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.  The C₃ process is more efficient when cooler temperatures are cooler and in shadier environments whereas the C₄ process is more efficient in hot, sunny, and drought-prone conditions.

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. This higher nitrogen requirement results in protein-rich leaves, which are more desirable to herbivores (large or small). Soil microbes more easily decompose CSG litter than WSG whose litter decomposes more slowly.

Boot stage

When the 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.

2000 Oregon State University©

  W WThere is no single recipe for battling cool season grasses. We have learned that it requires persistence and a combination of techniques. More so when it’s integrated into a high-quality, diverse prairie. I emphasize this because study results do not translate well from grass-dominated systems to high-diversity ecosystems.

Burning can stimulate non-native cool season grasses. Late spring burning is often cited as a good control method, but the studies are contradictory, and the trade outs are high. Studies citing late spring burn effectiveness are conducted in tallgrass-dominated prairies (not diverse prairies) and rarely address the trade outs, which can include damaging early spring forbs and native cool season grasses. It can also imperil herptiles and immobile and slow-moving insects as well as early migrating grassland birds, such as the American woodcock. Small mammals may also be in jeopardy if the ring technique for burning is used.

Fire is discouraged on monocultures of smooth brome as results are variable (12). Recall that fire is most effective when WSG exist to provide competition. Annual burns are touted are necessary (2, 18) but they harm desirable species (13, 14). Frequent burns create a less diverse ecosystem as the WSG grass thrives and outcompetes the forbs.

Seasonality of burns might make a difference in controlling CSG but there’s still much unknown. Dormant season spring burns enhance smooth brome growth (12); dormant fall burning reduced the amount of smooth brome but wasn’t as effective as herbicides (2). 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 (17) although it’s more difficult to graze specific areas than to mow them.  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). Mowing during either of these times is also the most effective at preventing seed production (10). So how do we know when the tillers have elongated? [Refer to the tiller diagram] The advantage of mowing is how precise one can be, but is a single mowing effective? It’s suggested 4 mowings are more effective than 2 or 3 mowings in a year (12). Yet, mowing this often is would damage native plants and Poa pratensis thrives on frequent mowings.

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 a spring mowing or burning as the CSG patches begin growing and WSG is dormant. Herbiciding the regrowth after a fall burn when the WSG are dormant is also effective. The regrowth needs to be about 4” tall for the herbicide to work. We have found working in patches rather than broadcast spraying large areas is more effective.

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 wanting a diverse ecosystem.

Our first experience: The 5-Acre Prairie

The largest infestation of CSG we had was (note the past tense) in the 5-Acre Prairie, our first high-diverse planted prairie. One reason the CSG had a good foothold was the time it took us to get educated on it 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 (because we didn’t want to encourage the tall C₄ grasses) and never in the late spring (because of the herptiles and early native plants). Complicating this was our desire to preserve the Prairie dropseed (Sporobolus heterolepsis) and Junegrass (Koeleria macrantha). Spraying herbicide had to occur before they emerged. We also planned to overseed the CSG areas in late fall to introduce competition. We prefer to let nature do the cold, moist stratification process that most native forb seeds require. To make this plan 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. A few months later, the CSG was 4-6” tall.  We sprayed a grass-specific herbicide on the large CSG patches.

We monitored these patches as the year progressed. Some needed a bit more management so we mowed a second time then resprayed when the CSG grew to 4-6” tall. We have continued monitoring for a patch here with the goal to not allow it to get a stronghold.

While this method doesn’t kill every blade of CSG, it reduces the patch sufficiently, allowing the overseeding and the surrounding native plants fill in.

This is the same prairie in about the same location in June of the 2017 (3 months after the CSG treatment)

I have found herbiciding sections (rather than the whole habitat area) with grass-specific herbicide to be effective. This can be done after a burn or a mowing once the CSG has had time to regrow.

Plan to overseed but wait for 30 days before doing so. This will give the herbicide time to reduce its residual in the soil.

You’ll want to check in early July to see if mowing is necessary. You’ll know by how much regrowth there might be.

Sections can be herbicided prior to a fall burn. To prepare for this, mow in July/Aug and let it regrow to 4-6” and then herbicide it. You can overseed it afterwards and before the burn as well, just sow the seeds prior to a rain and about 30 days after the herbicide treatment.

The photo on the right is an overview of our remnant where I mowed the CSG clones. The photo above is a close up of one of these mowed areas.

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’ 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

Tiller elongation charge. Willson, Gary D.©

This boat-shaped tip is a key ID piece for Poa pratensis.

The underneath side and one can see the roots and rootlets have not left room for others.

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

 References

1. 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.
2. 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.
3. Colorado Weed Management Association. www.cwma.org/Quackgrass.html
4. Espeby, Lin A., Hakan Fogelfors, Sara Sjodal, and Per Milberg. 2014. Variation in Elymus repens susceptibility to glyphosate. Acta Agriculturae Scandinavica 74: 211-219.
5. Gorman, Emily. 2012. Plant species differ in their ability to reduce allocation to non-beneficial arbuscular mycorrhizal fungi. Ecology 93(4): 711-718.
6. Illinois Nature Preserves Commission Vegetation Management Guidelines.
7. Korhammer, Siegfried A and Ernst Haslinger. 1994. Isolation of biologically active substance from rhizomes of Quackgrass. Journal of Agricultural Food Chemicals 42: 2048-2050.
8. 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.
9. NRCS News Release. 2012. Smooth brome grass.
10. Operation Grassland Community. Smooth brome in native grasslands. The Landowners Toolkit Series 8.
11. Penn State Extension, Quackgrass management: an integrated approach. Agronomy Facts 5.
12. 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.
13. Sather, N. 1996. Element stewardship abstract for Poa pratensis and Poa compressa. The Nature Conservancy, Arlington, VA. Assessed October 6, 2019.
14. 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.
15. 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.
16. 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.
17. 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.
18. Willson, G.D. and J. Stubbendieck. 1996. Suppression of smooth brome by atrazine, mowing and fire. Prairie Naturalist 28: 13-20.