(Some of these notes first appeared in the members only Gloucestershire Naturalists’ Society Facebook Group, GNS Playground, on 5 July 2019).
There are three water-dropworts (Oenanthe species) commonly found in the floodplain meadows of the Severn and Avon in Gloucestershire. They are difficult to tell apart, especially when the basal leaves have disappeared, which often will have happened by mid-June.
They have slightly different habitats and time of flowering, but this can’t be relied on. At the wettest end, in places where water lies all winter or the edge of a pond, is Tubular Water-dropwort (Oenanthe fistulosa). This tends to be the latest to come into flower, often in early July. Its characters include a very reduced number of partial umbels – sometimes just two or three, the stem is constricted at the nodes, and the upper stem leaves are very reduced – a bit like a stick drawing of a leaf. Narrow-leaved Water-dropwort (Oenanthe silaifolia) is the plant of the wet meadows and is the earliest to come into flower, often in mid-May. In the driest fields on the floodplain and unimproved fields and road verges off the floodplain, is Corky-fruited Water-dropwort (Oenanthe pimpinelloides) which flowers from about mid June.
It is easy enough to decide which of the the latter two you have if you are comparing them side-by-side, but when you only have one plant in front of you you can begin to doubt yourself. These notes may be helpful.
A consistent character is the stem at the base of the flowering shoot. If it squashes easily between the thumb and forefinger it is O silaifolia. The stem here is fairly silky to the feel and only gently ridged. If it gives very little when squashed between the thumb and forefinger and feels hard and ridgy, it is O pimpinelloides.
At the other end of the plant, looking at the biggest head of flowers, if you can imagine getting your fingers between them then it is silaifolia. If in the biggest head of flowers the partial umbels are touching or almost touching then it is probably pimp. This character is even better developed as the fruits ripen.
A clincher difference between these two – in silaifolia the fruits of the partial umbel splay out into a gentle dome. In pimp they stay pretty much upright, so that looking from the side you can see a row of individual fruits in profile.
21 June 2021