Cool season grasses, such as smooth brome (Bromus inermus), Kentucky bluegrass (Poa pratensis), and quackgrass (Elymus repens), are prevalent and wreak havoc on native ecosystems. As with most aspects of ecological restoration, you’ll find there isn’t adequate or definitive research nor is there a step-by-step formula one can use. The best way to deal with specific invasive issues is to learn everything possible about the plant, talk with other restoration practitioners, and then create a plan that works for your specific ecosystem and your goals.
To that end, I am sharing my initial research and our experience. I’ll update this and provide our lessons learned in the hopes it will be helpful. Please share your experiences and we can learn from each other.
Differences between C3 and C4 plants
Warm season grasses (WSG) are called C4 plants and cool season grasses (CSG) are called C3 to differentiate their photosynthetic pathways. They also differ in their optimal temperature ranges, their soil nitrogen needs, and how they produce seed heads. These differences are important to know when managing native ecosystems.
Their photosynthetic pathways differ in how they fix carbon. The “C” stands for carbon and the number represents the carbon molecules formed via photosynthesis. C3 plants reduce (fix) CO2 directly into a 3-carbon acid. C4 plants fix carbon dioxide by converting it to useable components before creating a 4-carbon acid. From this point forward, their photosynthetic processes are the same.
Seed production and germination occur at differing temperatures. The optimal temperature range for CSG is 65-75 degrees whereas the range for WSG is 80-95 degrees. CSG needs a period of cold for the tiller to reach a certain size before it produces seed heads, whereas WSG needs only a 70 degree period to germinate.
Soil nitrogen needs affect growth and decomposition. CSG require more nitrogen than WSG for growth. The additional protein in the CSG leaves makes them more easily digested by grazers. Soil microbes more easily decompose CSG litter than WSG so the mulching out effect of litter accumulations isn’t as prevalent as with WSG stands.
Tiller – A tiller is a new side shoot that grows vertically on the parent plant; it is sometimes referred to as a daughter plant. A tiller appears as if the grass is segmented. Each of these connection spots are points that new growth can occur. Tillers can produce flowers with the right growing conditions.
Rhizome – underground stem that stores nutrients and can reproduce vegetatively.
Boot stage – when flowering head is still enclosed in the sheath – usually at 5 leaves or 17-23” tall. It’s a time when root carbohydrates are low.
There are no studies that provide a recipe for battling cool season grasses. What we know is it requires persistence and a combination of techniques. I have found no studies that explore controlling cool season grass when it’s integrated into a higher-quality shortgrass prairie. Most study it as a monoculture or large part of a tallgrass-dominant prairie. This is a key distinction and needs to be highlighted. Management of tallgrass-dominant prairies differ from high-diversity prairies. I emphasize this because study results do not translate well from grass-dominated systems to high-diversity ecosystems or shortgrass ecosystems.
Late spring burning is often cited as a good control method but the studies are contradictory and the tradeouts are high. Fire doesn’t kill the smooth brome and can damage early spring forbs and native cool season grasses. It can also imperil herptiles and insects as well as early migrating grassland birds, such as the American woodcock.
Late spring burns can be effective when the field is mostly smooth brome. To produce desired results, it must be coordinated with the plant’s growth at that specific location rather than a calendar schedule Look at the plants before expending resources. Fire, like mowing, is only effective when there are fewer than 5 green leaves and there are more than 5 elongated tillers AND where there is sufficient WSG competition (17, 18). Studies citing late spring burn effectiveness are conducted in tallgrass-dominated prairies and rarely address the tradeouts of this practice.
Fire is discouraged on monocultures of smooth brome as results are variable and it doesn’t encourage growth of WSG (12). Recall that fire is most effective when WSG exist to provide competition. Annual burns are touted are necessary (2, 18) but they harm desireable species (13, 14). Frequent burns create a less diverse ecosystem as the WSG grass thrives and outcompetes the forbs.
