To determine the extent and nature of container-grown plant soil as a pathway for introduction of exotic plant species to Alaska, soil from container-grown ornamentals was obtained from vendors and was incubated in the greenhouse. Fifty-four plant species were identified growing in containers or germinating from the soil, and included Canada thistle—a prohibited weed in Alaska—and nine other species listed as invasive in Alaska. The number of species and estimated seed bank were very low for soil from vegetable starts/herbs and herbaceous bedding plants (< 2 seedlings/L soil), but was greater for soil from containers containing woody plants, especially balled and burlapped ornamentals (20 seedlings/L soil). Container alien plant seed bank size was strongly related to type of soil. Potting (soil-less) soil contained 1.2 germinating seeds/L, soil-based soil 5.5 seeds/L, and mineral soil 18.7 seeds/L. Growers and vendors were variables that also influenced the size of the container seed bank, suggesting that weed management practiced during production and at the point of sale can greatly influence seed banks of ornamental containers.

Growth chamber studies were conducted using 55-L aquariums to evaluate the efficacy of the herbicide triclopyr and the fungal pathogen Mycoleptodiscus terrestris, applied alone and in combination against Eurasian watermilfoil. Treatments included 0.15, 0.40, and 1.50 mg acid equivalent (ae)/L triclopyr, 0.08, 0.16, and 0.32 ml/L M. terrestris, combinations of both agents at all rates, and an untreated control. Plants were exposed to all treatments for a 24-h contact time and plant biomass (shoot and roots) was recorded 6 wk after application. For both triclopyr and M. terrestris applied alone, plant control increased with treatment concentration. Compared with untreated plants, 1.50 mg/L triclopyr and 0.32 ml/L M. terrestris reduced Eurasian watermilfoil by 100 and 91%, respectively. Lower doses of herbicide or pathogen were less effective and plant recovery was observed from surviving plant tissues (stems and root crowns). Although M. terrestris at 0.08 ml/L did not significantly reduce shoot or root biomass and 0.15 mg/L triclopyr provided only 53% control of plants, combining both agents at these rates reduced Eurasian watermilfoil by 90%. Results demonstrated that integrating low doses of triclopyr with an indigenous pathogen, M. terrestris, can improve control of Eurasian watermilfoil. Lower use rates of triclopyr would minimize impacts to sensitive nontarget vegetation, reduce application costs, and may minimize impacts of label-imposed use restrictions.

Prescribed grazing is an effective tool for controlling some rangeland weeds. Forage quality of eight nonnative rangeland weeds common to northern Idaho was determined. Five collection sites were established for each weed species: rush skeletonweed, meadow hawkweed, houndstongue, sulfur cinquefoil, yellow starthistle, Dalmatian toadflax, hoary cress, and tansy ragwort. Plants were collected at rosette, bolt, flower, and seed set stages; dried and separated into leaves, stems, flowers, or seed; and analyzed separately for crude protein (CP), neutral detergent fiber (NDF), and mineral content (ash). As expected, plants became more fibrous as they matured because stems increased in NDF value throughout the season and the leaf : stem ratio of most weeds we examined decreased throughout the season. In general, the weeds we examined expressed only moderate fiber levels, beginning the season with NDF values near 34% in the rosette stage and becoming near 52% NDF in the seed set stage. CP values were near 25% in the rosette stage for houndstongue, rush skeletonweed, and hoary cress whereas other weeds we examined had about 15% CP in the rosette. As the season advanced, all plants lost protein content and ended the season with CP values from 5 to 8% at seed set. Ash values declined for all species as the season progressed. Our results indicate that these weeds have forage values similar to many native plants and could be reasonable forage resources for livestock. We did not examine the content of secondary compounds in these weeds that may render them unpalatable and limit their forage value.

