Little mallow is becoming an increasing problem in no-till farming systems in southern Australia. There is little known about the germination ecology of this species, making management of the weed more difficult. Experiments were conducted to determine the effect of different factors on seed germination. Freshly harvested seeds had high levels of innate dormancy with only 5% of seeds germinating. Germination increased with storage time but did not reach more than 47% even 13 mo after seed harvest. Germination was not influenced by light. Germination was stimulated by scarification, which indicates inhibition of germination in this species is mainly due to the seed coat. Germination was moderately sensitive to salt and osmotic stress. Germination was completely inhibited at osmotic potentials of −0.6 to −1 MPa. Seeds of little mallow germinated over a broad range of pH between 4 and 10, but germination declined at higher pH. Potassium nitrate at 0.005 M stimulated seed germination of scarified seeds to a maximum level of 76%. Seedling emergence of little mallow was greatest at burial depths of 0.5 to 2 cm. No emergence occurred at 8 cm or deeper. Greater emergence in response to shallow burial and the requirement for scarification may both contribute to the increasing problem of small mallow in no-till farming systems in southern Australia by delaying emergence until after the crop has been sown.

Age distributions of sulfur cinquefoil populations were determined on sites that were historically grazed, cultivated, and mechanically disturbed. From 12 sites, a total of 279 reproductively active plants were collected and aged by using herbchronology (counting rings in the secondary root xylem of the root crown) to (1) estimate the age structure of the populations, (2) relate plant size and flower production to plant age, and (3) examine the relation of population age structure to environmental variables and disturbance history. Results indicated that the mean age for all sampled plants was 3.5 (± 1.74 SD) yr and ranged from 1 to 10 yr. Age was not related to number of flowers, plant size (number of stems per plant or plant height), or site disturbance type but was positively correlated with site elevation (P < 0.001). The pooled age distribution from all 12 sites was right-skewed with fewer old plants than young plants. We conclude that sulfur cinquefoil plants sampled in northeast Oregon are able to colonize, establish, and reproduce at disturbed sites rapidly. We suggest that herbchronology may be a useful technique to improve understanding of invasion biology and ecology for invasive plant species that form annual rings.

Pollen movement between individuals can increase the rate of herbicide resistance evolution by spreading resistance alleles within or between populations and by facilitating the rapid accumulation of resistance alleles within individuals. This study investigated the level of pollen-mediated gene flow between paraquat-resistant and paraquat-susceptible populations of the self-pollinated weed species hare barley. The experiment was conducted in both directions, from resistant to susceptible and susceptible to resistant, across 2 yr. To maximize the potential for pollen flow, individual plants were grown in a single pot. The level of gene flow was similar across years and between genotypes. The level of pollen-mediated gene flow ranged from 0.06 to 0.15%. Gene flow from resistant to susceptible plants was confirmed by demonstrating segregation for resistance in the progeny of suspected crosses. This study suggests that pollen-mediated gene flow will occur in this species at frequencies less than 0.16% and could assist the accumulation of resistance alleles within a population. These low levels of gene flow through pollen movement suggest that cross-pollination over larger distances would be unlikely and pollen movement probably does not contribute to gene flow between populations.

Field and greenhouse studies were conducted in 2002 and 2003 to evaluate mechanically stimulated compensatory growth response of ivyleaf morningglory, common waterhemp, and giant ragweed. Compensatory growth was initiated by the physical removal of the apical shoot to break apical dominance. The amount of apical shoot removed had an effect on mechanically stimulated compensatory growth of common waterhemp and giant ragweed. With these species, the more of the apical shoot removed from the plant, the less compensatory growth occurred. Removal of the shoot from above the cotyledons resulted in giant ragweed that were 48% shorter and weighed 41% less than control plants at 6 wk after shoot removal. However, the amount of apical shoot removed had no effect on the growth of ivyleaf morningglory compared with control plants at the completion of the study. The influence of plant height at the time of shoot removal on compensatory growth was specific to each weed species. Ivyleaf morningglory exhibited less compensatory growth when the plants were 10 cm at the time of shoot removal compared with 20-, 30-, and 40-cm-tall plants. Removal of the shoot when common waterhemp plants were 30 or 40 cm in height reduced plant weight by 23 and 21%, respectively, compared with control plants. However, no reductions in plant weight were observed when common waterhemp were 10 or 20 cm tall at the time of shoot removal. Giant ragweed subjected to shoot removal was smaller in most growth parameters than control plants, regardless of plant height at the time of shoot removal.

