K. George Beck

Combining Mowing and Fall-Applied Herbicides to Control Canada Thistle (Cirsium arvense)1

Abstract: Experiments were conducted in subirrigated and upland Colorado pastures to compare herbicides applied alone in fall to the same herbicides preceded by one, two, or three mowings. Picloram controlled Canada thistle (Cirsium arvense) well at both sites, and in general, mowing did not improve its performance. Picloram at 560 g ai/ha and picloram plus 2,4-D at 280 plus 1,120 g ai/ha were the lowest rates that eliminated Canada thistle. Mowing did not improve chlorsulfuron performance at either site, but chlorsulfuron eliminated Canada thistle at the subirrigated site. Dicamba controlled 97% of Canada thistle at the subirrigated site, and mowing did not improve its performance; two or three mowings before spraying dicamba at the upland site improved performance, but 37% remained uncontrolled. Control from the reduced rate and the two lowest recommended rates of clopyralid plus 2,4-D was improved by two or three prior mowings at the subirrigated site, but only the highest rate benefited from two or three mowings at the upland site. Mowing alone three times per year for 2 yr controlled 85% of Canada thistle at the subirrigated site, but failed to control it at the upland site. Variability in results between experiments likely was due to a high water table at the subirrigated site, which may have restricted Canada thistle root growth and possibly made it easier to control. Inconsistent results prohibit concluding that mowing before spraying will consistently improve Canada thistle control, and such a treatment combination should not be commonly recommended. Nomenclature: Clopyralid, 3,6-dichloro-2-pyridinecarboxylic acid; chlorsulfuron, 2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide; dicamba, 3,6-dichloro-2-methoxybenzoic acid; picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid; 2,4-D, (2,4-dichlorophenoxy)acetic acid; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR. Additional index words: Integrated management, noncrop weed management, CIRAR.

Genetic Variation in Invasive Populations of Yellow Toadflax (Linaria vulgaris) in the Western United States

Abstract Intraspecific genetic variation may contribute significantly to invasiveness and control problems, but has been characterized to date in relatively few invasive weed species. We examined 56 intersimple sequence repeat (ISSR) loci in 220 individuals from 11 invading populations of yellow toadflax sampled across five western states. All populations showed high levels of genetic diversity. Estimated values for Shannons diversity measure ranged from 0.217 to 0.388, and for expected heterozygosity from 0.178 to 0.260. Neis total gene diversity index (HT), on the basis of all individuals across all populations, was 0.267. Partitioning of genetic variance using analysis of molecular variance revealed 1.7% of genetic variation among regional population groups, 29.1% among populations within groups, and 69.2% within populations, consistent with expectations for an outcrossing species but suggesting little geographic differentiation. Pairs of adjacent individuals identical at all ISSR loci that appeared to be ramets of a single clone were detected in only one population. This indicates that patch expansion in yellow toadflax is driven more by sexual reproduction via seed than by rhizomatous clonal spread, at least at the spatial scale of sampling for this study. Eight populations had significant values for Mantels R at P  =  0.05, suggesting some fine-scale positive genetic structuring, possibly from restricted gene flow. Population clustering on the basis of Neis genetic distance between populations and unweighted pair group method with arithmetic mean did not reflect geographic location. It is likely that multiple introductions of this species have occurred across the Intermountain West, followed by extensive genetic recombination. High levels of genetic diversity within yellow toadflax populations pose management challenges, as already seen in reports of variable response to herbicide application and limited impacts of biocontrol agent releases.

Tumbling: Use of Diffuse Knapweed (Centaurea diffusa) to Examine an Understudied Dispersal Mechanism

Abstract Dispersal is a critically important process in the spread of invasive plants. Although knowledge of dispersal will be crucial to preventing the spread of invasive plants, little research has been performed within this context. Many important invasive or agricultural weeds disperse their seeds via tumbling, yet only one previously published paper investigated this dispersal mechanism. Field and wind tunnel experiments were conducted to quantify and model tumbling dispersal. We developed competing models for diffuse knapweed seed dispersal from wind tunnel experiments and compared predictions to data collected from a field site in Colorado. Seeds were retained in plants that had traveled hundreds to as much as 1,039 m (3,408 ft). Although neither model accurately predicted dispersal when compared with independent field data, surprisingly, seed retention with distance was somewhat better described as a linear process than as exponential decay. Wind tunnel trials showed no evidence that the number of seeds deposited per meter depended on plant size. Thus, fecundity might be a key factor determining seed dispersal distances; plants with higher fecundity might disperse seeds over longer distances than those with fewer seeds. Nomenclature: Diffuse knapweed, Centaurea diffusa Lam. CENDI

