What is Switchgrass?
Switchgrass (Panicum virgatum L.) is one of many warm-season perennial grasses native to North America. It is a natural component of the tallgrass prairie, which covered most of the Great Plains. It is also found on prairie soils from the southeastern U.S., westward to the Rocky Mountains, as far south as Mexico, and northward into Canada. After extensive evaluations of many plant species in multiple locations, switchgrass was selected in 1991 by the U.S. Department of Energy as a model herbaceous energy crop for the development of a renewable feedstock source to produce transportation fuel. While switchgrass has been promoted as a “savior” or the “answer to expensive, imported fuel,” much work remains. The work will require many years of research and development.
Switchgrass evolved in many different environments – cold northern and warm southern areas; upland sites with shallow soils and in creek and river bottoms with deep soils – and as a consequence has many ecotypes. Each ecotype is slightly different from the others, but all belong to the same species. Two morphological ecotypes are widely recognized - lowland and upland. Upland types usually grow four to five feet tall and are adapted to relatively shallow soils. Lowland types, which grow up to 12 feet tall, are typically found on deep soils.
Switchgrass is a cross-pollinated species and has two common ploidy levels (number of chromosome sets). Upland types are generally octoploid and lowland types are usually tetraploid. Another variation has to do with the latitude where a particular switchgrass evolved. Those from the north tend to grow slowly in the late summer due to shortening day length, while those from the south usually grow longer into the autumn; thus, producing higher yields if otherwise adapted.
Some switchgrass ecotypes and varieties can grow well in the northern U.S. and in Canada where the growing seasons are relatively short (2 months), but days are long during the summer. Likewise, switchgrass can grow well in the southern U.S. and Mexico where the summer days are much shorter, but the growing season may last six to eight months. Switchgrass grows rapidly after breaking dormancy (March in Oklahoma) and slows when it begins to produce seed heads (July in Oklahoma). It needs at least one month in the north to reach its reproductive stage and may use three months in the south to finish its vegetative growth stage. If not harvested, switchgrass spends the rest of the growing season developing seed and storing energy in its crowns, roots, and rhizomes. This energy is needed to remain alive during its dormant period in the winter and for regrowth the next spring.
Adapted lowland switchgrass varieties, suited to the southern U.S., frequently produce more than 10 tons/acre/year of dry matter, but it is more reasonable to expect 5 to 8 tons/acre under commercial conditions. Upland types growing in shallow soils are likely to average yields of 1 to 4 tons/acre/year. With the longer growing season in the southern latitudes, biomass yields in the south can be expected to be higher than in northern areas. High yields for all crops are dependent on good moisture conditions. As a component of rangeland, switchgrass yields are closely related to its growing conditions and are much lower in low rainfall areas and in shallow soils.
Even though switchgrass is tolerant of poor soil, droughty, and fl ooded conditions, it does have certain needs to produce economical biomass yields and persist for several years. High switchgrass yields have been recorded when the grass was grown under good conditions. These good conditions include a relatively deep soil with a pH level near neutral that can supply the nutrients needed for growth. Water must be available either through rainfall or irrigation. Temperatures must be relatively warm during the growing season, and the growing season must be long enough for the plants to fully develop.
Deep soil is needed for reasonable yields because it serves to store water and nutrients. Soil can store one-half inch to two inches of water per foot of depth. If the soil is only one foot deep, the available water will only be sufficient to maintain switchgrass for a few daysor weeks without additional rain or irrigation. High yields are impossible without adequate water. An extremely acidic or alkaline soil may be toxic and may hold nutrients in forms that are unavailable to switchgrass. Soils near neutral (pH 5 to 8) are required for good growth and high yields.
Switchgrass is a perennial warm-season grass. This means it possesses mechanisms that allow it to tolerate cold temperatures during winter as dormant plants, but needs warm temperatures for growth. When temperatures are below 60° F or above 95° F, the growth of warm-season plants slows or stops. Plants can remain alive at these temperatures, but they grow slowly, if at all.
Switchgrass, along with other native species, is generally nonresponsive to applied nutrients compared to introduced grasses, such as bermudagrass, tall fescue, wheat, etc. However, high yields of switchgrass cannot be sustained if adequate N and other nutrients are not applied because removing the switchgrass biomass also removes significant amounts of these nutrients. To sustain production, the nutrients removed from the soil need to be replenished.