Mark Brand, professor of horticulture at the University of Connecticut in Storrs, Connecticut, replies:
Grasses, sedges, and rushes can be difficult to tell apart. In the horticultural world, we often loosely refer to these three plants as “ornamental grasses,” but they all belong to different plant families. If you look closely, you can distinguish each from the other by structural differences as well as by general differences in their native habitat and distribution.
Grasses and rushes can be either annuals or perennials, while sedges are all perennials. Grass and rush stems are typically round or flat, while sedge stems are typically triangular. The stems of sedges and rushes are generally solid, while those of grasses are hollow. Grass stems also contain swollen nodes or joints; sedges and rushes do not. In addition, grasses often produce both vegetative and floral stems, whereas sedges and rushes only develop floral stems.
The leaves on grasses are usually two-ranked, which means they occur on two rows on opposite sides of the stems. Leaves on sedges are usually three-ranked, where they lie in three vertical planes along the stem. The leaves of rushes are mostly basal and spirally arranged, although this varies among the members of the family. The leaf sheaths (the part of the leaf that hugs the stem) of grasses are usually open, while those of sedges and rushes are usually closed.
The flowers of many grasses are relatively showy, while those of sedges and rushes are more inconspicuous. Most grasses have bisexual flowers (both male and female parts in each flower), but some are monoecious (separate male and female flowers on the same plant). Sedges can have flowers that are either bisexual or unisexual (only containing a male or female part in each flower, not both). Flowers of rushes are usually bisexual. Grasses and sedges produce single seeds from each flower, while rushes produce three seeds from each flower.
Grasses belong to the Gramineae (a.k.a. Poaceae) family and are common worldwide, although more species are found in tropical and warm temperate regions than in colder regions. Grasses are most abundant in dry, open habitats. The sedge family (Cyperaceae), which also has worldwide distribution, is more common in colder, wetter areas. Rushes comprise the smallest family, the Juncaceae, which is condo fined mostly to the colder, wetter, more northern parts of the world.
Grasses, sedges, and rushes can all be used to provide unique texture, movement, architecture, color, sound, and year-round interest to almost any landscape. As a general rule of thumb, remember that grasses prefer sunny locations with well-drained soils. Sedges and rushes, on the other hand, are at their best in shady, moist locations.