Soil characteristics, elevation, annual precipitation, potential evapotranspiration, length of frost-free season, and mean maximum July temperature were estimated for 116 established infestations of spotted knapweed (Centaurea maculosa Lam. # CENMA) in Montana using basic land resource maps. Areas potentially vulnerable to invasion by the plant were delineated on the basis of representative edaphic and climatic characteristics. No single environmental variable was an effective predictor of sites vulnerable to invasion by spotted knapweed. Only a combination of variables was effective, indicating that the factors that regulate adaptability of this plant are complex. This technique provides a first approximation map of the regions most similar environmentally to infested sites and, therefore, most vulnerable to further invasion. This weed migration prediction technique shows promise for predicting suitable habitats of other invader species.

Current and projected increases in the concentrations of CO2 and other radiatively-active gases in the Earth's atmosphere lead to concern over possible impacts on agricultural pests. All pests would be affected by the global warming and consequent changes in precipitation, wind patterns, and frequencies of extreme weather events which may accompany the “greenhouse effect.” However, only weeds are likely to respond directly to the increasing CO2 concentration. Higher CO2 will stimulate photosynthesis and growth in C3 weeds and reduce stomatal aperture and increase water use efficiency in both C3 and C4 weeds. Respiration, and photosynthate composition, concentration, and translocation may be affected. Perennial weeds may become more difficult to control, if increased photosynthesis stimulates greater production of rhizomes and other storage organs. Changes in leaf surface characteristics and excess starch accumulation in the leaves of C3 weeds may interfere with herbicidal control. Global warming and other climatic changes will affect the growth, phenology, and geographical distribution of weeds. Aggressive species of tropical and subtropical origins, currently restricted to the southern U.S., may expand northward. Any direct or indirect consequences of the CO2 increase that differentially affect the growth or fitness of weeds and crops will alter weed-crop competitive interactions, sometimes to the detriment of the crop and sometimes to its benefit.

The Arable Area Payments Scheme of the European Union has been in operation since 1992, paying farmers to set aside a portion of their arable land from production for 1 to 5 yr. Management of the land is prescribed, both in terms of timing and type. The scheme is potentially beneficial to the environment, as set-aside land can be used to improve bird, mammal, and insect habitats. However, environmental and agronomic objectives frequently conflict, particularly in the area of weed management. To predict weed growth and succession, a tool was required to model vegetation at set-aside sites using the key environmental factors, location, and soil that could account for differences in the weed species likely to be invasive. To this end, a Geographical Information System has been developed that links modeled species distributions to a model of successional change. The system has a simple menu structure allowing speedy manipulation of the data and models to predict the effects of management on the weed communities at known geographical locations.

A weed survey system methodology was developed and used annually for 4 yr in the province of Saskatchewan. Based on a stratified random sampling procedure, fields were selected in wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), oat (Avena sativa L.), rye (Secale cereale L.), rape (Brassica napus L., Brassica campestris L.), and flax (Linum usitatissimum L.) crops. The boundaries of the 43 agricultural extension districts in the province formed the strata. The provincial agricultural extension service provided staff to conduct the survey. Computer processing of the data enabled reports, containing summary tables and distribution maps, to be distributed to extension, research, and industry personnel within 3 months after completion of the field survey. The field surveys were conducted late in the growing season before harvest commenced; hence, the weed populations consisted of species that had been present during the early part of the growing season and may have contributed to yield losses. During the 4 yr of the survey, weeds were counted in 4423 fields. Of the 164 species recorded, 97 species occurred in more than three fields. Only 29 species were encountered frequently in the fields. Green foxtail [Setaria viridis (L.) Beauv. # SETVI), wild buckwheat (Polygonum convolvulus L. # POLCO), and wild oat (Avena fatua L. # AVEFA) were the top three species based on a ranking system using a measure called relative abundance.

A weed survey of 115 Nova Scotia lowbush blueberry fields was conducted during the summers of 1984 and 1985 to determine the most common and prevalent weeds associated with blueberry production. A total of 119 weed species were observed within the surveyed blueberry fields. The most common weed species were bunchberry, colonial bentgrass, poverty oatgrass, sheep sorrel, and false lily-of-the-valley. Weeds with low relative abundance in this study, but which may become more widespread, include three-toothed cinquefoil, Kentucky bluegrass, hair fescue, canker-root, and Canadian St. Johnswort.

