The invasive Kentucky bluegrass
Featured Image: A Kentucky bluegrass lawn. Photo by author.
I’m pretty sure we have all heard of Kentucky bluegrass, or at least one of those two terms, be it the commonwealth or the music genre. The grass species known as Kentucky bluegrass (Poa pratensis) – contrary to its name – is not native to Kentucky but is originally from Europe and northern Asia. It is also the most popular lawn grass in the Unites States. Unfortunately, it has not stayed confined behind property lines and has made its way into natural grassland ecosystems. Kentucky bluegrass is currently wreaking havoc in the Great Plains states (North Dakota, South Dakota, Montana, Wyoming, Nebraska, Colorado, Kansas) where it has escaped into grasslands and is now displacing native prairie species.
A recent study by scientists at North Dakota State University found that the prevalence of Kentucky bluegrass in grasslands was associated with decreased butterfly diversity, lower plant species diversity, and fewer overall species of flowering plants, also known as forbs (Fig. 1;Kral-O’Brien et al. 2019). The researchers determined this decline in biodiversity through surveying prairies with diverse management histories in North Dakota, South Dakota, and Minnesota from 2015 to 2017. Several other studies have confirmed the invasion of Kentucky bluegrass in other Great Plains states (Cully et al. 2003, Murphy and Grant 2005, Toledo et al. 2014), highlighting its problematic status in ecosystems ill-equipped to thwart this nonnative intruder.
This problem with Kentucky bluegrass is not new, nor is it surprising given the history of lawn grass in our culture. Well-kept lawns have always been a popular addition to U.S. households; this is partly because they tend to be associated with a higher social status due to the resources and labor required to maintain their appearance.
The deal with lawns in general
The concept of the immaculate lawn, or turfgrass, representing a high socioeconomic status has existed since the beginning of lawns themselves. The word “lawn” is derived from the old English (Brythonic) word “laune” meaning a woodland opening or clearing. The use of the term emerged in the 1500s when aristocracies in northern Europe first established lawns – which looked more like pastureland – around castles to easily view visitors or threats advancing from afar. Lawns were managed through grazing and human labor typically in the form of scything. The luxury of lawns was limited to society’s wealthier homeowners, for only they could afford to hire additional hands to maintain their lawns. But with the invention of the first lawn mower in 1830, lawn prevalence extended to the middle class. Lawn sports such as golf and bowling greens traveled from Europe to North America and accelerated the establishment of turfgrass across Canada and the United States.
With the migration of lawns from Europe to North America, so too, came the seeds of grasses used to create large swaths of low-cut greens. Because the eastern United States was primarily forested when the first European ships arrived, settlers lacked grazing grasses for their livestock. In response to this, European supply ships delivered several species of grasses ….along with many other tag-along nonnative species including dandelions and plantains (Fig. 2).
The nonnative grasses (plus the tag-alongs) spread.
Kentucky bluegrass has spread from coast to coast due to both unintentional and human-assisted migration. The traits that make Kentucky bluegrass such a desirable outdoor carpet are the same reasons it is problematic in prairies. It forms a thick layer of dense turf that provides a soft, lush green surface. That turf eventually becomes a thick layer of thatch – dead plant matter – that prevents many other species from germinating and establishing underneath it. Kentucky bluegrass spreading into grasslands means significantly less plant and animal diversity and researchers are only scratching the surface of the effects it is having in our grassland ecosystems (Fig. 3;Toledo et al. 2014).
Kentucky bluegrass is not the only nonnative lawn grass in the United States. Tall fescue (originally from Europe) and perennial ryegrass (also from Europe) have become widespread in northern states. Bermuda grass is from Africa and now established throughout southeastern states along with centipede grass (from Southeast Asia), zoysia grass (also from Southeast Asia), and St. Augustine grass (from Central America). Out west, in addition to Kentucky bluegrass, is the only true native turfgrass in the U.S. – buffalo grass.