Threehorn bedstraw is an important dicotyledonous weed in southern Australia that is particularly difficult to control in pulse crops. Knowledge of the germination ecology of this weed would facilitate development of effective weed-control programs. Experiments were conducted to study the germination of two populations, Roseworthy Campus (RC) and Yorke Peninsula (YP), of threehorn bedstraw from South Australia. In the absence of chilling, seeds germinated only in the darkness. Germination was considerably higher under an alternating day/night temperature range of 13/7 C compared with 20/12 or 25/15 C day/night temperature. Germination was inhibited by light; however, when seeds were subsequently transferred to complete darkness they germinated readily. Potassium nitrate (0.005 M KNO3) and gibberellic acid (0.001 M GA3) stimulated germination in the darkness in both populations. This concentration of KNO3 increased germination of the RC and YP populations from 26 and 37% to 56 and 68%, respectively; however, higher concentrations of KNO3 inhibited germination. GA3 added in combination with KNO3 further increased germination to 81 and 94%, respectively. Germination was also promoted by cold-stratification treatment (5 C). Complete germination (100%) was achieved within 4 wk of cold stratification, when seeds were incubated in sand. In the field, seedling recruitment of both populations was higher under minimum tillage (25 to 27%) than no-till (15 to 18%) conditions, reflecting greater exposure of seeds to light under no-till systems.

The historic maternal environment; the identity of competing, neighboring plants; and biotic stresses, such as fungal disease and herbivory, interact to influence the competitive ability, reproductive output, and plasticity of weed populations. A weed capable of altering its phenotype in response to environmental factors is better able to compete for resources in agroecosystems, thereby reducing crop yields and contributing more seeds to the seed bank. Velvetleaf is a highly competitive annual weed in many North American cropping systems, exhibiting a high degree of phenotypic plasticity in response to biotic and abiotic factors. We examined the effects of seed size (small and large), competitive environment (with and without soybean), and stress level (none = no treatment, moderate = pathogen inoculation, high = pathogen plus herbicide) on velvetleaf allometry and reproductive output during three field seasons. Only under the high-stress treatment was velvetleaf biomass, height, and reproductive output compromised (> 15% each), and these traits were further reduced with soybean competition. Soybean competition alone reduced velvetleaf biomass (> 50%), height (25%), and seed output (50%), but effects of competition varied with stress level. Velvetleaf plants that originated from large seeds (≥ 10 mg) generally outperformed plants originating from small seeds (< 10 mg) except under the high-stress treatment. These findings suggest that velvetleaf life-history traits and reproductive output in the current generation are a function of both present and historic factors. Management strategies for this species should involve tactics that not only reduce its competitive ability (e.g., biomass, height) but also decrease its seed production via synergistic stressors such as increased crop competitiveness, reduced-rate herbicide applications, and pathogen infection.

Jointed goatgrass is a troublesome weed in winter wheat in the Pacific Northwest of the United States. Wheat and jointed goatgrass (JGG) can cross and produce hybrids in the field that can serve as a potential bridge for gene migration between the two species. To determine the potential for gene movement it is important to be able to identify hybrids in the field. To study the effect of wheat genotype on hybrid phenotype, reciprocal crosses were made between JGG and two common wheat cultivars: ‘Brundage 96’, ‘Hubbard’, a common-type advanced breeding line: ‘87–52814A’, and a club wheat cultivar: ‘Rhode’. Hybrids and parents were measured for plant height, spike length, flag leaf length, flag leaf width, and number of spikelets. Reciprocal effects were nonsignificant for all characteristics measured, indicating that hybrid morphology was not affected by the direction of the cross. Hybrids were different from their wheat parents for spike length, plant height, and flag leaf width. Hybrids produced from each of the wheat parents were uniform in phenotypic characters. Spikes were intermediate in circumference (size) from crosses between JGG and common wheat lines; however, club wheat × JGG crosses resulted in spikes that were more similar to common wheat. Spike size and flag leaf width for all hybrids also were intermediate between their parents. Hybrids differed in spike size and awn characteristics because of unique characteristics of the wheat parent. Based on these results, it should be possible to identify hybrids in the field accurately, regardless of the wheat parent or direction of the cross unless the parent is a club wheat.

