Equisetum fluviatile (Water Horsetail)

Plant profiles
Picture of Equisetum fluviatile
Equisetum fluviatile

Grasses (see the last #SxPOTW) evolved around 60 million years ago. They are parvenus by comparison with the horsetails (Equisetidae), which trace their ancestry back to the Devonian period, some 400 million years ago. Horsetails were in the vanguard of plants that colonised the land. The three orders spanned hundreds of millions of years, with hundreds, if not thousands of species, ranging from tiny plants to huge ‘trees’ a hundred feet tall. Eventually these dwindled as the seed-bearing plants came to dominate terrestrial habitats, until only a single genus with around twenty species remained: Equisetum. It is placed within the ferns, forming one of the four eusporangiate families (as opposed to the dominant, more familiar leptosporangiate families).

Two species are still impressively large, with the Mexican Giant Horsetail (Equisetum myriochaetum) growing to an impressive twenty-four feet. There are nine species to be found in the UK and Stace allows that all may be native (hesitating over E. rammosissimum (Rough Horsetail)). There are twelve hybrids, which confusingly often grow in the absence of either or both parents: definitely a case for ‘Plant Crib to the rescue’. In Sussex five species and one hybrid are found. The hybrid is E. x litorale (E. fluviatile x E. arvense) (Shore Horsetail) which is thought to be under-recorded.

Distribution map of Equisetum fluviatile in Sussex
Distribution of Equisetum fluviatile in Sussex

Native. Sx: occasional, locally very common. Ditches; ponds and shallow lakes; slow streams. The map clearly shows its abundance in the river valleys and on the Levels. The decrease in the central and eastern Weald probably results from the loss of ponds in the latter part of the 20th century.

Source: E. fluviatile L. Water Horsetail, The Flora of Sussex (2018)

I’ve chosen Equisetum fluviatile because I often see it in the ditches and dykes of the Lewes Brooks floodplain. It also happens to be one of the easiest to identify on account of its enormous central cavity.

Picture of Equisetum fluviatile cross section
Equisetum fluviatile cross section

Horsetails don’t really have leaves. The scaly sheaths at each node of the stem are the equivalent of leaves and important for identification based on colouring and border characteristics. They do often have branched stems and E. fluviatile can be found with and without these. The length to the first internode of the branch, compared with the length of the sheath, is another important identification feature. The photo below shows E. fluviatile on the left and E. palustre on the right. The latter has branches. The teeth of the sheath show distinctive black tips with white, scarious margins. You can also see the spore-bearing cones at the top of the stems.

Picture of E. fluviatile vs E. palustre
E. fluviatile vs E. palustre

In our largest (up to 2m) Horsetail, E. telmateia (Great Horsetail), these appear in spring on smooth, ivory-white, succulent stems, before the green vegetative stems appear. Our commonest Horsetail, E. arvense also has cones appearing in spring on non-green more succulent stems, but they are 1cm – 4cm long, compared to the 4cm – 8cm of the former.

Picture of Equisetum telmateia
Equisetum telmateia

Both these species can produce small tubers on their rhizomes which assist in vegetative spread. They therefore multiply when the soil is disturbed and have gained a reputation as pernicious weeds. There is a nice summary of Equisetum biology here.

The internodes get closer and closer together up the main stem, a phenomenon which allegedly inspired John Napier to invent logarithms! Although used in ‘herbal medicine’ for centuries, there are no proven therapeutic properties of the plants, although the silica in the cell walls made them useful in the kitchen, giving them one of their vernacular names, ‘scouring-rush’.