A review of the biological information published after 1980 is provided for three species of the genus Amaranthus: A. retroflexus L., A. powellii S. Watson and A. hybridus L. The three species are noxious weeds introduced to Canada from southern North America. Their geographical distribution has remained almost unchanged since the original paper published in 1980. The plants exhibit a high phenotypic plasticity and genetic variability and they easily adapt to a multitude of agrestal and ruderal habitats. The seeds contribute to a persistent seed bank; they exhibit a variable dormancy and polymorph germination as a result of maternal, genetic and environmental factors. Growth is rapid and plants produce a large number of viable seeds. The three species have developed multiple resistance to triazine and acetolactate-synthase-inhibiting herbicides. They are alternate hosts to many insects, nematodes, viruses, bacteria and fungi that affect cultivated plants.

Spreading orach is an annual weed that colonizes roadsides, field edges, and increasingly, no-till agricultural fields. It produces dimorphic seeds with different levels of physiological dormancy, but little is known about the germination ecology of the two seed types. Field and controlled-environment studies were conducted to determine seed responses to light and stratification, the pattern of seedling emergence in the field, and the effect of soil water content on the length of cold stratification required to break dormancy for each seed type. The large, brown seeds have three times the mass of the smaller, black seeds, primarily because of a larger embryo, but have a thinner seed coat. Germination of brown and black seeds in petri dishes was 98 and 90%, respectively, after stratification for 3 mo at 5 C, whereas germination of unstratified seeds was 19 and 12%, respectively. Light stimulated germination of both stratified and unstratified black seeds but did not increase germination in stratified brown seeds. Up to 40% of brown seeds germinated in situ during stratification, compared with only 2% for black seeds. Germination in petri dishes and emergence in the field were more rapid for brown seeds than for black seeds. Maximum germination of black seeds occurred after stratification for 2 or 3 mo at 5 C on soil that was waterlogged (pore-water matric potential, psi = 0 kPa), wet (psi = -0-38 kPa), or at field capacity (psi = -10 kPa). For shorter periods of stratification, total germination and germination rate of black seeds declined as soil water content decreased from waterlogged to dry (psi = -500 kPa). Seed dimorphism in spreading orach may provide a mechanism to enhance survival in uncertain or variable habitats such as disturbed agricultural fields.

Control of weeds growing around field edges to limit seed production is an important component of preventative weed management. POST herbicide rates that are effective on weeds growing within a dense corn or soybean canopy may not be high enough to control weeds at the edge of a field. A study was conducted from 2004 through 2006 to compare velvetleaf growth and fecundity at the edge of the field as opposed to within the crop in response to a range of glyphosate rates. Treatments included position (plot center or edge), time of emergence (VE or V4 crop growth stage) and glyphosate rate (0 to 900 g ae ha −1 ). Without herbicide application, velvetleaf plants grown on the edge flowered earlier, had thicker stems, and produced more seed capsules than plants grown in the center of the plots. At glyphosate application rates of 200 to 900 g ha −1 , the percentage of plants surviving and reproducing was higher on the edge than within the crop. Edge plants treated with 900 g ha −1 of glyphosate produced more seeds than center plants that received no herbicide. Dose–response curves were used to estimate the glyphosate rate that would reduce seed production of surviving plants to 80% of the untreated plants. Plants emerging at the VE stage were estimated to require 300 g ha −1 within the corn or soybean canopy and 668 g ha −1 on the crop edge, whereas plants emerging at the V4 stage would require 0 g ha −1 within the canopy and 280 g ha −1 on the crop edge.

Two greenhouse trials were conducted over a 2 year period (2004 and 2005) to compare the relative growth potential and fitness of triazine‐susceptible (TS) and triazine‐resistant (TR) waterhemp biotypes. Waterhemp plants from each biotype were grown in 6 L pots in the greenhouse in a completely randomized design. There was no difference in leaf number, plant height, plant biomass, time to bud or time to flower between the two biotypes. The TR female waterhemp plants produced 30% less reproductive biomass and 39% less seed than the TS biotype. The fitness penalty associated with triazine resistance was related to reproductive capability, not to vegetative growth.

