The next developmental stage, known as elongation, is characterized by internode elongation and stem formation. This phase is often triggered by changes in day length and temperature. As days get longer in spring and temperatures warm, tillers will begin elongating. Notice in the picture on the left (Exhibit 12), the elongated tillers of big bluestem. If you were able to run your fingers along the stems, you would be able to feel nodes on each tiller. Also notice how the tillers are upright growing and taller than the vegetative tillers demonstrated in the picture on the right (Exhibit 13).
Although one would be able to harvest more biomass when tillers enter the elongation stage, it is at this stage where perennial grasses are most vulnerable to defoliation. Harvesting or grazing during the elongation stage increases the risk or removal of the stem apex or growing point as it is elevated above ground. Maintenance of sufficient residual stem is needed to promote growth following defoliation as carbohydrate and protein reserves maintained in stem bases, rhizomes and roots also are at their lowest level the elongation stage.
The extent of culm development differs among perennial grasses which can impact their persistence. After flowering in spring, the cool-season perennials tall fescue (Schedonorus arundinaceus (Schreb.) Dumort.), orchardgrass (Dactylis glomerata L.), Kentucky bluegrass (Poa pratensis L.), and crested wheatgrass [Agropyron cristatum (L.) Gaertn] produce culmless vegetative tillers in new summer and fall growth which protects the growing point at ground level. In contrast, smooth bromegrass (Bromus inermis Leyss.), reed canarygrass (Phalaris arundinacea L.), intermediate wheatgrass [Thinopyrum intermedium (Host) Barkworth & D.R. Dewey], and quackgrass (Elymus repens L.) produce culmed vegetative or sterile tillers in new summer and fall growth where apical meristems are susceptible to close, continuous grazing.