Early, dormant season spring burns enhance smooth brome growth (12) but they can hurt early native forbs. Fall burning reduced the amount of smooth brome but wasn’t as effective as herbicides, (2) yet no studies regarding fall burning have been conducted in shortgrass, high-diverse prairies (12).
Mowing is another control technique to explore. Grazing or mowing does the most to control CSG with fire having limited success (17). Mowing prior to flower development, called the boot stage, and mowing at the tiller elongation stage will damage the plant because of low carbohydrates in roots (1). This mowing timing is the most effective at preventing seed production (10). So how do we know when the tillers have elongated? [Refer to the tiller diagram]
How often should one mow? A single mowing is ineffective. It’s suggested 4 mowings are more effective than 2 or 3 mowings in a year (12). Yet, mowing this often is not healthy for other native plants and Poa pratensis thrives on frequent mowings. To mow this often, one assumes there is no other vegetation as it would frequency would damage native plants.
Haying is a good mimic of fire but haying in spring had little effect on controlling CSG (14). I could find no other studies about haying and CSG.
Grass-specific herbicide is effective when applied after mowing or burning in spring as the CSG patches begin growing and WSG is dormant. Application after a fall burn when the WSG are dormant is also effective. It will be effective as long as the CSG is photosynthesizing so whether you apply the herbicide in spring or fall or both is a personal decision.
As with other management routines, combinations of techniques are generally more effective than a single practice. Whichever technique or combination is used, overseeding into the CSG area is necessary if you’re not working with mostly tallgrass prairie. Research has also been lacking with studies of control methods in high-diverse ecosystems as well as studies relating to how and when to use various techniques to achieve different goals.
Our Experience Controlling CSG in High Diverse Ecosystems
The largest infestation of CSG we had was (note the past tense) in the 5-Acre Prairie. We planted this to be a shortgrass, high-diverse prairie. One reason the CSG had a good foothold was the time it took us to get educated on the possibilities and develop a strategy to fit our goal.
Our land goal is to increase and maintain biodiversity. This meant we did not want to burn often and never in the late spring. We did not want to encourage the tall C4 grasses. Complicating this was our desire to preserve the Prairie dropseed and Junegrass. Spraying herbicide had to occur early before they emerged. We also needed to overseed the CSG areas in late fall to introduce competition. We prefer to let nature do the cold, moist stratification process forb seeds need. To make any of this happen, we needed to either burn or mow. Weather was a factor and the final decision maker! It was too wet to burn so we mowed when the plants senesced, then overseeded, knowing the winter snow and freeze/thaw would work the seeds into the soil. In spring before the prairie greened up, we burned. In months the CSG was about 4-6” tall. We sprayed a grass-specific herbicide on the large CSG patches. Monitoring these patches as the year progressed those needing a bit more management were mowed once senesced and resprayed when they grew to 4-6” tall. Timing required this be done before the first freeze. The subsequent years we monitored for a patch here and there never allowing it to get a stronghold. That was 5 years ago. We still monitor it!!
In subsequent ecosystems, we tackle the CSG as soon as we discover it. It’s often as simple as walking about with a backpack sprayer working a small area here and there.