Field and garden experiments were performed to investigate if pathogens, generalist herbivores, or a specialist herbivore (the tortricid moth Pristerognatha fuligana) have the potential to control the invasive Himalayan balsam. Impacts of generalist herbivores and pathogenic fungi were excluded by using thiacloprid and trifloxystrobin. Results show no effect of any antagonist in the field experiment, though we found a modest impact of the combined influence of generalist herbivores and pathogenic fungi in the garden experiment, i.e., under suboptimal growing conditions. Limited information from the native area (the Himalayas) suggests a strong impact of specialist herbivores, thus confirming the enemy release hypothesis. We predict that impact studies of native specialized herbivores will clearly indicate antagonists from the native area with a strong impact on Himalayan balsam in its invaded area.

Riparian habitats are important components of an ecosystem; however, their hydrology combined with anthropogenic effects facilitates the establishment and spread of invasive plant species. We used a maximum-entropy predictive habitat model, MAXENT, to predict the distributions of five invasive plant species (Canada thistle, musk thistle, Russian olive, phragmites, and saltcedar) along the North Platte River in Nebraska. Projections for each species were highly accurate. Elevation and distance from river were most important variables for each species. Saltcedar and phragmites appear to have restricted distributions in the study area, whereas Russian olive and thistle species were broadly distributed. Results from this study hold promise for the development of proactive management approaches to identify and control areas of high abundance and prevent further spread of invasive plants along the North Platte River.

Within highway rights-of-way, native perennial grasses provide desirable services to support natural and human constructed ecosystems. However, native perennial grass establishment in annual grass dominated roadsides of semiarid and Mediterranean climates of the western United States requires specific cultural and chemical management treatments to control weeds. In 2004, field studies were conducted in Sacramento Valley, California to determine the effect of herbicide, disc cultivation, and species selection on native perennial grass establishment and annual weed persistence. Perennial grass species mixes common to drier and wetter upland areas in northern California were drill seeded at two sites (I-5 North and I-5 South) that had been burned in 2003 and received weed control (i.e., herbicide, cultivation, mowing) in spring 2004. Herbicides were the most important treatments for native perennial grass establishment and weed reduction. Native perennial grass species persistence was largely unaffected by cultivation or native plant accessions at these sites. Native perennial grass density increased at I-5 North in the second year of growth (2006) resulting in a plant density totaled across all herbicide regimes of 3.9 plants m−1 compared to 2.5 plants m−1 at I-5 South. Vigorous native perennial grass growth in the more fertile and less droughty soils of I-5 North helped to limit annual weeds through competition, which is anticipated to reduce the need for chemical and mechanical control in years following early establishment.

Medusahead is one of the most problematic rangeland weeds in the western United States. In previous studies, prescribed burning has been used successfully to control medusahead in some situations, but burning has failed in other circumstances. In this study, trials were conducted using the same protocol at four locations in central to northern California to evaluate plant community response to two consecutive years of summer burning and to determine the conditions resulting in successful medusahead control. During 2002 through 2003 large-scale experiments were established at two low-elevation, warm-winter sites (Fresno and Yolo counties) and two higher elevation, cool-winter sites (Siskiyou and Modoc counties). Plant species cover was estimated using point-intercept transects, and biomass samples were taken in each plot. After 2 yr of burning, medusahead cover was reduced by 99, 96, and 93% for Fresno, Yolo, and Siskiyou counties, respectively, compared to unburned control plots. Other annual grasses were also reduced, but less severely, and broadleaf species increased at all three sites. In contrast, 2 yr of burning resulted in a 55% increase in medusahead at the coolest winter site in Modoc County. In the second season after the final burn, medusahead cover remained low in burned plots at Fresno and Yolo counties (1 and 12% of cover in unburned controls, respectively), but at the Siskiyou site medusahead recovered to 45% relative to untreated controls. The success of prescribed burning was correlated with biomass of annual grasses, excluding medusahead, preceding a burn treatment. It is hypothesized that greater production of combustible forage resulted in increased fire intensity and greater seed mortality in exposed inflorescences. These results demonstrate that burning can be an effective control strategy for medusahead in low elevation, warm-winter areas characterized by high annual grass biomass production, but may not be successful in semiarid cool winter areas.

