By Charles Bryant, Huerfano County Noxious Weed Manager
After the prolonged drought conditions that our region has experienced this year, the tremendous amount of fall-time moisture received has triggered growth among noxious weed infestations that have largely remained dormant for most of 2026. The considerable amount of flash flooding has also created new, dense infestations as flood waters have deposited a variety of seeds from sites far upstream. While our native plant species have fairly strict seasonal growth\establishment schedules, our exotic noxious species make the most of every moment, with some being able germinate\sprout every month of the year. While many view the spring and early summer as the prime window to perform mitigation efforts, autumn is often overlooked. Autumn is actually the time when some of our most problematic species gain a foothold for the upcoming year or are in their most vulnerable state.
First, we will review a few of our common annual and biennial species that can flourish during the cooler months of the year. Perhaps the most easily identified is Scotch thistle. While often referred to singularly, the Scotch thistle in our area is actually comprised of two different species, Onopordum acanthium and Onopordum taricum. Originating from Eurasia, these biennial thistle species tend to stand out on the landscape compared to native thistle species as they have the ability to grow exceptionally large (over 8 feet in height) while the desirable native thistles seldom grow over 1.5 feet tall. Of the two Scotch thistle species, O. acanthium is the most cold tolerant, with seedling growth being observed all 12 months of the year. Right now in October and in the wake of the exceptional moisture we have received, seedling growth is in overdrive. Their stature this time of year is miniscule compared to that when they bolt and set flower during the summer months, with their physical size easily fitting within the footprint of a quarter-dollar. Thanks to their high germination rate, dense mats consisting purely of thistle seedlings develop in the fall, later turning into what has been described as a “living fence” when they finally bolt the following summer and grow to the aforementioned considerable heights. This is not to say that all Scotch thistle grows to a large size, as the availability of moisture largely dictates their ultimate size. In a drought year like we just experienced, many will bolt and flower at heights of one foot or less. Learn more about Colorado’s native and non-native thistles here.
Those choosing to use herbicides for controlling these infestations in the fall will find that much less product at a lower application rates can be used due to their smaller stature and enhanced translocation of herbicides. Acting much like trees in the autumn, most energy resources are channeled to the root system so that it can overwinter, allowing systemic herbicides to impact both above and below ground biomass. When considering mechanical controls, folks will also find that the tender stems of fall Scotch thistle growth are much easier to cut than the large diameter fibrous stalks of summertime bolting plants. By performing this control work in the fall, a considerable amount of money and effort can be saved, allowing land managers to divert their spring\summer efforts to more desirable activities.
Next, we will look at one of the most widespread, yet under recognized invasive grass species known as Cheatgrass (Bromus tectorum). This winter annual gets its common name due to the fact that it “cheats” other vegetation out of their environment due to its exceptionally early growth. Germinating in the late summer and continuing throughout the winter months so long as soil remains unfrozen, cheatgrass slowly establishes itself, exploding in the early spring, forming dense monocultures that outcompete desirable vegetation. Completing its life cycle in the early summer as our desirable native plant species are just emerging, cheatgrass quickly dries after seed set, creating an abundance of fine fuels that pose a high fire risk. Those that run livestock can actually utilize this problematic species prior to seed set as quality forage, with it having a nutritional profile that rivals that of common desirable pasture grasses. The crude protein levels in young cheatgrass can be as high as 19% and is often sought by livestock as it is often the only green growth early in the season. Higher intensity targeted grazing in the spring can greatly disrupt the lifecycle of cheatgrass as seed development is greatly limited. However, grazing should be avoided as the plants set seed and desiccate in the early summer months. The sharp textured seed heads have leant cheatgrass the alternative name of “rip-gut brome” as the seed heads have the ability to perforate the intestines of grazing animals. Concerning the fall time application of herbicides on cheatgrass, land managers will find a greater flexibility and efficacy of products utilized, being able to use non-selective products that have no residual soil activity since desirable native species are dormant.
Generally speaking, such non-selective product options like glyphosate are not the best tool to utilize when other desirable vegetation has emerged, but pose little concern of off-target damage when utilized in the right timeframe while desirable species are dormant. In addition, much like Scotch thistle, much less product at a lower application rate can be utilized to realize satisfactory results. Controlling cheatgrass in the fall serves the duel purpose of not only assisting native plant communities, but also by mitigating next summer’s fire hazards. Should the above listed control methods be impractical during the fall and\or winter months, land managers can also greatly disrupt cheatgrass through properly timed mowing, prior to seed set. As temperatures climb in the later part of May, cheatgrass that has been mowed has a much harder time developing new seed heads and will succumb as temperatures reach the mid to high eighties, leading to severe desiccation and plant death. Being an annual that only reproduces by seed, land managers thankfully do not have to contend with rhizomatous growth common with perennial grass types.
While we have covered only but a small portion of Colorado State List Noxious Species, many of the control methods described in this article can be applied to other annual and biennial species, though mechanical controls should largely be avoided for perennial species as they can actually stimulate growth. By performing this critical control work now, landowners can have a portion of their busy spring and summer months back, all while saving considerable money and effort.
Please contact the Huerfano County Noxious Weed Department for more information concerning the control of these or any other problematic noxious species that you may have growing on your property.
Noxious Weeds Contact: (719)738-2420 or [email protected]
