Cheatgrass - Scourge of the West

April 14, 2022

CHEATGRASS IN THE WEST

A COMPLEX CHALLENGE FOR RESOURCE MANAGEMENT 

By Lucy Bauer, PhD Ecologist

In the aftermath of the Snaking Gulch Fire of 2002, the author’s forested mountain land on the outskirts of Bailey, Colorado, was subjected to a wave of non-native plant interlopers that were carried in by wind, water and wildlife from the fire-ravaged public lands to the west. Such noxious weeds as were found at that time (musk thistle, Canada thistle, mullein and knapweed) were diligently removed and the land reseeded with a mix of native grasses and reforested with scores of conifer seedlings. Gradually, as the years passed, the bare slopes were recolonized by burgeoning vegetation and the land seemed to have healed. It was, therefore, a nasty surprise to discover, many years later, that one more unwelcome invasive plant species had insidiously infiltrated the recovering mountain slopes and was steadily gaining ground; that scourge of the western rangelands and partner-in-crime where wildfire is concerned; cheatgrass.

Matured cheatgrass amidst native vegetation on author’s property.

Cheatgrass, Bromus tectorum, is an unremarkable-looking grass that evolved in central and western Europe. In the Mediterranean region, cheatgrass occupies the decaying straw of thatched roofs (Leopold, 1949), and thus is also known as thatch bromegrass (the direct Latin translation: “ brome of the roofs”). It is thought to have been brought across to the United States in the 19th Century as an unintended contaminant in seed supplies, but it was also introduced purposely by the USDA as a chosen ground cover to stabilize soils and provide somewhat graze-resistant fodder for livestock (Mealor et al, 2013). Since its introduction it has spread to every state in the union, and in the West has been relegated to List C of noxious weeds - those that are so well-ensconced in their adopted home that completely eradicating them is considered next-to impossible. Even in the noted ecologist Aldo Leopold’s day, nearly a century ago, cheatgrass was widespread on the western plains and grasslands, for, as he notes, “A plant that can make a living on the roof of a house can also thrive on this rich but arid roof of the continent.”(Leopold, 1949, p.155) It is important to know the biology and ecology of this species to fully understand its impact upon the American scene, and especially in the West.

Cheatgrass illustration; adapted from Stubbendieck et al. (2011).

Cheatgrass is an annual grass, growing vigorously from seed every year in dense stands, and maturing early to then die off into a thick blanket of wispy dead material at the end of the season. The plant is usually of relatively short stature (less than 2 feet high) with drooping seed heads of prickly awns or spikelets of a reddish color when mature. Its stem and leaves are covered with a fine layer of hair which is a distinguishing characteristic for this species, and also leads to another of its names, “downy brome”.The seed heads are already maturing and the plant is turning brittle by early summer when other species are still putting on growth. The spiky awns catch and stick on fur, clothing, and vehicles, allowing them to be spread to new locations easily. Those seeds that stay in place typically fall within a few feet of the parent plant to germinate a couple of months later in the autumn, when other grasses are going dormant.  After a winter hiatus, the little seedlings resume growth in early spring, getting a jumpstart on neighboring native species. It is this precocious growth regime that has made cheatgrass so successful as an invasive, and so devastating to local landscapes.

Early growth phase of cheatgrass in Spring, at author’s property.

Historically, the western plains and mountain grasslands were blanketed by a rich diversity of grasses and forbs that had co-evolved over the eons to thrive under a unique regime of intense but intermittent grazing, and infrequent low-intensity wildfires. The ecosystem was in balance and all elements flourished. The extermination of the vast buffalo herds over a century ago and their replacement by domestic grazers, along with extreme fire-suppression for the past century, has upset the ecological balance of this system and provided the perfect niche for cheatgrass establishment. This grass species is now so entrenched in the western landscape that it is hard to conceive that it has not always been there. As Leopold mused back in 1949, “ the honey-colored hills that flank the northwestern mountains derive their hue not from the rich and useful bunchgrass and wheatgrass which once covered them, but from the inferior cheat which has replaced these native grasses. The motorist… is unaware of this substitution. It does not occur to him that hills, too, cover ruined complexions with ecological face powder.” 

Dry cheatgrass foreground) at Rocky Mountain Arsenal National Wildlife Refuge.

In the United States cheatgrass now exists in a plethora of habitats from disturbed sites, roadsides  and overgrazed rangelands to upland and riparian areas. It can adapt to a wide range of soil types, but is especially competitive and abundant on dry sites. The ramifications of this species’ introduction are significant for entire ecosystems, reducing species diversity and wildlife habitat, but also limiting forage quality and abundance, and altering wildfire regimes.