Triazine-resistant (TR) common waterhemp was reported and confirmed in Fillmore County, NE in 1990. A survey of 81 fields was conducted to characterize the occurrence of common waterhemp and grower practices. Sampled fields fell into three categories: suspected by growers to contain resistant plants (26 fields), randomly selected (28 fields), and adjacent to fields containing TR common waterhemp (27 fields). Resistant plants were found in 64% of all fields. Resistance was confirmed in 92% of the fields suspected by growers to contain resistant plants. Adjacent fields were no more likely to contain resistant plants than randomly chosen fields. Crop rotation did not significantly affect occurrence of resistance. Resistance was associated with the grower, indicating movement of resistant seed between fields via equipment. Atrazine, bromoxynil plus atrazine, and bentazon plus atrazine provided less than 75% postemergence control of TR common waterhemp, while primisulfuron, dicamba plus atrazine, primisulfuron plus dicamba, dicamba, 2,4-D ester, and metribuzin plus bentazon gave over 85% control.

A random mail survey of Ohio growers was conducted to determine the geographic distribution of weeds across Ohio and the influence of production practices on the severity of agronomic weed problems. Statewide, the ten most severe weed problems, in descending order, were Canada thistle > foxtail spp. > velvetleaf = common ragweed > common cocklebur > giant ragweed = annual morningglory spp. > quackgrass = common lambsquarters = Pennsylvania smartweed. Distribution of most weeds varied geographically. Infestations of common and honeyvine milkweeds, hemp dogbane, and tree seedlings increased as tillage was reduced. Infestations of common cocklebur and common ragweed decreased as tillage was reduced. As crop rotation became more diverse, infestations of velvetleaf, common cocklebur, giant ragweed, Pennsylvania smartweed, and jimsonweed decreased. Where rotations included forage or hay crops, infestations of large crabgrass, common chickweed, wild carrot, and common dandelion increased.

The distribution pattern of s-triazine-resistant biotypes of common lambsquarters (Chenopodium album L. #3 CHEAL) and smooth pigweed (Amaranthus hybridus L. # AMACH) in Virginia was determined. Seeds were collected from suspected triazine-resistant biotypes of both species. Triazine resistance was confirmed by measuring chlorophyll fluorescence in the presence of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine]. Greenhouse bioassay with whole-plant material and a sinking leaf disc assay were also performed as further confirmation of triazine resistance. Triazine-resistant smooth pigweed was confirmed in 19 counties and common lambsquarters in eight counties in Virginia. Triazine-resistant smooth pigweed and common lambsquarters were located mostly in the northern and southwestern highlands of the state where there has been a long history of triazine use in no-till corn (Zea mays L.) production. S-triazine-resistant biotypes were also cross-resistant to other representative s-triazine and as-triazine herbicides but susceptible to the substituted urea herbicide diuron [N′-(3,4-dichlorophenyl)-N,N-dimethylurea].

Knowing the distribution of weed seedlings in farmer-managed fields could help researchers develop reliable distribution maps for site-specific weed management. With a knowledge of the spatial arrangement of a weed population, cost effective sampling programs and management strategies can be designed, so inputs can be selected and applied to specific field areas where management is warranted. In 1997 and 1998, weeds were sampled at 612 to 682 sites in two center pivot irrigated corn fields (71 and 53 ha) in eastern Colorado. Weeds were enumerated when corn reached the two-leaf, four-leaf, and physiological maturity stages in a 76.2- by 76.2-m grid, a random-directed grid where sites were established at intervals of 76.2 m, and a star configuration based on a 7.62- by 7.62-m grid within three 23,225 m2 areas. Directional correlograms were calculated for 0, 30, 60, 90, 120, and 150° from the crop row. Fifteen weed species were observed across fields. Spatial dependence occurred in 7 of the 93 samples (a collection of sampling units for a particular weed species that was detected within a field at a particular sampling time and year) for populations of field sandbur, pigweed species, nightshade species, and common lambsquarters. Correlogram analysis indicated that 18 to 72% of the variation in sample density was a result of spatial dependence over a geographic distance not exceeding 5 to 363 m among the examined data. Because of the lack of spatial correlation for weed seedling distributions in these eastern Colorado corn fields, interpolated density maps should be based on grid sizes (separation distances) less than 7.62 m for weed seedling infestations.

