The spike-rushes (Eleocharis species, Cyperaceae) are nowhere near as difficult as their reputation suggests. If you want to learn them, there is nowhere better to start than with the commonest of them all (E. palustris) which you will find at the shallow end of your local pond.

All Eleocharis have leaf sheaths without any leaf blades, and there is a single, terminal inflorescence (the spikelet). At the base of the spikelet are glume-like bracts (these are usually referred to as the lowest glumes).

This is the common Eleocharis palustris (Common Spike-rush). There are three important questions: how big are the lowest glumes; how many flowers are there in the spikelet; and how sheathing is the base of the lowest glume ?

First the size of the lowest glume. This is what a small glume looks like. Technically, a small lower glume is defined as less than 2/5th of spikelet length (certainly less than half). You will see a big glume later.

Second, count the flowers: there are many in this case (defined as more than 10)

Third, look closely at the base of the lowest glume. It clearly doesn’t completely encircle the base of the spikelet (in fact, it encircles less than half the circumference).

That’s enough to identify it as E. palustris in most parts of Britain (we’ll talk about the local and rare E. mamillata subsp. austriaca later). So that wasn’t hard, was it ?

Another thing that helps with identifying spike-rushes is that their ecological preferences are relatively clear-cut, so the habitat where you find it is a very useful feature. Marsh or dune slacks. Bogs and peaty places on acid soils. Fens. Stony lake margins. Estuarine mud.

OK. Here we go. Let’s start with the plants that have big lower glumes (>2/5th spikelet length) and few flowers (<12). This is what a big glume looks like. Sorry about the low picture quality, but you get the idea.

The common species with big glumes is Eleocharis quinqueflora (bots call it ‘kinky flora’). This grows in wet places in fens, dune slacks and on moorland. It has relatively fat, stout stems (>0.4mm wide) and big spikelets (4-10mm); glumes are brown (there may be a green midrib)

The very delicate, tiny plant with big glumes growing on pond and lake margins is Eleocharis acicularis. This has narrow stems (<0.4mm) and tiny spikelets (2-5mm) and lower glumes (<3mm)

There is just one other species with big glumes: this is the very local and rare Eleocharis parvula. It has greenish (not brown) glumes, and very slender whitish rhizomes that end in tiny white tubers (2-5mm). It grows in wet, estuarine mud in SW Britain and in Co. Londonderry.

All of the other species have small glumes and many flowers. The next question is the most challenging when using your x10 lens in the field in wind and rain. Does the basal glume completely encircle the base of the spikelet (left) or not (right) ?

The two species with encircling glumes are easily told if you can count the stigmas. E. multicaulis has 3 (and 3-angled nuts); E. uniglumis has 2 (and biconvex nuts). Sod’s law suggests that neither stigmas or nuts are available.

Without nuts or stigmas you need to find the top of the leaf sheath: is the apex 45 degrees (Eleocharis multicaulis, left, the one most likely on acid peat), or truncate (E. uniglumis, right, the one you are most likely to find in dune slacks).

The rare Eleocharis mamillata subsp. austriaca was first found by Noel Yvri Sandwith by the R Wharfe below Buckden (Mid-west Yorks; SD9475) in 1947. It is told from E. palustris by having 5 perianth bristles (not 4), conical (not cylindrical) spikelets and tiny anthers (<1.3 mm)

So that's it. Just 7 species of which only 4 are common. The pairs you need to practice separating are big glumes

acicularis vs. quinqueflora (stem width)

and small encircling glumes

multicaulis vs. uniglumis (sheath angle)