Cover crops are important in organic arable cropping systems because they improve soil fertility and suppress annual weeds in the post-harvest period (the period between cash crops), but their effectiveness against perennial weeds is less clear. This study analyzed outbreaks of perennial weeds in two long-term organic crop rotations with and without cover crops in the period 2011–2022 in Denmark. In addition, the impact of including forage plantain (Plantago lanceolata) in cover crop mixtures on the growth of perennial weeds was studied from 2020 to 2022. P. lanceolata is known to contain secondary metabolites that inhibit nitrification and may influence nitrogen (N) availability. This study found that cover crops did not hinder outbreaks of Cirsium arvense, Sonchus arvensis, Elytrigia repens or other perennial weed species. E. repens actually proliferated more with cover crops whose presence prevented repeated mechanical post-harvest interventions. The crop rotation involving regular cutting of a one-year or two-year green manure crop provided the best management of perennial weeds if repeated post-harvest soil cultivation was also conducted. This could only be done in the absence of cover crops. Mixing P. lanceolata into the cover crop mixtures did not change the outcome of competition between cover crops and perennial weeds in the period between crop harvest in August and November. Instead, P. lanceolata significantly reduced the number of shoots emerging from perennial weeds in the subsequent growing season. The causality of this effect was not further elaborated in the study.

Three wash techniques, each with 1, 10, or 95% (v/v) ethanol:water were used to measure foliar absorption of 14C-glyphosate [N-(phosphonomethyl)glycine], 14C-3,6-dichloropicolinic acid, and 14C-chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in Tartary buckwheat [Fagopyrum tataricum (L.) Gaertn. ♯3 FAGTA], Canada thistle [Cirsium arvense (L.) Scop. ♯ CIRAR], and barley (Hordeum vulgare L. ‘Galt’). For the herbicides and species tested, the most suitable common procedure for determining absorption consisted of a double or triple rinse with or immersion in 10% ethanol. Wiping the treated leaves with cotton balls moistened with the solvent was much less effective. Efficiency of herbicide removal by a given solvent was not related consistently to solubility of the herbicide in the solvent.

Phytotoxicity of chlorsulfuron {2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino] carbonyl] benzenesulfonamide} in cell cultures of Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] was approximately 30-fold greater than in leafy spurge (Euphorbia esula L. # EPHES) cultures. Differences in chlorsulfuron phytotoxicity in these two species were attributed to large differences in cellular metabolism of the herbicide. Leafy spurge cells metabolized all of the applied 14C-chlorsulfuron within 72 h of treatment, while Canada thistle metabolized less than 2%. Acid hydrolysis of the metabolic products isolated from leaf disks and cell cultures of leafy spurge yielded a radioactive product that cochromatographed with a 2-chlorobenzenesulfonamide standard. Therefore, metabolic transformation occurred on the heterocyclic portion of the molecule. A major metabolite was further characterized as 2-chloro-N-[[4-(hydroxymethyl)-6-methoxy-1,3,5-triazin-2-yl]aminocarbonyl]-benzenesulfonamide by cochromatography with the authentic standard.

The freezing resistance of overwintering buds of four perennial weed species was evaluated in artificial freezing experiments conducted in the winters of 1986 and 1987. Survival, dry weight of surviving shoots, and number of shoots produced/surviving section were determined at −4, −8, −12, −16, and −20 C for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes. The temperature required to reduce survival (LT50) and total dry weight (GR50) by 50% was determined. The response to freezing temperatures varied among species and in some cases between years. The LT50 was −7, −13, colder than −20, −17, and colder than −20, and the GR50 was −5, −11, −14, −15, and −13 for Canada thistle roots, leafy spurge roots, leafy spurge crowns, perennial sowthistle roots, and quackgrass rhizomes, respectively. The relative susceptibility to freezing among species appears to be directly related to the depth at which the overwintering buds generally are located.