Facts about Bromus inermus (Smooth brome)
- Tiller density and tiller mass are stimulated by soil nitrogen addition
- Rapidly establishing perennial grass reproducing by tillering, rhizomes, and seed production
- Rhizomes and seed production are profilic
- Forms a dense monoculture with little open space
- Drought tolerant
- Tolerates a wide variety of soil conditions
- Peak productivity occurs early in the growing season with seed set and flower occurring before WSG
- Spring tiller emergence depletes smooth bromes’s root carbohydrate reserves
- Root carbohydrates are replenished by mid-summer and are greatest during winter
- 2nd growth phase in fall
- Becomes dormant after seeding
- Produces large amounts of litter – this can affect microclimate of sites
- Litter accumulation facilitates its persistence by recycling the high levels of Nitrogen back into the soil
Diagram of Smooth brome tillering
Facts about Poa pratensis (Kentucky bluegrass)
- Forms a dense mat of short creeping rhizomes
- Prefers moist soil and tolerates flooding
- Likes soils with a high pH
- By midsummer, the plants are nearly dormant
- Produce tillers (aboveground shoots) and rhizomes (horizontal belowground stems)
- Shallow rooted
- Intolerant of drought
- Short rhizomes increase under fire injury, high temperatures, close mowing or grazing
- New rhizome growth begins when flower stalks start to lengthen (tiller elongation)
- Thrives on repeated mowings
Facts about Elymus repens (Quackgrass)
- Reproduces mainly by rhizomes in early spring
- 2nd growth in fall
- May be allelopathic, releasing a plant growth inhibitor
- Mowing does not prevent resprouting
- A small patch can increase 6’ in diameter in one growing season
- Once introduced, it is nearly impossible to eradicate
- Moderately shade tolerant
- Cross pollination must occur for seeds to produce
- The 2nd growth can allow plants to produce seeds two times in a year
- Seeds may be viable for up to 10 years
- Forms a dense clump with extensive tillering
- Bahm, Matt A, Thomas G Barnes, and Kent C Jensen. 2011. Restoring native communities in smooth brome dominated grasslands. Invasive Plant Science and Management, 4:239-250.
- Bahm, Matt A, Thomas G Barnes, and Kent C Jensen. 2011. Herbicide and fire effects on smooth brome and Kentucky bluegrass in invaded prairies remnants. Invasive Plant Science and Management, 4:189-197.
- Colorado Weed Management Association. www.cwma.org/Quackgrass.html
- Espeby, Lin A., Hakan Fogelfors, Sara Sjodal, and Per Milberg. 2014. Variation in Elymus repens susceptibility to glyphosate. Acta Agriculturae Scandinavica 74: 211-219.
- Gorman, Emily. 2012. Plant species differ in their ability to reduce allocation to non-beneficial arbuscular mycorrhizal fungi. Ecology 93(4): 711-718.
- Illinois Nature Preserves Commission Vegetation Management Guidelines.
- Korhammer, Siegfried A and Ernst Haslinger. 1994. Isolation of biologically active substance from rhizomes of Quackgrass. Journal of Agricultural Food Chemicals 42: 2048-2050.
- Mahaney, Wendy M, Kurt A Smemo, and Katherine L. Gross. 2008. Impacts of C4 grass introductions on soil carbon and nitrogen cycling in C3 dominated successional systems. Oecologia 157: 295-305.
- NRCS News Release. 2012. Smooth brome grass.
- Operation Grassland Community. Smooth brome in native grasslands. The Landowners Toolkit Series 8.
- Penn State Extension, Quackgrass management: an integrated approach. Agronomy Facts 5.
- Salesman, Jessica Balwahn and Meredith Thomsen. 2011. Smooth brome in tallgrass prairies: a review of control methods and future research directions. Ecological Restoration 29(4):374-381.
- Sather, N. 1996. Element stewardship abstract for Poa pratensis and Poa compressa. The Nature Conservancy, Arlington, VA. Assessed October 6, 2019.
- Tix, Daniel, Jo Anna Hebberger, Elizabeth Vaughan, Iris Charvat. 2003. The effects of fire versus mowing on prairie plant communities. University of Minnesota, Department of Plant Biology.
- Ulrich, Emily and Lora Perkins. 2014. Bromus inermis and Elymus canadensis but not Poa pratensis demonstrate strong competitive effects and all benefit from priority. Plant Ecology 215: 1269-1275.
- Vinton, Mary Ann and Erin M Goergen. 2006. Plant-soil feedbacks contribute to the persistence of Bromus inermis in tallgrass prairie. Ecosystems 9: 967-976.
- Willson, Gary D. 1990. Morphological characteristics of smooth brome used to determine a prescribed burn date. Proceedings of the Twelfth North American Prairie Conference: 113-116.
- Willson, G.D. and J. Stubbendieck. 1996. Suppression of smooth brome by atrazine, mowing and fire. Prairie Naturalist 28: 13-20.