Maturing cheatgrass seed head at author’s property

Cheatgrass, though a non-native, provides good fodder for cattle and sheep and many native herbivores in the spring while it is green and actively growing, but it quickly becomes unpalatable and even a danger to livestock as the seed heads mature and dry out and can become lodged in the grazer’s jaw, causing painful abscesses. Because of this, rangeland with a high level of cheatgrass infestation is rendered unproductive and even unusable for a large portion of the year, with any remaining native grass species quickly becoming overgrazed as a side consequence. Once dry, cheatgrass is low in nutritional value (Stewart & Hull, 1949), and must be eaten in large quantities to provide adequate nutrients, and herbivores that frequent cheatgrass-dominated meadows will be likely to increase its spread as they migrate between ranges and carry seed into previously uninfested pasture. By midsummer, a cheatgrass patch is tinder-dry and its seeds are already safely in the surface soil, but its dead above-ground foliage is very susceptible to lightning strikes and ground fires. Cheatgrass is a prolific producer of extremely flammable litter: in Utah, research into the litter production of grass communities of native perennials versus that of communities invaded by cheatgrass, showed that there was an increase of over 125% in the latter, compared to the former grassland communities (Evans et al, 2001).  A distinct cyclical and synergistic relationship exists between cheatgrass and wildfire: cheatgrass abundance and spread is increased by frequent fire as fire-ravaged land is deprived of its native vegetation and the cheatgrass takes advantage of the barren eroding soil for new colonization. The dense accumulation of further cheatgrass litter then fuels new fires, and so an ever-expanding cycle of destruction ensues. Moreover, with cheatgrass providing a perfect fuel source for wildfire early in the season, any remaining perennial native grasses that are still growing and have yet to set seed are likely to be destroyed; roots, shoots and all. Combining this effect with the fact that cheatgrass litter can also act as a ladder fuel to shrubs and other low fire-susceptible vegetation in its vicinity results in an ever-decreasing species diversity as these native species are ousted from their accustomed ranges over successive fire seasons. Thus the disruptive spread of cheatgrass is exacerbated, and prospects for its complete elimination grow ever slimmer.

The complex ecology of a cheatgrass-dominated landscape makes weed-control a tricky issue. Leopold (1949) already noted a fatalistic attitude amongst land  managers nearly seven decades ago, “I listened carefully for clues whether the West has accepted cheat as a necessary evil, to be lived with until kingdom come, or whether it regards cheat as a challenge to rectify its past errors in land-use. I found the hopeless attitude almost universal” But does that approach still hold true today? Most current natural resource managers have realized that while there is no panacea for cheatgrass eradication, there is still value in tackling the problem to reduce the pernicious impact this grass has on its local and regional environment. In cheatgrass control, a multi-pronged approach is needed, with different methods being employed for different scenarios. The universities of Wyoming and Colorado State recently joined forces to develop a detailed manual for cheatgrass management (Mealor et al, 2913) and a schematic from this handbook is shown below.  

Chart showing successional processes to follow in managing cheatgrass

The researchers advocate a comprehensive process of assessing the level of a cheatgrass infestation by inventorying and monitoring, adopting realistic goals for treatment based on the extent of the invasion and the potential of the land for recovery, followed by designing a treatment plan that is then implemented and monitored to determine the level of success in controlling the problem, This process, with its integrated feedback loops for both management, and for prevention in instances where cheatgrass is currently absent, allows for a well-informed and flexible treatment of a complex invasive species (and can be applied to other such species besides cheatgrass). 

Largely cheatgrass-free upland grassland at author’s property,  dominated by native species such as Blue Grama. A healthy sward may prevent cheatgrass from getting established.

Cheatgrass eradication by any method is difficult, not least because a significant seed bank can be built up in the soil over the years, meaning that persistent long term treatments will be necessary for many years in a row to deal with this reservoir of viable seed. Mechanical methods may work in some instances:  In the case of an initial invasion of cheatgrass in small and discrete patches, hand-pulling of plants before seed-ripening may be effective, as around the author’s garden and outbuildings. More mature plants can also be removed and bagged for disposal. Repeatedly mowing a larger expanse of cheatgrass in its early stages can severely weaken the plants, but cheatgrass is capable of producing seed on stunted plants only two inches high, so it is not a foolproof method. Reseeding with native grasses once the cheatgrass has been removed and the soil has been disturbed is a good policy.  Selective grazing by livestock in early spring can help reduce cheatgrass densities (Hempy-Mayer & Pyke, 2008), especially if done in successive years. Allan Savory’s holistic approach to grazing livestock in high densities on select areas of degraded land for a short burst of time might have some value here. Combining this approach with a fall prescribed burn is also an appealing option. Other biological controls under study include using fungi and bacteria to attack the seeds and roots of cheatgrass, which could also be combined with adding nitrogen-poor mulches to the soil surface, which reduces the availability of nitrogen to cheatgrass and gives it a competitive disadvantage compared to other native grasses that can thrive in lower-nitrogen conditions (Perry et al, 2010). 