Weed management systems in carrot are limited in part by a lack of fundamental understanding of crop–weed interactions. Irrigated field studies were conducted to quantify the effect of volunteer potato density and duration of interference on carrot yield and to determine relationships among weed density, duration of weed growth, and volunteer potato tuber production. A season-long volunteer potato density of 0.06 plants m−2 produced from 150 to 230 g tubers m−2 and resulted in an estimated 5% crop yield loss. At two volunteer potato plants m−2, the same level of crop loss was estimated with a duration of interference of 430 growing degree days (GDD), a time at which the weed had already produced 130 g tubers m−2. Volunteer potato height at the time of weed removal predicted carrot yield loss (R 2 = 0.77) and may be useful for timing of management strategies such as hand weeding. Functional relationships describing carrot–volunteer potato interactions provide simple information that is useful for developing weed management recommendations for carrot, a crop that relies on multiple tactics for managing weeds, and rotational crops that are negatively affected by persistence of volunteer potato.

Germination of African mustard seeds collected from southern Australia was not influenced by light conditions at the optimum temperature of 20/12 C. However, seed germination was inhibited by light at the lower temperature (15/9 C). Presence of light increased the sensitivity of seeds to low temperature, as well as salt and osmotic stress. In dark conditions, seed germination was relatively unaffected at a low level of salinity (80 mM NaCl) but decreased even at 10 mM NaCl in light/dark conditions. In the dark, seed germination was unaffected up to an osmotic potential of −0.6 MPa but declined thereafter. Seeds of African mustard germinated over a broad range of pH from 4 to 10. Seed germination was stimulated by potassium nitrate (from 0.005 to 0.04 M) and gibberellic acid (0.001 M). Seedling emergence of African mustard was the greatest (51%) for seeds buried at 1 cm but no seedlings emerged from seeds placed at a depth of 5 cm. At the end of the growing season, seed decay (77 to 87%) and dormant (12 to 18%) components were similar among different seed burial depths. Information gained in this study will be important in developing a better understanding of the requirements for African mustard germination and emergence.

Perennial pepperweed is an aggressive, exotic weed invading wetland and riparian areas in California, including the San Francisco Bay/Sacramento–San Joaquin Delta Estuary. Effective management will require detailed and accurate maps of its distribution. Remote sensing technologies offer the capability to map weed species over broad areas and with rapid return intervals. As a first step in assessing the potential to map perennial pepperweed with hyperspectral remote sensing data, this study determined its spectral uniqueness relative to co-occurring species. Spectral measurements were conducted during summer drought conditions in the Sacramento–San Joaquin Delta region. Reflectance spectra of perennial pepperweed and seven co-occurring species were collected with a portable spectrometer. Nineteen physiological indexes were calculated from the reflectance data. Physiological indexes are sensitive to narrow spectral features and encapsulate reflectance information in ecologically relevant ways. Classification trees generated from these indexes were able to discriminate both flowering and fruiting perennial pepperweed from co-occurring species with high levels of cross-validated accuracy when using the original spectrometer data and also when this data set was resampled to simulate the spectral resolution of two widely used airborne hyperspectral imagers. Perennial pepperweed's characteristic white flowers are the major component of the spectral uniqueness of this species. Phenological state influenced reflectance spectra more strongly than variation in intraseasonal maturity. Field spectrometer spectra were qualitatively and quantitatively similar to perennial pepperweed spectra extracted from airborne image data. These results suggest that hyperspectral remote sensing will be a powerful tool for the mapping and monitoring of perennial pepperweed. Future work will extend these analyses to image data encompassing the San Francisco Bay/Sacramento–San Joaquin Delta region.