A field study was conducted at two sites in southwestern Ontario, Canada, from 2002 to 2005 to assess the cumulative effects of different weed management strategies applied to the same plots over time in a corn–soybean rotation. These strategies included a POST broad-spectrum herbicide applied at recommended or reduced rates and use of a computer decision aid to select the POST herbicide having the greatest net return (economic threshold approach) based on weed populations before treatment or seed-bank estimates. Four years after study initiation, increased weed populations were observed only in the nontreated control. The reduced rate and economic threshold approaches did not lead to increased weed seed banks compared with the recommended rate of a broad-spectrum herbicide. Reductions in herbicide rate were more effective than the use of economic thresholds for reducing the risk of environmental impact in corn and soybeans. Weed populations in this study were generally above threshold levels, and herbicides selected by the computer decision aid tended to be older, less-expensive herbicides with high use rates. Recommendations by the computer decision aid were generally more effective and more profitable when based on weed populations before treatment than when based on seed-bank estimates.Nomenclature: Corn, Zea mays L, soybean, Glycine max (L.) Merr

Prickly lettuce is an annual weed that germinates in both the fall and the spring. It is often found in no-till soybeans and winter wheat in Ontario, Canada, as well as along the edges of fields. Field studies were conducted from 2001 to 2004 to estimate crop yield losses, and to characterize the phenology and seed production of prickly lettuce in relation to time of emergence. Prickly lettuce had a large impact on soybean yield, with losses of 60 to 80% at densities of 50 plants m(-2) or more. Prickly lettuce density estimated to cause a 10% soybean yield loss varied from 0.2 plants m(-2) in 2002 to 1.2 plants m(-2) in 2003 and 2004. In winter wheat, prickly lettuce at densities up to 200 plants m(-2) caused no detectable yield loss in this study. Plants that emerged in the fall generally were larger, flowered earlier. and produced more seeds than those emerging in spring, but size and fecundity were strongly density-dependent. The number of flowers produced per plant could be estimated from the height of the main stem. Seed production per plant ranged from 2,200 to 67,000 in soybeans, and up to 87,000 in a noncrop area at the edge of the field. Winter wheat harvest interrupted prickly lettuce flowering, and only about 25 to 30% of the plants present in the wheat crop survived harvest and flowered in untreated stubble. These plants produced less than 4,000 seeds per plant. Postharvest control with glyphosate, mowing, or cultivation prevented prickly lettuce seed production in wheat stubble. This study suggests that prickly lettuce populations could build up quickly in continuous no-till soybeans, but rotation with winter wheat and control of plants at the edge of the field would help to limit population growth.

Amaranthus tuberculatus var. rudis (common waterhemp) was first identified in Ontario, Canada in 2002. Eight trials were conducted over a 2-year period (2003 and 2004) at two locations to determine the efficacy of pre-emergence and post-emergence herbicides for the control of A. tuberculatus in maize (Zea mays L.). A. tuberculatus was resistant to the acetolactate synthase inhibiting herbicides at both locations and was resistant to the photosystem II inhibiting herbicides at one location; therefore, results varied by location. A. tuberculatus interference in maize resulted in yield loss of up to 38%. Isoxaflutole plus atrazine, s-metolachlor/atrazine, mesotrione, or S-metolachlor/atrazine plus mesotrione when applied pre-emergence all resulted in greater than 96% control of A. tuberculatus at both locations at 70 days after emergence. Dicamba, dicamba/atrazine, and mesotrione plus atrazine when applied post-emergence all provided greater than 90% control of A. tuberculatus at both locations at 70 days after application. Herbicide treatments which effectively controlled A. tuberculatus resulted in maize yields which were equivalent to the weed-free check. Herbicide selection will have to be site-specific depending on the resistance pattern of A. tuberculatus at each site.

Root traits and growth of spreading orach and common lambsquarters were compared in response to soil compaction, drought, and waterlogging under controlled environment conditions. On the basis of the typical habitats occupied, the hypothesis was that spreading orach Would be more tolerant of compaction and waterlogging and common lambsquarters more tolerant of drought. When grown in buckets with soil bulk densities (1.2 and 1.6 g cm(-3)) two sol for 8 wk, the two species responded similarly to compaction, with the fraction of fine roots reduced by 10%, total root length by 70%, root and shoot dry weight and leaf area by 50 to 60%, and plant height by 30% at the high compared with the low bulk density. When grown for 6 wk in soil columns I m long, which were watered daily or allowed to dry, common lambsquarters was deeper rooted than spreading orach at both moisture levels and better able to sustain growth in the drying columns. The watering regime did not alter the rooting depth of either species. Total root length in successive 10-cm increments declined exponentially from the top to the bottom of the watered columns, but root proliferation was reduced in the upper 20 cm of the drying columns. The average root diameter of both species decreased with drought and increased with soil compaction. When gown in waterlogged soil at 10 or 20 C for 4 wk, seedlings of spreading orach survived with little reduction in growth, whereas survival and growth of common lambsquarters were drastically reduced, particularly under cool soil conditions.

