Besides seed production, perennial grass can also propagate or spread through production of vegetative tillers. We have already learned about upright tillers that arise from the seed itself. Upright tillers also develop from axillary buds on the crown of the parent tiller as demonstrated in Exhibit 21.
As perennial grasses get larger, many produce horizontally-spreading tillers known as rhizomes and stolons that extend the plant outwards along the soil surface. Rhizomes are stems that grow horizontally below the soil surface, enabling the lateral spread of perennial grasses (Exhibit 22-23). They serve as a vegetative organ that will overwinter and support growth of new tillers. Stolons have similar functions, but they grow horizontally above ground (Exhibit 25). They too are stem structures. Both rhizomes and stolons originate from axillary buds on the parent tiller. New plants establish through production of upright tillers and adventitious roots at the nodes along rhizomes and stolons. The following pictures demonstrate rhizome formation on a grass seedling (Exhibit 22) and rhizomes from well-established grassland plants (Exhibit 23 and 24). Exhibit 25 shows stolon structures.
Exhibit 22. Rhizome initiated from an axillary bud on a big bluestem seedling (Andropogon gerardii Vitman). Photo credit: John Guretzky, University of Nebraska-Lincoln
The extent of horizontal spread and production of tillers within a canopy is an important characteristic for classification of grasses as either bunchgrasses or sod-forming grasses. Sod-forming grasses are characterized as horizontally spreading and may contain rhizomes and/or stolons (Exhibit 26). Rhizome and stolons provide a mechanism for thin stands to develop into dense stands. Bunchgrasses, on the other hand, do not form rhizomes and have little lateral spread (Exhibit 28). Some grasses may be classified as intermediate types as they may have only short rhizomes which give the canopy a loose, bunch-like appearance (Exhibit 27). Some of the intermediate types, such as tall fescue, can form rather dense canopies through prolific tillering.
Exhibit 26. Smooth bromegrass (Bromus inermis Leyss.) produces rhizomes and spreads horizontally; on left, birds-eye view; on right, side-view.
Switchgrass produces short rhizomes, but has limited lateral spread. Whereas, little bluestem [Schizacharium scoparium (Michx) Nash] does not produce rhizomes and has a tight, bunch-like appearance.