Invasions of many different plants have occurred in ecosystems around the world and theories of the mechanisms of these invasions abound. All the proposed theories have value, and many of the proposed mechanisms may at least serve as facilitating factors, but no overarching conceptual framework for the mechanisms of plant invasion has emerged. One common theme in all invasions is that the invading plant, in the process of geographic displacement, has been dislocated from its coevolved biota and relocated with a less-familiar biota. The impacts of dislocation from coevolved mutualists, parasites, and competitors are different but follow general principles. The impacts of relocation with new mutualists, parasites, and competitors are also variable and will change as the introduced plant coevolves with its new biotic environment. I propose some hypotheses to guide predictions of the outcomes of the dislocation of plants from coevolved relationships and, hence, the outcomes of plant geographic displacement. Invasiveness in plants is not determined by their life history traits or the nature of the ecosystem they are invading. Invasiveness is primarily a result of the process of invasion itself. When plants are dislocated from coevolved relationships and confronted with new relationships, they can become ecologically transformed. This transformation can affect the ability of a plant population to become established, invasive, and naturalized in a new environment.

Garden huckleberry, a member of the Solanaceae family and a close relative of black nightshade, is an exotic plant introduced from Africa. Information on garden huckleberry response to a new environment and to herbicides is useful for determining the potential of this species to become an invasive weed, predicting the potential range of this species in the United States, and developing an optimum garden huckleberry management program. Germination and survival of garden huckleberry seed, as affected by environmental factors, were studied under greenhouse and controlled-environment growth chamber conditions. Garden huckleberry seed became viable between 2 and 3 wk after anthesis and was nondormant when separated from fresh berries. Garden huckleberry seed was not photoblastic and germinated equally well under both a 14-h photoperiod and continuous darkness. Seed germinated at constant temperatures from 17 to 35 C, with optimum germination between 22 and 30 C. Germination of garden huckleberry seed markedly declined as the osmotic potential of the germination medium decreased. The optimum pH for germination of garden huckleberry was between 5 and 9. Paraquat, dicamba, and bromoxynil provided excellent garden huckleberry control (95 to 100%); atrazine and glyphosate were more phytotoxic (90%) than imazamox (80%); and acifluorfen and foramsulfuron gave inadequate control.

The influence of environmental factors on germination and emergence of horseweed was examined in growth chamber experiments. Germination was highest (61%) under 24/20 C day/night temperature under light. Horseweed seed germination was observed under both light (13 h photoperiod) and complete darkness (24 h), but germination under continuous darkness was only 0 to 15% compared with 0 to 61% under light. All other experiments were conducted under 24/20 C and 13-h light conditions. Germination was 19 to 36% over a pH range from 4 to 10, with a trend toward higher germination under neutral-to-alkaline conditions. Horseweed germination was > 20% at < 40 mM NaCl concentration and lowest (4%) at 160 mM NaCl. These data suggest that even at high soil salinity conditions, horseweed can germinate. Germination of horseweed decreased from 25% to 2% as osmotic potential increased from 0 (distilled water) to −0.8 MPa, indicating that germination can still occur under moderate water stress conditions. Horseweed seedling emergence was at its maximum on the soil surface, and no seedlings emerged from seeds placed at a depth of 0.5 cm or higher.

Numerous studies have quantified the developmental responses of wheat to vernalization, but its response compared to a weedy relative, jointed goatgrass, remains relatively unknown. Six paired jointed goatgrass collections gathered from Washington and Oregon fields, and winter and spring wheat, were grown in field studies to quantify yield and germination in response to vernalization. Monthly planting dates initiated in October and concluded in March were used to vary the vernalization durations for plants sown at three Oregon locations (Corvallis, Moro, and Pendleton) over two growing seasons. Minimum vernalization requirements to produce reproductive spikes were similar among plants of six jointed goatgrass collections. Jointed goatgrass collections grown at Corvallis required a minimum of 89 and 78 vernalization days (January 17, 2003 and January 22, 2004 sowing, respectively) to produce reproductive spikes, and plants grown at Moro required 60 vernalization days (March 3 and February 23) in both years, and 48 and 44 vernalization days (March 3 and February 24) were required by plants to produce spikes at Pendleton. Jointed goatgrass spikelet and winter wheat seed yield were positively influenced by vernalization days, experiment location, and year. The strength of the interactions among these main effects differed among jointed goatgrass collections and winter wheat. The effects of vernalization on jointed goatgrass yields and seed quality were more pronounced at Pendleton, OR, a location where jointed goatgrass has adapted, compared to Corvallis, OR, where it has not adapted. The minimum vernalization days required to produce germinable seed differed among jointed goatgrass collections, winter and spring wheat. There was not a selection of spring-adapted jointed goatgrass populations in the populations tested. Yet if spring temperatures are cool, minimum conditions for vernalization may be satisfied, and the benefits of planting spring crops to control jointed goatgrass would be reduced.