Predicting yellow toadflax infestations in the Flat Tops Wilderness of Colorado

Understanding species–environment relationships is important to predict the spread of non-native species. Yellow toadflax (Linaria vulgaris Mill.) is an invasive perennial recently found in the Flat Tops Wilderness of the White River National Forest on the western slope of the Colorado Rocky Mountains. We hypothesized yellow toadflax occurrence could be predicted from easily measured site characteristics. We used logistic regression with stepwise selection to generate a model to predict yellow toadflax occurrence on a particular plot based on that site’s physical characteristics. The experimental design was a paired-plot study in two locations using circular 1,018-m2 plots. Sixty-eight plots that did not contain yellow toadflax and 65 plots that contained yellow toadflax were sampled at the Ripple Creek site in 1999. In 2000, 54 non-toadflax plots and 55 toadflax-containing plots were sampled in the Marvine Creek site. Site characteristics sampled included: vegetation type; under-canopy light level; slope; aspect; soil properties; presence of disturbance, trails, and/or water; and total species richness. A model that correctly classified >90% of the 242 plots sampled included two vegetation type parameters, the presence of trails, and total species richness. Yellow toadflax is most often found in areas that were open-canopy sites, along trails, and with higher species diversity plots (>23 species). This approach can be used for other species in other areas to rapidly identify areas vulnerable to invasion.

Spatial prediction of invasion success across heterogeneous landscapes using an individual-based model

The limited resources available for managing invasive plant species in native ecosystems and the magnitude of the problem make it essential that we develop methods to prioritize sites for management efforts. We used the individual-based simulation model ECOTONE in conjunction with climate and soil texture data to identify grassland site types where the invasive perennial forb Acroptilon repens is likely to be successful, and to create a threat map indicating the most vulnerable regions of Colorado. Acroptilon repens has the potential to become most abundant in dry areas with fine-textured soils. This information can be used to direct management efforts towards the areas at greatest risk, allowing the most effective use of limited resources. The most common approach for identifying invasible regions has been to extrapolate from the locations of existing invasions to find similar sites. Two major drawbacks to this method are the lack of consideration of the role of the existing plant community in inhibiting or facilitating invasion, and the assumption that the invading species is at equilibrium with the environment. The combination of an individual-based simulation model and a geographic information system provides a flexible tool to investigate the community and regional dynamics of invasive plant species.

A Potential New Herbicide for Invasive Annual Grass Control on Rangeland

ABSTRACT Downy brome (Bromus tectorum L.), a winter annual grass, is considered one of the most invasive non-native rangeland species in the United States. Although glyphosate, imazapic, and rimsulfuron are herbicides commonly recommended to control invasive, annual grasses, their performance is inconsistent and they can injure desirable perennial grasses. Indaziflam is a recently registered cellulose biosynthesis inhibiting herbicide, providing broad-spectrum control of annual grass and broadleaf weeds. Indaziflam is labeled for winter annual grass control in citrus, grape, and tree nut crops and could represent a new mode of action for selective winter annual grass control on rangeland. Three field experiments were conducted to compare indaziflam with imazapic, rimsulfuron, and glyphosate, three herbicides commonly used for downy brome control. Multiple herbicide application timings were evaluated. At all three sites, glyphosate and rimsulfuron provided less downy brome control than indaziflam 1 year after treatment (YAT). Percent downy brome control with imazapic decreased significantly 2 YAT (45–64%) and 3 YAT (10–32%). Across all sites and application timings, indaziflam provided the greatest downy brome control 2 YAT (89–100%) and 3 YAT (83 – 100%). Indaziflam did not significantly reduce species richness. This study demonstrates that indaziflam can provide extended downy brome control compared with currently used herbicides.

