Grass is simple for most people. It’s what cows and horses eat. It’s tall and slim in a field, short and in frequent need of mowing in a lawn. But it’s not really that simple. There are many species of grasses, some that don’t look much like grasses at all, and there are other plants that look like grass but are not grasses.
Let us introduce the rushes and the sedges. Many — but not all — species in both groups fit the commonly held view of tall and slender, upright or gently bending plants of open habitats such as hay fields, sand dunes, prairies, marshes, meadows, lawns and golf courses. In fact, all three groups — grasses, rushes and sedges — come in many forms and inhabit every sort of natural and unnatural place.
These plants are grouped together by botanists as graminoid plants (resembling grass). One thing the graminoids have in common is a thin, stiff flower-bearing stem tightly enclosed by the lower portion of the leaves, called a sheath. Many graminoids have multiple stems that form tufts or whorls.
When I point out a sedge or a rush to friends, they often ask, “What is a sedge?” “What is a rush? I never heard of one.” But they have. Think of Moses among the bulrushes. Chinese “water chestnuts” are not nuts of any kind, but rather the sliced inner stems of a rush (Eleocharis dulcis). Papyrus is a sedge (Cyperus papyrus). Besides the leaves they made paper from, the Egyptians used the mature stalks to build reed boats, and ate the soft centers of the stems.
The diversity and ubiquity of grasses, sedges and rushes are ripe for immersive exploration, wonderfully and precisely presented on the internet, if you don’t relish delving into massive volumes of printed sources. Here we present the basics — what makes a grass a grass, a rush a rush, and a sedge a sedge.
A staple phrase of botany students, “Sedges have edges,” is true, but the tactile test of rotating the stem between thumb and forefinger may not feel convincing. “Rushes are round,” is often added, sometimes further aided by a rhyming adage such as “Grasses are hollow, What have you found?” or “Grasses have nodes all the way to the ground.” (A node is a swelling at the base of a leaf sheath.)
In grasses (family Poacae) leaf sheaths envelop (wrap around) the stem, but are split along the side of the stem opposite the blade, the extension of the leaf above the sheath. In grasses the split is visible, or separable by pulling back on the leaf. A grass stem may feel hard and round like a rush, angular like a sedge, or soft and flattish. So, just feeling the stem is not enough. Observe and manipulate the specimen to get a full impression. Pull the leaf away from the stem, and cut a cross-section to better see the shape of a leaf stem.
In sedges (family Cyperaceae), the leaf sheaths encircle the stem, but are unsplit. In some species the leaves are angled horizontally where the sheath ends.
In rushes (family Juncaceae) also, leaf sheaths encircle the stem, but are not split. What distinguishes them from the sedges is the way the flowers and fruit are arranged along the stem.
The flowers or fruit (one after the other in season) of all three families are borne on one or more of the upright stems referred to above, and form the inflorescence. Aside from shooting up, blooming and fruiting, the inflorescence changes little over the growing season, meaning a plant can be studied over a longer time.
The inflorescences illustrated here are similar in form, the better to show the differences that matter. All are common species of fields, parks, roadsides, and yards. All are in fruit at the date of this article.
The grass is orchard grass (Dactylus glomerata), a non-native species imported for hay and fodder. It long ago escaped farm country, and now occurs in abandoned city lots and unkempt corners of peoples’ yards.
What makes this a grass is the branching of the stem bearing the florets of the inflorescence. Rushes and sedges don’t branch this way.
The rush is path rush (Juncus tenuis), also non-native, but probably an accidental introduction. Path rush has no human uses, but it takes the abuse of trampling so well that it is often the only thing growing in the middle or edge of a footpath. Note that each stem has one branched inflorescence with single fruit (in this case) subtended by two or three slender leaves, typical of rushes.
The sedge is greater straw sedge (Carex normalis), a member of “the complex and difficult section Ovales.” (Flora of North America, https://www.efloras.org). Ovales sedges occur only in the North American temperate zone. Our specimen came from a power line cut in Saugerties. The lanky, bristly look is grass-like, but each inflorescence is on a single stem subtended by its uppermost leaf, marking it as a sedge. In rushes an inflorescence can be subtended by several tightly spaced leaves. In grasses the inflorescences are borne on separate stems branching from the main stem, or only from the main stem.
Join the family! 
