The diversity of switchgrass’ morphology parallels the diversity of sites to which the species is adapted (Fig. 1). Robust lowlands can be taller than 3.0 m, while some uplands may only reach 0.5 m; root depths can extend to 3 m where soils are not restrictive (Porter 1966; Moser and Vogel 1995). Upland plants also typically have thicker roots and longer root internodes, with rhizomes long enough to support sod formation (Beaty

Color image of this figure appears in the color plate section at the end of the book.

et al. 1978). In contrast, lowland plants, with shorter rhizomes, more often exhibit the characteristic growth habit of a bunchgrass (Vogel 2000). New shoots originate from rhizome buds on lowland plants but also from basal nodes of culms in the upland ecotypes (Porter 1966). In addition to being smaller in stature, upland ecotypes typically have finer leaves and stems and smaller panicles; lowlands, in addition to their larger size, also often have a distinctive bluish coloration (Casler 2005). Both ecotypes generally have erectophile leaves, although this characteristic generally is stronger in lowlands.

Although seed size can differ by cultivar, this generally will have little agronomic consequence except in cases of excess seeding depth (Zhang and Maun 1991; Smart and Moser 1999). Seedling morphology, however, has some potential to affect switchgrass development and survival, as some seedlings have elevated crowns, which limits adventitious root formation. This appears to have little actual effect, however, as research in the field gives little evidence that this is an issue in stand establishment (Elbersen et al. 1999). Researchers have also selected for increased tillering of seedlings, which may be useful in the development of divergent genotypes, but this has not been a successful approach for improving plant establishment or yield (Smart et al. 2003, 2004).

Switchgrass is a determinate plant and it produces multiple tillers that become reproductive after exposure to the right environmental signals. Biomass accumulation comes to an end in conjunction with inflorescence development. Research suggests that daylength is the chief signal for floral development, although this response may not be completely under photoperiod control (Esbroeck et al. 2003). Reproductive development may also be delayed or inhibited by flooding, excessive K fertilization, and low temperatures (Porter 1966; Balasko and Smith 1971; Friedrich et al. 1977).

For a given cultivar, the critical photoperiod (some minimum night length, actually) is genetically determined and linked to the plant’s latitude of origin. Differences in photoperiodic flowering responses among cultivars have important implications for selection and production in the field. For example, moving southern-adapted cultivars to higher latitudes delays their reproductive development. This promotes continued vegetative growth (and greater biomass yield) as the plant does not experience the appropriate signal for reproductive development (sufficient night length) until later in the year. In the opposite way, a northern type will be less productive when moved south because the photoperiodic trigger occurs earlier in the season at lower latitudes. This attribute also has important implications for survival, and we will give both issues further consideration in the section below on cultivar selection.

Along with latitudinal differences among cultivars, switchgrass can display variable longitudinal morphology and adaptation (Hopkins et al.

1995a, b; Madakadze et al. 1998; Vogel 2000; Casler and Boe 2003). Cultivars adapted to conditions of the humid east are generally taller but less tolerant of the drier and windier conditions of the Great Plains (Cornelius and Johnston 1941). Conversely, productivity of the shorter, coarser-stemmed western-adapted switchgrass can be negatively affected when moved east. This is thought to be largely related to the lower pathogen resistance in cultivars adapted to drier climates (Vogel 2000).