It has been proposed that cropping systems can be managed to promote the development of soil microbial communities that accelerate weed seed mortality. We examined soil fungal and bacterial communities, soil C:N ratio, soil particle size fractions, and weed seed mortality in soil from fields with over 10 yr of five contrasting management histories with the objective of determining if seed mortality could be explained by differences in soil properties. Seed mortality of giant foxtail and velvetleaf were greatest in soil from the conventionally managed systems and lowest in soil from a reduced input system. Principal-components analysis of soil microbial communities, as determined through denaturing gradient gel electrophoresis of polymerase chain reaction–amplified ribosomal RNA genes (PCR-DGGE), showed distinct differences in the composition of fungal and bacterial communities among the study soils. The first principal component of the 18S rDNA PCR-DGGE analysis of fungal community composition showed a strong negative correlation with both giant foxtail (− 0.52, P < 0.05) and velvetleaf (− 0.57, P < 0.01) seed mortality, as did ordination with nonmetric multidimensional scaling (NMS) [giant foxtail (− 0.54, P < 0.01) and velvetleaf (− 0.60, P < 0.01)], suggesting that seeds of the two species were affected similarly by changes in the soil fungal community. For giant foxtail, weed seed mortality was also positively correlated (r = 0.48, P < 0.05) with the first NMS axis of the bacterial 16S rDNA analysis. None of the other measured soil properties were significantly correlated with weed seed mortality. Thus, for the soils tested here, management history, microbial community composition, and weed seed mortality were linked. To extend these results to the field, more work is needed to identify components of the fungal and bacterial communities that are active in seed degradation, and to develop conservation biocontrol recommendations for these species.

Controlled-environment experiments were conducted to predict the dispersal distance of horseweed seed. Seed were released from a fixed height and collected at three distances from the introduction point along a 6-m wind tunnel. Dispersal potential was assessed at wind speeds of 8 and 16 km hr−1 and release heights of 50.8 and 76.2 cm. In separate experiments, settlement velocity was determined to be 0.323 m sec−1 (SD = 0.0687). These data were used to parameterize a mechanistic model and compared to a quantile extrapolation (QE) of wind-tunnel results. The QE method predicted a greater mean dispersal distance than the mechanistic model, with large disparities between maximum dispersal distances. Quantile extrapolation predicted dispersal distances over 100 m, whereas the mechanistic model predicted a maximum distance of approximately 30 m. Air turbulence within the wind tunnel and complex dynamics of seed flight may have contributed to the discrepancy between models. Predicting the mean and numerical distribution of seed dispersal distance is crucial when estimating the spread of wind-dispersed seed and for the design of a field-sampling protocol. Although controlled-environment experiments lack the wind variability present in natural systems, predictions from wind-tunnel studies provide a better first approximation of dispersal distance than the mechanistic model. Field experiments designed on the basis of these outcomes are more likely to capture the true dispersal distribution. This should provide more accurate data to inform management decisions for wind-dispersed species.

