What is a Grass?

Technically, the word grass refers to species in the Poaceae (a.k.a. Graminae) plant family. In this book, we have also included grass-like plants, sedges from the Cyperaceae family, and rushes from the Juncaceae family.

The Plant

Grasses, sedges, and rushes are in a botanical group referred to as monocotyledons, often called monocots for short. The name monocotyledon is derived from the single (mono) embryonic leaf (cotyledon) contained in the seed of a monocot. Graminoids and other monocots share this characteristic, along with narrow leaves, parallel venation, and fibrous root systems. Use the image on the below to learn the basic anatomy of a graminoid plant.

Flowers

Neither grasses, sedges, nor rushes have colorful, large, or showy flowers. All are wind pollinated and so do not need bright petals or nectar to attract animal pollinators. For this reason, the flowers are simplified and usually small in size.

In grasses, the petals and sepals have been reduced to very small scales called lodicules that enclose the ovary, which contains one ovule. From the ovary rise two styles ending in feathery stigmas; adjacent are typically three anthers that open to shed pollen held on long, thin filaments. 

The ovary and anthers are usually protected and enclosed by two small papery bracts called the palea and the lemma. Outside the lemma and palea are two more larger, yet still small, papery bracts called glumes. When the anthers have ripe pollen, the very small lodicules help to open and close the papery bracts, thus assisting with wind pollination. All of these small parts make up the floret, an individual grass flower. A large seedhead or grass inflorescence usually contains hundreds of florets.

The arrangement of the one or many florets and the branching patterns of these florets determines how grasses are identified, named, and classified. If there is only one floret subtended by the two glumes, the floret is called a one-flowered spikelet. If there are two or more florets above the glumes, it’s a multi-flowered spikelet. Different genera of grasses have specific numbers of florets in a spikelet. The arrangement of spikelets in an inflorescence is also characteristic of specific genera. To see these small flower parts, you need a hand lens or, ideally, a microscope. This is often where many people give up on learning grass identification! However, you can easily learn the differences in the large seedheads that will help you identify many grasses.

Sedge inflorescences are simpler. However, they are just as small, and, again, the identification may require a hand lens or microscope. Sedges can be comprised of unisexual or bisexual flowers. Sedge female flowers are each enclosed by a small sack-like structure called a perigynium. The shape of the perigynium is often used in sedge identification. Many sedges have female flowers in one section (often lower on the flowering stem), with the male flowers that contain only the pollen in another section, (often on top of the stem). If you look closely at sedge flowers in early spring when they are shedding pollen, you can see the larger, more conspicuous female flowers growing below the male flowers. The flower (male or female) and bract is called a spikelet. Spikelets are attached directly to the axis that forms the inflorescence. Explore the image below to see the inflorescence of a sedge.

Rushes have the simplest inflorescence. The flowers are typically bisexual. From beneath each flower emerge six small green or brown, petal-like structures called tepals.  Typically, there are many rush flowers held together in a cluster at the end of a stem.

Stems and Leaves

To quickly tell the difference between grasses, sedges, and rushes, look at the stem and leaves. Grass stems (culms) are normally round like a straw, and their leaves come off in two ranks, meaning one on either side of the plant—whereas the stems of sedges are usually triangular, and their leaves are three-ranked. It can be hard to feel the three edges when you roll the stem between your fingers.

Rushes have round stems as well, but unlike grasses the leaves of rushes grow from the base of the plant, so the stems lack the nodes that arise from leaf joints and are completely smooth.

This famous poem can help you remember the differences between the three:

Sedges have edges;

Rushes are round;

Grasses have bumps all the way to the ground.

In the poem, “edges” refers to the triangular stem of a sedge. “Round” refers to the smooth round stem of a rush. “Bumps” refers to the nodes or joints where leaves attach to the stem of a grass.

Illustration credit for this section in order:

American beakgrain: Hitchcock-Chase Collection of Grass Drawings, on indefinite loan from the Smithsonian Institution, courtesy of Hunt Institute for Botanical Documentation, Carnegie Mellon University, Pittsburgh, Pa.

Angiosperm flower, grass flower, grass spikelet, inflorescence type:  Clark, Lynn G., and Richard W. Pohl. Agnes Chase’s first book of grasses: the structure of grasses explained for beginners. Smithsonian Institution, 2012.

Threadleaf sedge, rush flower, rush inflorescence: United States Department of Agriculture Forest Service Collection, courtesy Hunt Institute for Botanical Documentation, Carnegie Mellon University, Pittsburgh, PA.