Abutilon theophrasti is one of the worst agricultural weeds in North America, yet it has not reached that status in California in the 80 yr since it was first reported. The research reported here examined the distribution and modeled climatic requirements of A. theophrasti to determine whether it is likely to spread more widely in the state. Herbaria records and weed literature were surveyed to determine the historical occurrences of A. theophrasti in the state; current distribution was assessed through surveys sent to University of California personnel in each county. Combined results showed 42 counties out of 58 with A. theophrasti present historically or currently. A plot of the cumulative number of counties containing A. theophrasti by decade fit a logistic equation. The maximum rate of spread of this species occurred in 1962 and it is likely that its final distribution by county in California is leveling off and not likely to increase further. The climate-matching/mapping software CLIMEX® was used with observed and estimated parameters of environmental requirements of A. theophrasti to model its current distribution from India through China to Japan. The same model parameters were then used to map its potential distribution in California. Areas where A. theophrasti has been reported were predicted by CLIMEX to be poorly suited for its growth and development without added soil moisture in the form of irrigation. It appears that the Mediterranean climate is a deterrent to the integration of A. theophrasti into California. The climate-matching approach provided a biologically reasonable assessment of potential distribution of A. theophrasti in California. The approach also allowed assessment of the effects of common agricultural practices on potential distribution given the environmental requirements and limitations of A. theophrasti.

The occurrence of 36 arable weed species across 13 counties in the eastern Arkansas–Mississippi Delta area on 489 randomly selected road sites was surveyed in 2012. Palmer amaranth, johnsongrass, large crabgrass, barnyardgrass, prickly sida, and broadleaf signalgrass were the top six weed species, with occurrence noted at 313, 294, 261, 238, 176, and 136 sites, respectively. Factors found to affect weed occurrence along Mississippi Delta roadsides included topographical characteristics, weed species, ditch slope, road type, and nearby land use. Among roadside topographical characteristics, road shoulder was found to strongly affect weed occurrence. In addition, paved and gravel road types with moderate roadside slope explained most of the variability of weed occurrence at each sampling site. Additionally, nearby arable land use affected weed occurrence more so than natural, residential, and pastoral land. Barnyardgrass, johnsongrass, and Palmer amaranth were 3.6 to 4.3 times more likely to occur than all other species identified. An effective weed management plan along eastern Arkansas–Mississippi Delta roadsides should focus on road shoulder, adjacent arable land use, road type, and specific weed species (e.g., Palmer amaranth, johnsongrass, and barnyardgrass). The inclusion of these parameters in future weed control programs can prove invaluable for preventing the spread of the herbicide-resistant Palmer amaranth, barnyardgrass, and johnsongrass.

A survey was conducted in 2012 across 13 counties in the eastern Arkansas–Mississippi Delta area on 489 randomly selected road sites to assess the distribution of the most commonly occurring arable weeds. Among the 36 species recorded, Palmer amaranth, johnsongrass, large crabgrass, barnyardgrass, prickly sida, and broadleaf signalgrass were the top six weed species, occurring at 313, 294, 261, 238, 176, and 136 sites, respectively. Barnyardgrass, johnsongrass, and Palmer amaranth were present at 34, 32, and 31% of all sampling occasions (site by roadside topographical characteristic). Habitat preferences varied between weed species. Palmer amaranth, large crabgrass, and johnsongrass exhibited a preference for disturbed habitats as well as field shoulders. Conversely, barnyardgrass, yellow nutsedge, hemp sesbania, and giant ragweed exhibit a preference for moist environments similar to these found in roadside ditches. Herbicide use on roadsides is subject to many environmental regulations and public concerns that, in combination with the evolution of herbicide resistance, necessitate an effective plan for managing agronomically important weed species on eastern Arkansas–Mississippi Delta roadsides.

Greenhouse, field, and laboratory experiments were conducted in northern Greece during 1996, 1997, and 1998 to study possible metribuzin resistance in Amaranthus retroflexus (redroot pigweed) and Chenopodium album (common lambsquarters) biotypes found in potato fields. The greenhouse experiments indicated that the suspected resistant biotypes (R) of both species were not controlled by metribuzin applied either pre- or postemergence at rates of 245, 490, 980, and 1,960 g ai ha−1 (the higher rate is eight times greater than the rate recommended for weed control in Solanum tuberosum [potato]). However, susceptible biotypes (S) were completely controlled by 245 g ai ha−1. Also, both R- and S-biotypes of either species were effectively controlled by prometryn applied either pre- or postemergence at 1.5 kg ai ha−1. The field trials confirmed that metribuzin applied either pre- or postemergence at rates of 490, 980, and 1,960 g ai ha−1 gave fair or partial control of the R-biotype of C. album, whereas prometryn applied preemergence at 1.5 kg ai ha−1 gave excellent control of this weed. Chlorophyll fluorescence measurements performed in laboratory experiments indicated that photosynthetic electron transport in metribuzin-incubated leaves detached from plants of the R-biotypes was not affected, but it was inhibited in leaves detached from plants of the S-biotypes. Electron transport was inhibited by prometryn in leaves detached from both S- and R-biotypes of either species. These results show clearly that the biotypes of both species developed resistance to metribuzin, but they were not cross-resistant to prometryn.