Field experiments were established in 1998 and 1999 to evaluate the effect of giant smutgrass competition and hexazinone application on bahiagrass forage yield. The experimental design was a split-plot, with low (< 20% groundcover), medium (20 to 70% groundcover), and high (> 70% groundcover) giant smutgrass density as the main plot factors and hexazinone application or no hexazinone application as the subplot factors. In 1998, without hexazinone, bahiagrass biomass accumulation was 1,164 kg ha−1 mo−1 under low giant smutgrass infestation but 590 and 154 kg ha−1 mo−1 under medium and high giant smutgrass densities, respectively. From harvests occurring 1 yr after hexazinone application, bahiagrass yield in the weed-free area was similar to that growing under low giant smutgrass density. However, as giant smutgrass density increased to moderate or high levels, bahiagrass yield was reduced relative to the weed free. Giant smutgrass biomass accumulation was also measured over time. Giant smutgrass biomass, in both years, increased dramatically in the late summer months at the medium and high densities but not at the low density. It was concluded that bahiagrass was competing with the giant smutgrass at low density and depressed late season growth but was not capable of doing so at higher infestation levels. A rapid increase in late-season giant smutgrass growth was partially explained by the fact that bahiagrass is a short day plant that begins to flower in mid- to late summer, and aboveground biomass production decreases in late summer. This shift in carbon allocation in bahiagrass would exert less competition on giant smutgrass and thus partially be responsible for the late season increase in giant smutgrass growth. Economic analysis performed on these data illustrated that a net loss of $15.20 per stocking unit (cow–calf pairs) would be realized if hexazinone were used to control low densities of giant smutgrass. However, a net gain of $29.28 and $55.75 per stocking unit was observed if hexazinone was used to control giant smutgrass that had reached medium or high levels of infestation, respectively. It was concluded from these data that giant smutgrass should not be controlled until densities reach approximately 35% infestation.

Horseweed is a winter or summer annual plant, native to North America and distributed worldwide in temperate climates. This plant is considered an important agricultural weed because it can reduce agricultural yields by 90% at high densities and becomes problematic under low-tillage agriculture. Seed production is robust with an estimated 200,000 seeds produced per plant, and seed dispersal is wind-assisted. The confirmation of glyphosate-resistant horseweed in Delaware in 2001 and the rapid spread of the resistant biotype, currently covering more than 44,000 ha, has necessitated a change in the discussion about weed dispersal. Large radio-controlled airplanes were used to sample the lower atmosphere for the presence of horseweed seeds during a 3-d period in early September 2005 in southern Delaware. The collection of multiple seeds at heights ranging from 41 to 140 m above ground level strongly suggests that horseweed seeds are entering the Planetary Boundary Layer (PBL) of the atmosphere, where long-ranged transport of aerial biota frequently occurs. With wind speeds in the PBL frequently exceeding 20 m s−1, seed dispersal can easily exceed 500 km in a single dispersal event.

A trait-based community assembly approach to weed management may enhance our understanding of how weed communities respond to specific management practices and increase the utility of weed management based on ecological principles. Therefore, identifying management practices that operate as assembly filters and the species traits upon which they act is an important first step in developing a more predictive weed science. Here, I report results from a 3-yr investigation of the effects of timing of annual tillage (spring vs. fall) on the annual assembly of arable weed communities. The timing of tillage had consistent and dramatic effects on the composition of weed communities; spring tillage led to weed communities dominated by early emerging spring annual forbs and C4 grasses, and fall tillage led to communities dominated by later-emerging forbs and C3 grasses. Traits determining a species' susceptibility to tillage time likely include germination syndrome and life cycle, both of which influence how species respond to changes in soil resource levels and light availability driven by seasonal disturbance regime. Manipulating the timing of tillage and other major soil disturbances may therefore be an important tool for managers interested in influencing community composition or targeting species with similar germination and life-history traits.

Roads and streams are important conduits for the spread of invasive species, and croftonweed is an invasive plant in southwest China. We established six transects along one road and stream in Huili County, Sichuan Province to determine the pattern of invasion and identify major factors potentially influencing it. Results demonstrated that the invasion (cover, abundance, and number of clusters) of croftonweed declined significantly with distance from the road and stream. However, after compensation for environmental factors, only distance from the road was significantly negatively related to number of clusters of this plant. Instead, native plant species cover and richness were significantly negatively associated with croftonweed invasion along the road; elevation and native species cover were negatively correlated with the invasion along the stream. These findings indicated that roads and streams were main conduits for the spread of croftonweed in southwest China, and reduced native species cover could facilitate invasion by croftonweed in habitats along roads and streams.