Bunchgrass Growth

Bunchgrasses (aka tussock grasses) are cespitose, meaning they form clumps; they grow in tufts from a single root crown. The bunchgrasses (aka shortgrasses), that most of us are familiar with include side oats grama (Bouteloua curtipendula), little blue stem (Schizachyrium scoparium), and prairie dropseed (Sporobolus heterolepsis). What’s been confusing as I’ve researched this is that sometimes they are classified as short rhizomatous. While I’m not sure if this is a scientifically accurate definition, I’ll leave it here so you’ll know you might encounter this term when doing your own research.

Here is another great example of how understanding biology has led to better management decisions – better in the sense of meeting our goals, reducing collateral damage, and conserving our resources.

Side oats grama, photo by Allendan Seed

Side oats grama, photo by Allendan Seed

We had a couple of small areas and knowing shortgrasses were more difficult to get established, we started by seeding with only them. We burned and mowed the first 2 years but we weren’t getting the establishment we thought we should. At that time, we did not know that these grasses only spread via seeds. We made the assumption they were like the tallgrasses and spread via rhizomes. We also hadn’t learned of previous research demonstrating that bunchgrasses are minimally affected if burned during the dormant season and change little in frequency due to fire (Tober & Jensen 2013). This was a severely overgrazed remnant area where the side oats was thick but so was the smooth brome. We knew mowing side oats wouldn’t kill it, so we mowed in late May to set back the smooth brome. Then later in the summer we wondered why the side oats didn’t set seed. Keep reading and you’ll see why this happened. Knowledge gave us the ability to fulfill our goals and not waste our resources. 

3 Types of Grass Growth

Knowing how our native grasses grow is important to knowing how to manage them. Not all grasses are created equal in their growth habits. There are 3 classifications of grass growth: rhizomatous, stoloniferous, and cespitose. Rhizomatous and stoloniferous are often referred to as sod-forming grasses; examples of these are big bluestem (Andropogen gerardii) and Indian grass (Sorghastrum nutans). Little is ever mentioned about these differences, yet they have different management strategies and growth implications.

These 2 diagrams show the distinction among the 3 grass growth classifications. What they have in common is the initial growth comes from basal buds in the crown. In bunchgrasses, these buds remain within the surrounding leaf sheath of the mother stem, resulting in minimal lateral spreading (Oregon State). Perennial grasses develop these buds each fall, storing energy for them to survive winter. These buds are the sites for tillering, a shoot that arises from the base; it’s where all shoots arise after the initial seed germinates.

Grass growth diagram, Oregan State University

Source: Oregon State University

Sodgrass vs bunchgrass diagram, Source, Karen Launchbaugh

Source: Karen Launchbaugh, Grasses and Grasslike Plant Presentation

Cespitose growth

There are 5 structures that allow grasses to grow and not every grass has all 5 of these: intercalary meristem, basal buds, stolons, rhizomes, and apical meristem.  

Bunchgrasses regenerate from buds and clusters of small leaves crowded on a crown of stems and roots just below the ground level (Judziewicz et al. 2014). These are protected by the soil surface and are usually too low to be accessible to grazing animals. The removal of these do not stop leaf growth nor does it stimulate further growth. Any additional regrowth comes from the intercalary meristem (see diagram below), located above the crown. It’s what pushes leaves upward and forms the leaf base. A bud will develop within the sheath that surrounds the node; this is known as intravaginal development or tillering (see photo below) (Chapman 1996). Bunchgrasses, unlike sod-forming, rhizomatous grasses, spread very little each year.

Intravaginal tillering. Source: Oregon State University

Source: Oregon State University

Intercalary stem diagram

Source: Thermo Fisher Scientific

The upward growth (vs lateral growth of rhizomatous grasses) means the growing points are susceptible to removal (Branson 1953). Each bunchgrass species may have growing points at different locations. For example, little blue is about 1” off the ground and side oats is near the soil surface throughout the growing season (Branson 1953). The removal does not stop leaf growth or damage the plant, but it will stop seed production. This is what we experienced with our late mowing to control the smooth brome mixed with the side oats. Since bunchgrasses expand their presence by seeds (Cheplick 1998), this can reduce their spread – a definite management consideration. Because they spread by seed, they will also cover the prairie more slowly than rhizatomous grasses and with the “empty” spaces between the tussocks, weeds can grow. Cespitose grasses will be more prevalent in low moisture habitats compared to rhizomatous grasses.

As tussock grasses grow, their centers “hollow out,” creating fairy rings. These grasses expand their clumps by producing new tillers around the outside edges, leaving older tillers in the center (Derner et al. 2012; Cheplick 1998). These fairy rings will eventually break into separate segments and start the clump-forming process over (Arber 1934). While this takes years, it Is another way cespitose grasses spread.

References

Arber, Agnes. 1934. The Gramineae: A study of cereal, bamboo, and grass. New York: The Macmillan Company.

Branson, F. A.  1953. Two new factors affecting resistance of grasses to grazing. Journal of Range Management, 6(3), 165-171.

Chapman, G. P. 1996. The Biology of Grasses. Wallingford UK: CAB International.

Cheplick, G. P. 1998. Population Biology of Grasses. New York: Cambridge University Press.

Derner, Justin D., David D. Briske, and H. Wayne Polley. 2012. Tiller organization within the tussock grass Schizachyrium scoparium: a field assessment of competition-cooperation tradeoffs. Botany 90: 669-677.

Judziewicz, Emmet J., Robert W. Freckmann, Lynn G. Clark, and Merel R. Black. 2014. Field Guide to Wisconsin Grasses. Madison WI: University of Wisconsin Press.

Oregon State University. 2023. Discuss the basics of grass growth. https://forages.oregonstate.edu/regrowth/developmental-phases/vegetative-phase/crown

Tober, D. and N. Jensen. 2013. Plant guide for little bluestem (Schizachryium scoparium). USDA Natural Resources Conservation Service, Plant Materials Center, Bismarck, ND.