Seed production of velvetleaf, lamb's-quarters, common ragweed, giant ragweed and giant foxtail was studied in corn and soybeans in relation to distance from the crop edge. Individual weeds were grown 0.5 in outside the last crop row, in the last crop row, 0.5 in into the crop, or 2.5 in into the crop. Plots either had no herbicide applied, or were treated with half rates of a pre- or post-emergence herbicide. Survival of emerged weeds to maturity was recorded. Basal stem diameter of the broad-leaved weeds, and number and length of inflorescences of giant foxtail were measured in late summer. Seed production per plant was estimated from these traits. Seedlings were more likely to survive to reproduce at or near the crop edge than within the crop, particularly in corn after herbicide application. Fecundity of all weeds was two to four times higher 0.5 in outside the last crop row, compared to 2.5 in within the crop. Seeds from weeds close to the crop edge are likely to be dispersed into the crop by the combine. These studies highlight the potential value of border sprays to control weed seed production where it is likely to be highest.

Horseweed is a winter annual weed that has evolved resistance to multiple herbicide modes of action in 11 countries worldwide. A paraquat-resistant horseweed population in an Ontario orchard that was being managed by a rotation of herbicides began to show increased tolerance to the herbicide linuron. Experiments were conducted in the greenhouse to compare the response of this population and several paraquat-susceptible populations to linuron, diquat, and oxyfluorfen. Plants were sprayed with a range of doses of each herbicide when they were from 5 to 10 wk old, and the ED50, or dose at which shoot dry weight was reduced by 50%, was estimated. There was a sevenfold difference in the ED50values of the paraquat-resistant and -susceptible populations in response to diquat and a threefold difference in response to linuron. The response to oxyfluorfen was age dependent. The ratio of resistance to susceptible ED50values was estimated as 57 for 5-wk-old plants and 11 for 8-wk-old plants in response to oxyfluorfen. Ten-week-old plants from both populations showed no response to oxyfluorfen at rates up to 4.8 kg ai ha−1.

An historical analysis of vapour pressure deficit was done to determine the frequency of very low vapour pressure deficits during June, July and August of the growing seasons, in Harrow, Ontario. The impetus for the analysis was a lack of yield response of processing tomatoes to fertigation in 1995 compared to other years or sites. In the historical analysis, 2 years, 1964 and 1995, were identified as having a high frequency of very low daily vapour pressure deficits during the growing season. The site of the experiment where low VPD was recorded was about 5 km north from the shore of Lake Erie. A site 27 km away from Harrow, and about 10 km north of Lake Erie, near Leamington, Ontario, had a normal, increased yield response to the fertigation treatment. Key words: Relative humidity, vapour pressure deficit, yield, tomato

Populations of horseweed and Virginia pepperweed resistant to the herbicide paraquat were found in several fruit orchards in Essex County, Ontario, Canada, where paraquat had been applied four to five times per year for at least 10 yr. Dose–response experiments were conducted in the greenhouse for each species, using 2-to 4-mo-old plants from populations that had received repeated exposure to paraquat and those that had not. The paraquat dose that reduced shoot dry weight by 50% (ED50) was estimated for each population. Resistant populations of horseweed had ED50 values 25 to 35 times higher than susceptible populations. A resistant population of Virginia pepperweed had an ED50 10 times higher than susceptible populations.

To test if the high nutrient inputs of agroeosystems select for specialized agroecotypes or for phenotypic plasticity, Ontario populations of the northwardly migrating annual weed Solanum ptycanthum from ruderal (beach) and agricultural habitats were compared over a nutrient gradient. Temporal variation of total available nitrogen was determined in both types of habitats. As gene flow via seed contamination of tomato transplants from S. United States was detected, variation in response to nutrient (N) levels was also compared between agrestal populations from the northern (Ontario) and southern (Georgia) ends of the species range. Five families from six populations (two northern agrestal, two northern ruderal and two southern agrestal) were grown in the greenhouse at low, medium and high nutrient levels, and plant growth and traits associated with reproductive success measured. All populations displayed significant levels of plasticity in the majority of vegetative and reproductive traits. There were no detectable differences over the levels of nutrients tested between individuals sampled from northern agrestal and ruderal populations, even though variation in available nitrogen is greater in agroecosystems. Southern agrestal populations were genetically differentiated from the northern populations, and exhibited almost twice the overall plasticity of northern populations, measured by the Mahalanobis distance. Canonical discriminant analysis showed complete overlap in the northern populations over all nutrient levels, suggesting that colonization of new habitats is via a general-purpose genotype, rather than by selection for specialized agroeotypes.