Litter Reduction by Prescribed Burning Can Extend Downy Brome Control

ABSTRACT Downy brome (Bromus tectorum L.) is a highly successful invasive species primarily because it fills an open niche in native plant communities. It also produces large amounts of litter over time. We hypothesized that removing accumulated litter with a prescribed burn before applying herbicides would improve herbicide efficacy, extending the duration of control. In January 2012, two downy brome-infested sites were burned. In March 2012, postemergent applications of glyphosate, imazapic, and tebuthiuron were made in a split-plot design. Above-ground biomass was collected at 6, 18, and 27 months after treatment (MAT) to evaluate treatment effects. In nonburned areas, all herbicide treatments were similar to the control 27 MAT; however, burning combined with imazapic or tebuthiuron reduced downy brome biomass 27 MAT by 81% ± 4.6 SE and 84% ± 19.3 SE, respectively. Remnant species responded positively to burning and herbicide treatments. Native cool-season grass biomass increased after burning whereas native warm-season grass biomass increased following tebuthiuron treatments. The impact of litter on imazapic and tebuthiuron availability was also evaluated. Herbicide interception increased in a linear relationship with increasing litter. For every 50 g · m-2 increase in litter there was a 7% increase in the amount of herbicide intercepted, meaning that 75% of the applied herbicide was intercepted by 360 g · m-2 of litter. A simulated rainfall event of 5 mm, 7 days after application, removed a significant amount of herbicide. This indicates that in sites with surface litter, timely precipitation could be critical for herbicide efficacy; however, when burning was used to remove litter and was followed by herbicides with residual soil activity, downy brome control was extended. Due to downy bromes relatively short seed viability in the soil, extending herbicide efficacy to several years could help to reduce the soil seed bank.

The Weed Tunnel: Building an Experimental Wind Tunnel

Dispersal is a key component of plant population and community dynamics and the spread of weeds. Although many species of economic concern disperse via tumbleweed mechanisms, our ability to estimate relevant dispersal parameters can be hindered by the lack of a controlled environment that can be provided by a wind tunnel. Established wind tunnels are typically closed-circuit, clean systems and are therefore unsuitable for biological or ecological research. We designed and constructed a wind tunnel to estimate dispersal parameters for diffuse knapweed. Our design was a tunnel that utilizes the Venturi effect to obtain maximum flow velocity while pulling, rather than pushing, air through the test section. Flow velocity was continuously variable from 0 to 8 m/s, and the tunnel was equipped with instrumentation for measuring the force exerted on plants by wind. Our modular design provided a way to effectively estimate key parameters that govern the dispersal of tumbleweeds, and was readily constructed and stored in research facilities. Nomenclature: Diffuse knapweed, Centaurea diffusa (Lam.) CENDI

Seed Bank Depletion: The Key to Long-Term Downy Brome (Bromus tectorum L.) Management

ABSTRACT Invasive winter annual grasses such as downy brome (Bromus tectorum L.) are a threat to native ecosystems throughout the United States. Downy brome is able to exploit moisture and nutrients throughout the fall and early spring before native plants break dormancy. This results in decreased native species abundance and development of monotypic downy brome stands. Short-termdowny brome management has been shown to be effective; however, the soil seed reserve has often been overlooked, although its the mechanismresponsible for rapid reestablishment. This field study was conducted at two sites in Colorado to evaluate the longevity of the downy brome soil seed reserve and its implications on long-termdowny brome control. Glyphosate plus adjuvant applications were made for 0, 1, 2, 3, 4, or 5 consecutive years. Downy brome and perennial grass biomass harvests were conducted yearly to determine changes in species composition. In addition, soil cores were collected to evaluate the yearly variation and depletion of the downy brome soil seed bank in response to consecutive glyphosate applications. We found that 1–3 yr of consecutive glyphosate treatments were insufficient to deplete the downy bromesoil seed bank. Downy brome biomass and the soil seed bank recoveredwithin 1–2 yr after glyphosate treatmentswere terminated; however, 4 and 5 consecutive yr of glyphosate applications were sufficient to control downy brome through depletion of the soil seed bank. Managing downy brome for 4–5 consecutive yr resulted in a 4- to 9-fold increase in perennial grass biomass. These data suggest that long-term management of downy brome is dependent on eliminating the soil seed bank using a multiyear approach.

Preemergence Control of Nine Invasive Weeds with Aminocyclopyrachlor, Aminopyralid, and Indaziflam

There are an estimated 400 million hectares of non-cropland in the United States primarily designated as rangeland and pastureland, and there are more than 300 invasive weeds found on these sites, causing an estimated annual loss of