Timber Press Pocket Guide to Ornamental Grasses (Timber Press Pocket Guides) - Softcover

Though technically all grasses increase in width or spread to some degree by lateral shoots, for garden purposes it is common and practical to group grasses as either runners or clumpers.

Running grasses spread rather rapidly by rhizomes, in which case they may also be called rhizomatous grasses, or by stolons, in which case they may be called stoloniferous. Most running ornamental grasses are rhizomatous. Stoloniferous growth is more common among turf grasses and weedy species, such as crab grasses, Digitaria species.

When used appropriately, running grasses can minimize maintenance. Their ability to knit together and cover large areas often makes them the best choice for groundcover use and soil stabilization. Running grasses are able to fill in gaps that may appear in a planting due to physical damage or disease, and many are so dense and strong in their growth that they keep weeds from establishing. In extremely difficult sites, such as urban traffic islands, running grasses are often the most practical choice. Some running types, such as gold-edged cordgrass, Spartina pectinata 'Aureomarginata', are tolerant of moist or wet soils and can be ideal for holding streambanks and margins of ponds or storm water retention basins.

When planted in the wrong situation, however, running grasses can cause serious problems in the garden. They can completely overpower less vigorous neighbors and turn once-diverse borders into monocultures. Before planting a strongly running grass, carefully consider whether adjacent plantings and hardscapes are sturdy enough to contain its spread and whether someone will have the time and energy for removing its advances into unwanted areas.

The vigor of running species varies radically with climate and cultural conditions in the garden. For example, a warm-season spreader like giant reed, Arundo donax, may be unmanageable in a small garden in sunny Georgia, whereas the short, cool season of a Connecticut garden may slow it to the point that it behaves more like a clumping grass.

Clumping grasses essentially remain in place. They may slowly increase in girth, but new shoots will not appear at distances from the clump. Grasses that produce tight clumps are also referred to as tufted, caespitose, or bunchgrasses. Though clump-forming grasses may take many years to reach mature size, the ultimate space they consume in the garden is more predictable, and for this reason they are often easier to design with than running types. Because they are not able to fill in large gaps between individual plants, however, they can sometimes require more longterm maintenance than running grasses when used as groundcovers, as happens with groundcover plantings of blue fescue, Festuca glauca.

A few grasses do not fit neatly into either the running or clumping categories. The growth habit of Hakone grass, Hakonechloa macra, has been variously described as caespitose and spreading. This grass increases by rhizomes and is capable of continuous spread if cultural conditions are ideal, yet its rate of increase is often so modest that, for most gardeners, it is a clump-former in the practical sense. Switchgrass, Panicum virgatum, is also somewhere between strictly clump-forming and running. Its rhizomes occasionally stray noticeably from the clump, yet for most intents and purposes it is a clumping grass. The running or clumping nature of grasses can vary between species belonging to the same genus. For example, Miscanthus and Pennisetum each include strictly clumping species as well as aggressively running species.