Somerton and the Oxford Canal

Botanists botanising by the Oxford Canal. Remark the use of modern technology. Image by Fay Banks

Recording for Atlas 2020 in Oxfordshire continues. The target of this Sunday's meeting was the county's most under-recorded 10km square, SP42. Earlier in the spring I wrote about a recording outing of mine in SP42 at Tackley, and this week the venue was the parish of Somerton in SP42Z, further north along the River Cherwell/Oxford Canal. There will be another meeting in SP42Z on 8th July at Bestmoor SSSI, when we will survey the huge population of the nationally scarce Oenanthe silaifolia (narrow-leaved water-dropwort).

Sporangia of P. interjectum x100, with few thick-walled (indurated) cells forming the annulus and two basal cells between this and the stalk

The five of us who met at Somerton were blessed with wonderful weather and some fabulous plants. We started at the churchyard of St James’ which was a tidy disappointment, however. An exhibitor at the church exhorted us to come in afterwards for coffee and artwork, but we conveniently forgot and instead went to the Bell Inn in Lower Heyford! The only interesting native species in the village was a large amount of the fern Polypodium interjectum (intermediate polypody) on a shaded wall. Contrary to previous records I am finding that this is a common fern of limestone walls in Oxfordshire. I’d be grateful if other recorders would check any Polypodium they find microscopically, or record it as P. vulgare sens. lat. rather than attempt an identification based on frond morphology as this is not reliable.

We left Somerton southwards across bright green agricultural grassland and wheat fields infested with Alopecurus myosuroides (black grass). At this point I began to worry about our species list. A short detour out of SP42Z into SP42Y gave us Senecio viscosus (sticky groundsel) by the railway, from where we looped back into our tetrad and picked up the Oxford Canal. From here our fortune turned.

Despite being churned into a brown soup by boats, the canal was quite rich and we recorded many riparian species. Among the more frequent was Rumex hydrolapathum (water dock), the huge ornamental leaves of which were much admired. There were also many tussocks of that crazy sedge Carex paniculata (greater tussock sedge). Perhaps this species is favoured by the slow-flowing water and erosion caused by boats, for we saw none on the River Cherwell a short distance to the west. In the river we had species of more consolidated, tall-herb vegetation, e.g. Symphytum officinale (common comfrey), Rorippa amphibia (great yellow cress) and Lysimachia vulgaris (yellow loosestrife). We also had two of our best finds here.

The first of these, Oenanthe silaifolia, I walked past twice, keener eyes picking out solitary plants in dense nettles. I should now now have my eye in for the July visit to Bestmoor SSSI! These plants are perhaps relicts of a once larger population of this much-declined plant. Oenanthe are also adept colonizers having buoyant fruits, and so they could be arrivals from upstream.

The most exciting find of the day (for me anyway) was another Oenanthe, not that it was much to look at. We found one plant of O. fluviatilis (river water-dropwort) growing in fast-flowing water in a narrow meander of the river. I was initially circumspect about this identification as the species had been presumed extinct in the county, not having been recorded since 1990. The aquatic Oenanthe are also not easy to identify vegetatively, and O. fluviatilis is shy to flower which doesn’t help.

O. fistulosa and O. aquatica both develop many-pinnate leaves with extremely delicate filiform segments when permanently submerged, but the Somerton plant had compound leaves with rather irregular segments, which were laminar, more-or-less linear or cuneate-based and obtuse. It also lacked the swollen stem of O. aquatica, and I have never encountered these other species in strongly-flowing water. Thus while confident of my identification, I will grow on a fragment of leaf and stem removed to examine in the field with the hope of providing certain identification in the form of fruits.

The meadows between the river and the canal were also quite productive, though surely a shadow of their former glory. There were drifts of Conopodium majus (pignut), a good colonizer of grassland, and an overgrown ditch supported Carex actua, C. disticha, and C. nigra (slender tufted, brown and common sedges, respectively), and Equisetum fluviatile (water horsetail). A Scottish attendant new to the county was surprised to learn that C. nigra is a fairly uncommon species with us. Such has been the fate of our wet grasslands.

That was not the end of it! Retracing our steps along the towpath, we marveled at Saxifraga granulata (meadow saxifrage) in full bloom, presumably extirpated from the meadows but hanging on outside the fence by the path. This was a double delight for me as I had not yet seen this species in Oxon. At Somerton bridge we also saw Cardamine amara (large bittercress) on the opposite side of the canal, showing it can be useful to retrace ones steps and look the other side of the path on the return journey!

Back at the cars the final tally was of 235 taxa for SP42Z, rather good going for mid-May with much more to come and a limited selection of habitats. And of course the Oenanthe bonanza (the above plus O. crocata (hemlock water-dropwort)), one of which is a county resurrection. Many thanks for the help, enthusiasm and pleasant company of everyone who attended the meeting and made it such a success. Thank you also for the photographs — I will charge my camera next time!