Clods of three sizes (1.5-, 3.3-, and 5.3-cm diameter) and two hardnesses [soft (0.3 MPa) and hard (2.2 MPa)] were made by incorporating seed from one of the five following weed species: corn poppy (Papaver rhoeas L. # PAPRH), shepherdspurse [Capsella bursa-pastoris (L.) Medic. # CAPBP], smooth crabgrass Digitaria ischaemum (Schreb.) Muehlenb. # DIGIS], common lambsquarters (Chenopodium album L. # CHEAL), and Canada thistle [Cirsium arvense (L.) Scop. # CIRAR], listed by increasing seed size. Under conditions of this experiment, higher germination occurred with larger seed. The higher germination was connected with fewer dormant seed remaining in the clods. Germination significantly decreased (13%) in the larger soft and hard clods with the smaller seed sizes (less germination associated with more dormant seed remaining in the clod). Germination of the two larger seed decreased (23%) only in the larger soft clods but was less significant with both small seed. Non-emerged seedlings increased with larger clods and was most significant with the three large seed in the hard clods and absent with the three large seed in the soft ones. On the average, 70% of the germinated seed emerged. Differences in germination as well as in nonemerged seedlings caused decreased emergence on larger clods. Clod influence in germination and emergence was explained by the volumes of the clod layers involved, and in some comparable aspects corresponded with volumes (depths) of soil layers.

A root washer was constructed which rapidly separated Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] roots larger than 1.3-mm diam with 96% or greater efficiency from cohesive clay soil. The washer removed 93 ± 15% of 43 200 cm clay soil from Canada thistle roots in 2 h. Small residual soil clods and organic debris were removed by hand after washing. The washing action did not damage the root system. From 4 to 6 h were required to wash the large Canada thistle roots from an equivalent volume of clay soil by hand, without using the washer. Rhizomes, seeds, and tubers also may be separated from soil with the washer. The tank and frame of this washer are durable, and moving parts such as bearings can be replaced easily.

Studies were conducted in 1977 and 1978 to evaluate the impact of surface irrigation water on the dissemination of weed seed in the North Platte River Project from May through September each year. Seventy-seven different plant species were found in irrigation water, with germination averaging 26%. Two to five times more weed seed were found in two irrigation canals than in the North Platte River. As water moved through the canals, the weed seed content of the water increased dramatically. The majority of seed collected was found floating on the water surface. Amounts of seed collected varied with time with the largest amounts being collected in June and July and gradually declining in August and September. Samples also were taken to determine the amount of seed entering the field with irrigation water during the growing season. During the 1978 growing season irrigation water disseminated 48,400 seed per ha in the sampled fields.

The response of yellow nutsedge (Cyperus esculentus L. #3 CYPES) and Canada thistle [Cirsium arvense (L.) Scop. # CIRAR] to combinations of low rates of fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} and bentazon [3-(1-methylethyl)-(1H)-2,l,3-benzothiadiazin-4(3H)-one 2,2-dioxide] was determined in a controlled environment chamber and on Canada thistle in the field. In the growth chamber, injury to yellow nutsedge was increased with 24 g ai/ha fluridone combined with 323 g ai/ha bentazon. Either 2.7 or 5.4 g/ha fluridone combined with 27 g/ha bentazon caused uniform injury to Canada thistle. In the field, the combinations of 5.6 g/ha fluridone with either 28 or 2800 g/ha bentazon was phytotoxic to Canada thistle.

Invasive species such as Canada thistle pose a significant threat to ecosystems. The risk of introducing invasive species has increased with human activities, and the effects of such events have economic and aesthetic impacts. Native to Europe, Canada thistle is now established throughout temperate North America. Although there is documentation of early occurrences to North America, little is known on how it has become established in diverse habitats or how it continues to spread. We examined genetic diversity within and among nearly 1,700 Canada thistle individuals from 85 North American locations with the use of seven microsatellite markers in order to address these questions. PAUP and STRUCTURE programs were used to assess genetic diversity and relationships within and between populations. Populations exhibited greater within-population diversity (> 60%) than expected for a reported clonally reproducing species. Total diversity of sampled locations in North America (0.183) was less than previously reported for European locations (0.715), but the greater mean difference between North American populations (0.264 relative to 0.246 from England) suggests strong founder effects or restriction of gene flow influencing individual populations. Furthermore, analyses identified numerous instances where individuals from geographically distant regions clustered together, indicating long-distance translocation of propogules. However, isolation by distance analysis showed significant correlation between location and population genetic distances (r = 0.1917, P = 0.006). Within populations, nearly 92% of individuals sampled harbored unique multilocus genotypes, strongly suggesting that sexual reproduction is common. Within populations, analysis of genetic structure indicated significant admixture of genotypes throughout the invasive range in North America. The recurrent distribution of seed throughout North America has led to a highly diverse gene pool and increased the adaptive success Canada thistle to a wide variety of habitats. Future technologies developed for control of Canada thistle should consider this diversity.

