Reeded varieties of grass look good in floral arrangements because they provide body and character to the arrangement. Feather reed grass, for instance, is fairly stiff and will stand upright in the vase. Strong grass arranged around flowers with softer stems can help the flowers stay upright.
Grasses that have colored leaves, stems, heads or flowers provide a florist with more options in floral arrangements. The colors the grass provides can be used to complement or contrast the flowers. Purple fountaingrass, for example, has both purple tops and purple leaves. This provides a nice option outside the standard green of many ornamental grasses.
Arranging flowers is often about the geometry of the finished product. Most flowers have a number of curves in them, including round or oval shaped blooms. In order to contrast this and give a shape to the floral arrangement, straight lines need to be used. Very straight grasses, such as bear grass, are used in a floral arrangement to craft the overall shape of the arrangement. The straight lines give the bouquet definition and an outline that is then filled in by leafier greens and the focal point of the flowers.
Lightweight ornamental grasses provide a decorative fringe around the edge of a bouquet. These create a delicate border around the principle arrangement, and are especially attractive when made with wild ornamental grasses that flower. Arizona fescue, for example, has a very fine texture and will frame a bouquet nicely. Silky threadgrass is another fine option that looks good in both live and dried floral arrangements.