Grasses comprise a gigantic family of plants including grains, turf as well as ornamental grasses--those used solely for beautifying landscapes. Ornamental grasses include species with attractive foliage or flower plumes and upright or arching habits that form tidy clumps and tufts or spread and invade surrounding soil. In addition, some grasses proliferate with copious production of seeds, eventually forming a thicket.
Gardeners often regard upright clumping ornamental grasses as "neat and tidy", since they form pronounced clumps or tufts of foliage and do not spread roots outward. Distinguishing themselves from other clumping grasses, upright types bear flower and seed plumes that grow and mature with a rigid, vertical habit. Some examples include pampas grass (Cortaderia selloana), big bluestem (Andropogon gerardii) and feather reed grass (Calamagrostis x acutifolia).
Arch clumping ornamental grasses have weeping stems or pendant flower and seed plumes, forming a clumping plant with a vase shape or arching shape. While the arch or weeping stems, flower and seed plumes or foliage may cause the plant to look large and massive, the roots remain centralized, not spreading outward to form thickets of plants. Pink muhly grass (Muhlenbergia capillaris), fountain grass (Pennisetum alopecuroides, Pennisetum setaceum) and some non-invasive types of tropical bamboos (Bambusa spp., Dendrocalamus spp., Otatea spp.) form graceful clumps.
Perhaps the most troublesome type of ornamental grass has a root system with aggressive, colonizing stolons or rhizomes. While they serve a good purpose of soil stabilization or may have very ornate stems, foliage or seed plumes, gardeners need to contain the roots to prevent widespread colonization. Blue lyme grass (Leymus arenarius) and many temperate bamboos (Phyllostachys spp.) manage to spread their roots and create thickets of plants, often invading adjacent garden areas or wild native ecosystems. These grass types become difficult to eradicate through intensive physical removal or repeated, frequent treatment with herbicide.