Calamagrostis, or Small-reed or (Am.) Reedgrass, is a genus in the Grass family Poaceae with about 230 species that occur mainly in temperate regions of the Northern Hemisphere. They are commonly adventive. These tufted perennials usually have hairless narrow leaves. The ligules are usually blunt. The inflorescence forms a panicle. Some may be reed-like.
The roots may be rhizomatous (underground stems with shoots), stoloniferous (with runners), or caespitose (growing in tufts or clumps).
The bisexual spikelets have a single flower and are purple or purple-brown. They appear in early summer on long culms ( = stems). They can be very decorative.
The word "Calamagrostis" is derived from the Greek word kalamos (reed) and agrostis (a kind of grass).
Many species of "Calamagrostis" are morphologically similar, but they generally occur in distinct habitats, and they have unique geographical distributions. Given the subtle distinctions between many closely related taxa, there are several species complexes that could benefit from additional systematic study. Even the generic boundaries of the genus are controversial. For example, species in the genus "Deyeuxia", distributed largely in the southern hemisphere are morphologically very similar to species of "Calamagrostis". It is probably appropriate to recognize all of these species in a single genus, but this will require detailed scientific study of DNA of species from around the world.