Chemical control is the preferred and most extensively used method of cheatgrass management in rangeland and pasture. Its approach is often needed, especially in larger infestations, and can be quite effective if timed correctly and applied appropriately. It has the advantage over other methods of not significantly disturbing the soil, being fairly easy to apply, and giving considerable flexibility in exactly how, when and where it is used. Disadvantages include potentially spraying and killing non-targeted native plants, allowing chemical residues to get into soil and water, health concerns over contamination for people spraying or being in the vicinity of the herbicide application, and the not-insignificant costs of purchasing and applying the chemical. Plateau is one popular choice out of a number of options, having activity both as a pre-emergent on seeds, and post-emergent on newly-emerged seedlings. However, it must be applied judiciously either in late fall when new cheatgrass seedlings are emerging but native species are now dormant, or in early spring (before March) when cheatgrass growth, but not yet that of native perennials, has resumed; poor timing can result in native vegetation being destroyed along with the cheatgrass, as the author discovered when using this chemical on the driveway verge a little too late in the Spring.

Driveway edge at author’s property, treated with Plateau in early Spring. The non-selective herbicide killed cheatgrass but also some native grasses and forbs.

Since then, the author has discovered a newer and far more effective tool for chemical cheatgrass management: The pre-emergent herbicide, Rejuvra, which is rapidly becoming the favored tool for cheatgrass control across the West. It is a version of indaziflam that is approved for use on rangeland and pasture (as opposed to Esplanade SC 200, that is labeled for use on more industrial sites such as railroads, roadsides, airports etc). If applied correctly at the right stage in the cheatgrass lifecycle, it not only prevents seed germination for the year of application, but has residual activity for a year or more into the future, thus steadily depleting the weed seed bank, especially if two applications are made on the same site several years apart.

Established native vegetation, including perennial grasses and forbs, are not affected by the application of Rejuvra, and may even experience a “release”, growing more vigorously once the cheatgrass competition for light, space, water and nutrients is removed. In disturbed bare-ground areas, native seeds can be drilled into the soil at a lower depth where the herbicide does not penetrate, and help to restore the site as they emerge and flourish. Once the Rejuvra has ceased its activity after a few years, broadcast seeding of native or desirable plants can be done to revegetate a site too.

To conclude, cheatgrass is a supremely successful exotic addition to the western landscape of the United States;  a fact that is extremely troubling and frustrating for the land managers that have to deal with the consequences of this aggressive, opportunistic, unpalatable and fire-prone noxious weed. Leopold, back in 1949, reflected on whether cheatgrass would either be tolerated forever as a “necessary evil”, or regarded as a “challenge” for the rectification of past poor land use. Today it seems the way forward is to hold both options in a realistic but not fatalistic, balance; by applying a creative multi-faceted management approach that is carefully crafted to deal specifically with each situation in which cheatgrass is found, the negative impact of this weed can be reduced as much as possible, even if it is here to stay. On the author’s upland property this will be manifested as continued careful monitoring and judicious application of mechanical and/or chemical methods of control, dictated by the nature of the cheatgrass infestation, for many years to come.

References

Cheatgrass Management Handbook - managing an invasive annual grass in the Rocky Mountain region. University of Wyoming and Colorado State University. 2013.

Mack, R.N. Invasion of Bromus tectorum L. Into Western North America: An Ecological Chronicle. Agro-Ecosystems 7, 145–165 (1981).

Evans, R.D., Rimer, R., Sperry, L. & Belnap, J. Exotic Plant Invasion Alters Nitrogen Dynamics in an Arid Grassland. Ecological Applications 11, 1301–1310 (2001).

Stewart, G. & Hull, A.C. Cheatgrass (Bromus tectorum L.)—An Ecologic Intruder in Southern Idaho. Ecology 30, 58–74 (1949).

Hempy-Mayer, K. & Pyke, D.A. Defoliation Effects on Bromus tectorum Seed Pro- duction: Implications for Grazing. Rangeland Ecology and Management 61, 116–123 (2008).

Perry, L.G., Blumenthal, D.M., Monaco, T.A., Paschke, M.W. & Redente, E.F. Immo- bilizing Nitrogen to Control Plant Invasion. Oecologia 163, 13–24 (2010)