Preharvest applications of glyphosate can be useful in controlling perennial weeds. Experiments were conducted from 1996 to 1999 to determine whether preharvest glyphosate applications are affected by differences in the amount of soybean canopy present at the time of application by measuring spray deposition and subsequently horsenettle or Canada thistle control. Soybean leaf interference levels were achieved by use of three soybean cultivars with different maturity groups to achieve no leaf interference, moderate leaf interference, and maximum leaf interference, and soybean was planted in three row spacings ranging from 19 to 76 cm. As soybean leaf interference increased, spray coverage of spray deposition cards decreased. There was a similar trend for relative spray volume, determined by intensity of the color change with water-sensitive cards. Row spacing did not influence spray coverage or relative spray volume. Percent change in horsenettle or Canada thistle stems from fall to spring counts was inconsistent. Differences detected in spray coverage did not influence weed control or weed stem density the following spring.

We assessed the ability of climatic, environmental, and anthropogenic variables to predict areas of high-risk for plant invasion and consider the relative importance and contribution of these predictor variables by considering two spatial scales in a region of rapidly changing climate. We created predictive distribution models, using Maxent, for three highly invasive plant species (Canada thistle, white sweetclover, and reed canarygrass) in Alaska at both a regional scale and a local scale. Regional scale models encompassed southern coastal Alaska and were developed from topographic and climatic data at a 2 km (1.2 mi) spatial resolution. Models were applied to future climate (2030). Local scale models were spatially nested within the regional area; these models incorporated physiographic and anthropogenic variables at a 30 m (98.4 ft) resolution. Regional and local models performed well (AUC values > 0.7), with the exception of one species at each spatial scale. Regional models predict an increase in area of suitable habitat for all species by 2030 with a general shift to higher elevation areas; however, the distribution of each species was driven by different climate and topographical variables. In contrast local models indicate that distance to right-of-ways and elevation are associated with habitat suitability for all three species at this spatial level. Combining results from regional models, capturing long-term distribution, and local models, capturing near-term establishment and distribution, offers a new and effective tool for highlighting at-risk areas and provides insight on how variables acting at different scales contribute to suitability predictions. The combinations also provides easy comparison, highlighting agreement between the two scales, where long-term distribution factors predict suitability while near-term do not and vice versa.

Many hypotheses have been developed to explain the adaptive nature of insect galls. One of these, the nutrition hypothesis, states that gall formers have advantages over other insects because gall tissue provides a better (higher quality) food source than unmodified tissue. However, this has rarely been experimentally tested. In a test of this hypothesis, we grew plants of Cirsium arvense (L.) Scop. in a factorial design with two main treatments: the addition of nitrogen (to enhance foliar N levels) and of fungicide (to reduce colonization of roots by arbuscular mycorrhizal fungi). Mycorrhizal fungi have been shown previously to reduce the N concentration of host plants. Plants were exposed to adult gall flies, Urophora cardui L., and maintained through one season to allow maturation of galls.

Reduction of the percentage mycorrhizal colonization by fungicide resulted in an elevation of total stem N comparable to that achieved by N addition, but gall N concentration remained unchanged in all treatments. Nitrogen application elevated stem N levels when mycorrhizal fungi were present, but application of both compounds together did not result in any increase over either single treatment. Fungicide application resulted in larger galls, which contained more larval chambers, with more live, and heavier, larvae. However, the main effects of N were not significant, as N addition only increased fly performance on plants where mycorrhizas were not reduced.

It is suggested that U. cardui gall inhabitants can manipulate N at an optimal level and thus might conform to a modified version of the nutrition hypothesis. Mycorrhizal colonization might reduce gall fly performance by delaying the appearance, or impairing the quality, of secondary nutritive tissue in the gall. Future tests of the nutrition hypothesis should include a consideration of the plant's mycorrhizal status.