Cool season grasses, such as Smooth brome, Kentucky bluegrass, and Quackgrass are prevalent and wreak havoc on prairies. Their phenology gives them a competitive advantage in the spring but in the fall, the phenology works against them. As with many aspects of ecological restoration, you’ll find there isn’t adequate or definitive research. I have learned that the best way to deal with specific invasive issues is to learn everything I can about the plant, talk with other restoration practitioners, and then figure out a plan that works for my prairies in my area. To that end, I am sharing my initial research. As we move through the management plan, I’ll update this and provide our lessons learned in the hopes it will help others. Please feel free to provide any experiences you have had with managing cool season grasses.
Warm season grasses (WSG) are called C4 plants and cool season grasses (CSG) are called C3 to differentiate their photosynthesis pathways. The “C” stands for carbon and the number is the number of molecules of the carbon that are formed via photosynthesis. C3 and C4 plants also differ in leaf anatomies (structure or morphology) for carrying out the removal of atmospheric carbon dioxide for the plants’ use.
The differences between these plants are how they fix carbon, optimal temperature ranges, soil nitrogen needs, and how they produce seed heads. 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 processes of photosynthesis are the same. WSG utilize oxygen much more efficiently during the photosynthesis than CSG, especially in warmer temperatures. The optimum temperature range for CSG is 65-75 degrees whereas the range for WSG is 80-95 degrees. CSG needs a period of cold and for the tiller to reach a certain size in order to produce seedheads, whereas WSG need only a 70 degree period to germinate. CSG have more protein in their leaves than WSG and are more easily decomposed and/or digested by grazers. Litter decomposes more rapidly in CSG than WSG or native forbs, creating a faster nitrogen cycling meaning that CSG are more abundant on plots without spring litter removal.
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.
Facts about smooth brome (Bromus inermus):
- 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
Control based on studies:
There are no studies that provide a recipe for battling cool season grasses. What we do know is that it requires persistence and a combination of control methods.
Burning is often cited as a good control method but the studies are contradictory toward this. Some personal communication to one study author stated that a schedule of rotational 3-year burns in early May showed reduction in CSG coverage from 98% to <1%. (Salesman) In talking with others who have dealt with CSG invasions, many have had similar success with late spring burns. A literature review shows that fire is discouraged on monocultures of smooth brome as results are too variable and it doesn’t encourage growth of WSG. (Salesman) Although one study by Willson & Stubbendieck states that annual burns for several years are necessary (Bahm, Herbicide…) there are many other studies that say fire has had limited success (Willson). And yet another study has explained how frequent burns do not actually reduce smooth brome because of the linkage of soil nitrogen levels. Since CSG thrives in soils with high levels of nitrogen and since frequent burning causes nitrogen depletion one would think frequent burning would be the answer, yet depletion of soil nitrogen did not always reduce CSG growth and density. (Vinton)
The litter of CSG, in general, decomposes faster than native WSG. This rapid decomposition creates a faster nitrogen cycling process. Carbon has been used to deplete the soil nitrogen but data from Vinton’s study do not show a “compelling decrease in smooth brome growth under carbon additions.” Vinton also commented that atmospheric nitrogen is increased “nearly 10-fold” in the past 100 years. This is just one reason why cool season grass invasions are on the rise.
We do know that early, dormant season spring burns enhance smooth brome growth (Salesman) but it can hurt early native forbs. One study states that fall burning reduced the amount of smooth brome but wasn’t as effective as herbicides, (Bahm, Herbicide…) yet a literature review states that no studies regarding fall burning have been conducted. (Salesman)
Mowing is another control technique to explore. Several studies demonstration that 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. (Bahm, Restoring…) The next question to answer is how does one know when the tillers have elongated? Willson suggests that elongated tillers have five or more leaves. A literature review of this subject says a single mowing is ineffective and that 4 times is more effective than 2-3 times in a year. (Salesman) Yet, cool season grasses are favored on plots that are mowed because the litter is not removed. Information on Poa pratensis states that it thrives on frequent mowings so perhaps control of it is done solely by herbicide.
Haying is a good mimic of fire. Hayed areas are most similar to burned plots in terms of soil moisture, temperature, and litter quality, yet haying in spring did not have much effect on controlling CSG. (Vix) I could find no other studies about haying and CSG.
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
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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.
Grman, 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.
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.