Russian knapweed is an invasive weed of rangeland, pasture, and natural areas throughout western North America. Aminopyralid is a new pyridine carboxylic acid herbicide that has activity on Russian knapweed at lower use rates than current standard treatments. The objectives of this study were to compare aminopyralid efficacy at the bud to early flower timing and the fall timing with commercial standards for Russian knapweed control. Studies were conducted at five locations in California, Utah, and Wyoming in heavily infested pastures or rangeland. When applied in summer at the bud to early flower timing, aminopyralid at 0.07 kg ae/ha controlled Russian knapweed effectively and was comparable to picloram + 2,4-D amine (0.56 + 1.12 kg ae/ha) at 12 and 24 months after treatment (MAT). The addition of 2,4-D with lower rates of aminopyralid did not improve control. When applied after seed set at the fall timing, control from aminopyralid at 0.05 kg ae/ha and higher was also comparable to picloram (0.56 kg ae/ha) and better than clopyralid (0.42 kg ae/ha) and imazapic (0.18 and 0.21 kg ae/ha) 12 and 21 MAT. Aminopyralid controlled Russian knapweed effectively at lower use rates than current commercial standards and good control lasted for at least 21 to 24 MAT.

It is essential that we improve our ability to predict which nonnative species will become invasive in order to prevent their introduction and spread. Past attempts to foresee invasions have met with limited success, but increased computing power, increased availability of information about exotic species, and comprehensive evaluations of invasion potential are improving our ability to predict which species are likely to invade most successfully. We used data from Colorado and other states to develop an effective means of predicting the spread of invasive plant species among states. Qualitative criteria were used to develop a numerical threat index, which rates potential invaders based on distribution and abundance with respect to climate, biological characteristics, and preferred habitats of the species. Out of a compiled list of 388 species, we identified six invasive nonnative plants that are highly likely to invade Colorado, 10 with medium invasive potential and five with low potential. Species found to be likely to invade Colorado included garlic mustard, smooth distaff thistle, and Syrian beancaper.

Invasive plants are a common problem in the management and restoration of degraded lands in the semiarid western United States, but are often not the primary focus of restoration ecologists. Likewise, restoring native vegetation has not been a major concern of weed scientists. But trends in the literature demonstrate increasing overlap of these fields, and greater collaboration between them can lead to improved efficacy of restoration efforts. Succession and ecosystem development are the products of complex interactions of abiotic and biotic factors. Our greatest restoration and invasive plant management successes should result when we take advantage of these natural processes. Recent shifts in management objectives have generated approaches to directing plant community development that utilize species that are strong competitors with invasive species as a bridge to the establishment of native perennial vegetation. Soil water and nutrient characteristics and their interactions can affect desired and undesired plant species differentially and may be manipulated to favor establishment and persistence of desired perennial plant communities. Selection of appropriate plant materials is also essential. Species assemblages that suppress or exclude invaders and competitive plant materials that are well adapted to restoration site conditions are important keys to success. We provide guidelines for restoration based on the fundamental ecological principles underlying succession. Knowledge of the complex interactions among the biotic and abiotic factors that affect successional processes and ecosystem development, and increased collaboration between weed scientists and restoration ecologists hold promise for improving restoration success and invasive species management.

Preamble: Executive Order (EO) 13112—defines an invasive species as “an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health.” In the Executive Summary of the National Invasive Species Management Plan (NISMP) the term invasive species is further clarified and defined as “a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health.” To provide guidance for the development and implementation of the NISMP, the National Invasive Species Council (NISC) and the Invasive Species Advisory Committee (ISAC) adopted a set of principles outlined in Appendix 6 of the NISMP. Guiding Principle #1 provides additional context for defining the term invasive species and states “many alien species are non-invasive and support human livelihoods or a preferred quality of life.”

Jamaican nightshade is a prickly, perennial, invasive shrub in central and southern peninsular Florida. It was first seen in Florida in 1930 near St. Cloud, and has since been reported at several other locations in the state. Jamaican nightshade is primarily found in wooded habitats, where it can quickly become dominant in the understory, but it also occasionally grows in isolated patches in the open. Although the distribution of Jamaican nightshade does not appear to be rapidly expanding in Florida, land mangers should be made aware of the potential of this weed to establish at new sites, and initiate control efforts during the early stages of colonization at newly invaded sites.