Most of the nonnative grass species are now considered naturalized, meaning they have become so pervasive in their new environment that eradicating them is financially and logistically unrealistic. Even Kentucky bluegrass is now considered “native” by the USDA in the northeastern U.S., and no one alive today will fully understand what this species has displaced and how it’s changed the northeast ecosystems (for more on this concept see this article about Shifting Baseline Syndrome). Because the invasion of Kentucky bluegrass into the Great Plains states is relatively recent, there is a better opportunity to curb and control its spread and potentially restore native grasslands in this region.
Alternatives to lawns
Despite their nonnative status, grass lawns should not be condemned; in fact they play an important role in our economy. The lawn industry in the U.S. generated an estimated revenue total of over 99 billion dollars in 2019 and is growing at an annual rate of 4.8%. Indeed, Americans love their lawns – the average household spends roughly $500 annually on lawn care and gardening (Mazareanu 2019). Lawns provide areas to recreate, nap, read, picnic, barbecue, play with our dogs, and people still take pride in their appearance. What is more American than mowing your lawn on a sunny weekend day, riding your John Deere mower, beer in hand, wearing a red ball cap? (Kidding! …kind of.) There are many intangible values from having your own lawn. One of my chores as a kid from elementary school through high school was helping push mow the yard, and my first paid job was mowing lawns when I was 12.
Perhaps the best solution to consider is downsizing our lawns, if possible. Residential lawns contribute significantly to watershed pollution through excess nutrient runoff (see Hobbie et al. 2017). Smaller lawns would likely result in less nitrogen and phosphorus pollution in both surface water and groundwater bodies. In lieu of turfgrass, there are many aesthetically pleasing and ecologically beneficial native plants that require minimum maintenance, if any. Pollinator gardens (Fig. 4) are becoming more popular to address declining bee populations; Minnesota has become the first state to provide residential homeowners with financial incentive to replace lawns with bee-friendly habitat. In western states where drought is common, many homeowners have adopted a natural yardscape of succulents, drought-tolerant grasses, and forbs in place of a sod lawn (Fig. 5), which reduces their annual water use. I do not have a lawn myself but am currently encouraging my parents to reduce their lawn size (which, sorry Mom and Dad, is an ensemble of Kentucky bluegrass, dandelions, and plantain) and establish their own pollinator garden.
Lawns are an extension of a garden requiring constant maintenance and care. A smaller lawn complemented by a native garden would not only add ecological value, it would reduce the amount of area requiring mowing and irrigation. Economically and ecologically, it is a win-win.
Cully, A.C., J.F. Cully, Jr., and R.D. Hiebert. 2003. Invasion of exotic plant species in tallgrass prairie fragments. Conservation Biology 17:990–998.
Hobbie, S.E., J.C. Finlay, B.D. Janke, D.A. Nidzgorski, D.B. Millet, and L.A. Baker. 2017. Contrasting nitrogen and phosphorus budgets in urban watersheds and implications for managing urban water pollution. PNAS 114(16): 4177-4182.
Kjelgren, R., L. Rupp, and D. Kilgren. 2000. Water conservation in urban landscapes. HortScience 35(6):1037-1040.
Kral-O’Brien, K.C., R.F. Limb, T.J. Hovick, and J.P. Harmon. 2019. Compositional shifts in forb and butterfly communities associated with Kentucky bluegrass invasions. Rangeland Ecology and Management 72(2): 301-309.
Leinauer, B., E. Sevostianova, M. Serena, M. Schiavon, and S. Macolino. 2010. Conservation of irrigation water for urban lawn areas. Acta Horticulture 881:487–492.
Mazareanu, E. 2019. Landscaping services in the U.S. – Statistics & Facts. Statista, accessed August 2020.
Murphy, R.K. and T.A. Grant. 2005. Land management history and floristics in mixed-grass prairie, North Dakota, USA. Natural Areas Journal 25(4):351–358.
Toledo, D., M. Sanderson, K. Spaeth, J. Hendrickson, and J. Printz. 2014. Extent of Kentucky bluegrass and its effect on native plant species diversity and ecosystem services in the Northern Great Plains of the United States. Invasive Plant Science and Management 7(4): 543-552.