Stem shape, flowers and fruit help identify grasses from sedges | Mystery Plant

It’s not a grass, and it’s not a sage. It’s a “sedge.”

This week’s column is devoted to a certain plant family, and especially a certain genus within the family. This would be the family named “Cyperaceae,” an enormous group, in terms of number of species, all of which are called “sedges” of one sort or another.

You may have heard that “sedges have edges, and grasses are round,” a simplistic statement describing the stems of these plants. Indeed, sedge species commonly have their stems (which we call “culms”) somewhat triangular in cross-section, and sometimes sharply so. Grasses, on the other hand, have culms which are generally circular in cross-section. Rolling the stem between your thumb and index finger often bears this out.

The distinction between these groups is better based upon characteristics of the flowers and fruits, however, rather than on stem shape. Both sedges and grasses have highly reduced flowers, and hard, one-seeded fruits.

In grasses, the fruit is what we call a grain, which most people already know. Each individual grain of a grass plant (for instance, corn or wheat) will have a single seed inside, whose wall is completely fused to the inner wall of the grain.

Sedges, on the other hand, have fruits which we call “achenes”: in these, the interior seed is more or less free from the wall of the fruit. Of course, there are other differences, too, but they can be part of a different story.

There are many thousands of different species of sedges, and these species are found nearly world-wide, and in a variety of habitats. There are quite a number of sedge species in the Southeast, and a lot of them are weedy, some rather notorious and problematic, while others are quite rare.

Our Mystery sedge (Thicket sedge, Carex abscondita) is a species placed in the genus Carex, as it turns out, and which is by far the largest genus in the family, if you go by species number. There are nearly 3,000 different species of Carex, and to some botanists, they represent one of the more difficult groups for identification, due to the small size of their reproductive structures, and to their great variability.

Species in this genus are perennials, and nearly all have their flowers segregated into spikes as either staminate (male) or pistillate (female). The female flower is represented by just a single pistil, and it will be found inside a little bag of tissue which we call a “perigynium."

Here, the perigynia are rather blunt-tipped (in some species of Carex the perigonium can be long and pointy), and the male flowers are crowded into a spike at the very tip end of the culm.

This particular species of Carex is common in the Southeast. I saw it in some considerable abundance the other day, growing in a ditch, and showing off its rather pronounced yellow-green foliage. The plants themselves are attractive, in a sort of sedge-y way, and I bet they would look good growing in a sunny bog garden, if given the chance.

John Nelson is the retired curator of the Herbarium at the University of South Carolina, in the Department of Biological Sciences. As a public service, the Herbarium offers free plant identifications. For more information, call 803-777-8175 or email johnbnelson@sc.rr.com.

This article originally appeared on Tallahassee Democrat: Mystery Plant: How to distinguish sedges from grasses