yellow Sedge, however I am guessing there was either a mis-identification, or is it possible for them to cross easily?
There are other characters to check too on the plants themselves - which it's difficult to see from a photograph. As I said leaf length is really just a starter. But it doesn't have the look of C.v.brachyrrhynca. I don't know whether the subsp. "hybridise" but really the split is as much on ecological as any other basis and there really isn't yet any agreement as to whether they should be species or subspecies. And just to confound the issue, the latest edn. of Stace has changed the names again! We're back to C. lepidocarpa and C. demissa. But not for some reason C. serotina which has become C. oederi. Hey ho!
Lat/Lng: 51.216653057964, -1.4640151259664
OS grid ref: SU375465