Weed of the month submitted by Charles Bryant Scotch thistle
Scotch thistle is perhaps one of the most easily identifiable noxious plant species in Colorado. Having a distinct physical profile with rather large pale blue/green-colored spiny leaves covered in cobweb like hairs, plants can easily grow to over eight feet tall with many beautiful purple/pink disc flowers up to three inches in diameter. This biennial species produces an abundance of seeds (up to 40,000 per plant) that contain a water-soluble germination inhibitor. These seeds also remain viable in the soil seed reserve for a considerable amount of time; upwards of 39 years (Weed Control in Natural Areas in the Western United States, p.292). These reproductive characteristics can make Scotch thistle a particularly difficult and frustrating species to control. Huerfano County alone has more than 400 infested acres documented in the EDDMaps system. Scotch thistle is one of the most abundant and problematic noxious plant species in Huerfano County. During seasons when precipitation is limited, Scotch thistle infestations can seem somewhat sparse and insignificant. Rather than growing into “Christmas trees” during drought years (this is how Huerfano County staff affectionately refer to exceptionally large plants), plants generally grow to a height of less than two feet and often times only have one flower head. During seasons of limited precipitation, there is also a fairly high mortality rate for seedlings.
When we have an exceptionally wet season similar to 2019, Scotch thistle infestations expand dramatically. Seedlings and rosettes are extremely cold-hardy and can be found any given month of the winter in areas such as Huerfano County. Starting in the late winter and early spring, Scotch thistle can be easily detected as it is one of the only broadleaved plant species to emerge, while most other species are still dormant. In late winter through spring, the rosettes begin to grow up to three feet in diameter before bolting. Wildlife and livestock will actually graze tender Scotch thistle growth during the early part of the season, but will generally avoid it when leaves more fully develop with spines. As springtime progresses, seedlings continue to rapidly germinate and can create a dense mat within infested sites. In Huerfano County, rosettes usually begin to bolt around the middle of May through early June. The large physical profile of Scotch thistle begins to form what has been described as a “living fence” which can impede livestock access to grazing areas and water sources. Due to the defensive physical characteristics of the plant, livestock and wildlife will generally avoid heavily-infested sites. Plants will begin to flower in warmer, lower elevation areas around the middle of June while plants in cooler high elevation areas begin flowering in late June through the middle of July. As plants begin to flower and ultimately set seed, a continuous growth of indeterminate flower buds often grow from the intersections of the main stalk and its numerous branches. Plants that flower and set seed in the early summer will continue to produce flowers throughout the summer and into the fall if enough moisture is present. During rather dry seasons, this additional flower regrowth is somewhat limited or does not occur at all. Seasonal monsoon rains seem to stimulate this additional flower growth. Abnormally dry summers will also provide a somewhat unexpected non-host specific biological control in the form of grasshoppers. In the absence of other green vegetation, grasshoppers readily consume the lower leaves and entire flower of Scotch thistle which effectively prevents any seed formation. While Scotch thistle may seem like a formidable enemy, there are a number of extremely effective methods for controlling it. Traditional mechanical methods such as hand-digging and pulling can be used. Scotch thistle is a biennial species that only reproduces by seed. When hand-dug, the root must be severed at least 3 inches below the soil surface, otherwise the plant can regenerate from growth points that lie just below the soil surface. When hand-pulling,target smaller plants, try to recover as much of the taproot as possible, and wear thick leather gloves. Scotch thistle is also very susceptible to chemical treatments when properly timed. Huerfano County staff have experienced exceptional results using 2.5oz/acre Opensight herbicide tank mixed with 0.5% V/V 2,4-D and a non-ionic surfactant at 0.25% V/V. Applications should ideally be performed early in the season to seedlings or rosettes prior to bolting, but effective control can still be maintained when plants are treated all the way up to flower break. While Scotch thistle infestations may seem overwhelming during a particularly wet season, it provides an excellent opportunity to exhaust the long dormant seeds within the soil seed reserve and prevent new plants from contributing to the seed bank. The Huerfano\Custer County Noxious Weed Department and it partners of the Upper Arkansas Cooperative Weed Management Area (UACWMA) are fully committed to addressing the threat that invasive plant species pose to our area and Colorado as a whole. Please do not hesitate to contact the UACWMA with any questions, weed management success stories, or to report sightings of List A species. Be sure to review the list of UACWMA cooperating members on the Home page of this website to find contact info for your local cooperating entity. All photographs were taken within Huerfano County and provided by Charles Bryant, Huerfano/Custer County Noxious Weed Manager.