Oenanthe silaifolia

Narrow-leaved water-dropwort is a hairless, tuberous perennial growing up to 100 cm tall with a stem that is solid below and hollow in the upper parts, grooved and striated, 0.7 cm in diameter. The upper leaves are once to twice pinnate with linear or lanceolate (sword-shaped) leaflets up to 3 cm long; the lower ones are 2-4 pinnate, with similarly narrow leaflets. The leaf stalks of the lower leaves are long, but the upper leaves have stalks that are shorter than the leaf blade. They do not form a sheath around the stem at their base.[1][2]

It flowers in June in northern Europe, with primary umbels of 4-8 smaller rounded umbels about 2 cm in diameter, each of which has numerous white flowers. There are no bracts on the main umbel and many (10-17) small, lanceolate bracteoles at the base of each of the individual secondary umbels. Plants are monoecious, with hermaphroditic and male flowers. Each flower has 5 unequal petals with the outer ones slightly larger, 5 stamens and 2 prominent styles arising from a swollen base (stylopodium) at the top of the ovary. After flowering, the flower stalks thicken, and the umbels do not become flat-topped in fruit. The fruit are 3-3.5 mm long, cylindrical, with prominent ridges.[1][3]

The first description of narrow-leaved water-dropwort was by the German botanist Friedrich August Marschall von Bieberstein in his Flora Taurico-Caucasica[4] in 1819. It has dozens of synonyms (i.e. other authors have subsequently named the same plant, but Bieberstein's name has precedence), including O. biebersteinii Simon (1903), O. peucedanifolia Heuff. ex Boiss. (1873) and Phellandrium lobelii Bubani (1899). A full list can be found in the Synonymic Checklists of the Plants of the World.[5] A few forms and varieties have also been named, but none are currently accepted. It is not known to hybridise with any other species.[3]

Its chromosome number is 2n = 22 (based on British specimens).[1]

The specific epithet "silaifolia" means "with leaves like silaum" and refers to its uniformly narrow leaflets.

The global range of narrow-leaved water-dropwort is in Europe, the main populations being in France, England, Spain, Italy and Greece. It extends as far north as The Netherlands, east to the Caspian Sea, and southwards as far as Israel and North Africa. It is not recorded as an introduction beyond its natural range.[6]

In Britain, it is found from southern England as far north as Yorkshire, and as far west as SE Wales. It is a plant of wet meadows, typically those which are flooded with calcareous river water during winter, but it is not tolerant of agricultural intensification and is common only in unimproved grasslands. It is also found on river banks and ditch sides.[7]

Its Ellenberg values in Britain are L = 8, F = 9, R = 7, N = 5, and S = 0.[8]

In a study of grasslands in the Marais Poitevin region of France, Oenanthe silaifolia was found not to contribute to the soil seed bank, which the authors of the study considered was because it primarily reproduces vegetatively. Any seeds that are produced may be short-lived.[9] In Britain, populations in unimproved fields can number tens of thousands of plants, but the meadows are typically mown before flowering, so seed production is rare.[10]

In Britain and France, it is classified as Least Concern, meaning that it is not rare nor declining at a particularly high rate. In certain counties and regions, however, it is judged to have a higher threat status.[11][12] Overall, it appears to be increasing slightly in Britain, although there is uncertainty about the veracity of some records.[7]

It is considered an axiophyte in several British counties.[13]

The aerial parts of narrow-leaved water-dropwort are not poisonous to livestock, although the tubers may be.[14] No part of the plant is edible by humans, and it appears to have no commercial uses.