Small-seeded vegetable crop production is challenged by poor emergence, stand establishment and canopy development, as well as a lack of effective weed control options. The potential use of plant growth regulators such as gibberellic acid (GA) could enhance early emergence and growth rates while potentially synchronizing weed germination with control tactics. In response, a controlled environment study was conducted to investigate the effects of GA on garden beet, cabbage, carrot, and onion. At 7 d after seeding (DAS) carrot emergence was greater when carrot seeds were treated with 2, 4, 8, 16, or 64 ppm GA compared with nontreated seeds. Total carrot emergence over the study period was 14% greater when seeds were treated with 4 ppm GA compared with nontreated seeds. Treatment of cabbage with as low as 2 to 4 ppm GA increased cabbage emergence rate and total plant emergence over the study period relative to nontreated seeds. Onion response to GA treatment was variable and unremarkable and was hypothesized to be influenced by seed dormancy because emergence was also low with the nontreated seeds. The GA rates that stimulated vegetable crop seed germination and emergence were then explored with three common weed species to determine whether a similar response was observed. If so, GA could be used to stimulate weed emergence in synchrony with management tactics. Palmer amaranth emergence was strongly affected by GA treatment, whereby the total number of emerged plants was 48% greater when 4 ppm GA was applied than in the nontreated check. Velvetleaf emergence at 3 DAS with the 4 and 8 ppm GA was 2.9 and 3.0 plants pot−1, respectively, compared to no emergence in the nontreated pots. Redroot pigweed emergence was not affected by GA treatment at any rate.

Weed control in vegetable production is especially challenging, because few registered herbicides simultaneously offer excellent crop tolerance and broad-spectrum weed control. We report here the response of several vegetables and weeds to 37.5 and 50 g ai ha−1 of the new herbicide bicyclopyrone (BCP). Vegetable crops showed good tolerance to BCP PRE and post-directed (POST-DIR) in high organic matter content muck soil. POST BCP severely injured all crops. Soil type and the rate of BCP PRE significantly affected response of vegetable crops, and variety of onion was significant. POST BCP controlled hairy galinsoga and small common purslane plants (>80% injury). Hairy galinsoga was not controlled by BCP PRE application in muck soil but was controlled in a 2:3 (vol/vol) blend of Wooster silt loam and a commercial potting mix. Common purslane was slightly injured in the muck soil and was well controlled in the soil and potting mix blend by PRE BCP. The herbicide did not control prostrate pigweed in either soil type or at any growth stage.

Weed management strategies differ in their ability to control weeds, and often have unique agroecological implications. To provide growers with an improved sense of trade-offs between weed control and ecological effects, we implemented several prominent organic weed management strategies in yellow onion in 2014 and 2015. Strategies included cultivation of weed seedlings during the early, weed-sensitive “critical period” of the crop; frequent cultivation events to ensure “zero seed rain”; and weed suppression with polyethylene or natural mulches. As expected, end-of-season weed biomass and weed seed production were greatest in the critical period system and nearly zero for the zero seed rain system. Weeds were also well controlled in natural mulch systems. Average onion yield per treatment was 50.7 Mg ha−1. In 2014, the critical period system and the polyethylene mulch systems demonstrated yield loss, likely due to weed competition and excessive soil temperature, respectively. Onion soluble solids content was also diminished in these systems in 2014, but bulb firmness was greatest in unmulched systems. Carabid beetles, earthworms, soil compaction, soil nitrate, and microbial biomass were affected by weed management strategy, with natural-mulched systems generally performing most favorably. However, these effects were not substantial enough to affect yield of a subsequent sweet corn crop grown in weed-free conditions. In contrast, sweet corn managed with only early-season cultivations demonstrated yield loss (P=0.004) in plots where the critical period treatment was implemented the prior year, indicating that weed competition resulting from abundant weed seed production in that system was the most influential legacy effect of the weed management strategies.

Field studies were conducted in 1984, 1985, and 1986 to compare standard flat fan nozzles, low-volume flat fan nozzles, rotary controlled droplet applicators, and air-assisted nozzles for postemergence weed control in onions with oxyfluorfen and fluazifop-P. Herbicides applied with flat fan nozzles at 47 or 190 L/ha controlled broadleaf and grass weeds as good or better than with the other nozzles. Oxyfluorfen applied with flat fan nozzles controlled common lambsquarters and ladysthumb best. None of the nozzles improved control of the difficult marsh yellowcress. Carrier volume had minimal effect on weed control. Nozzle and carrier volume did not affect onion yield.

Studies were conducted to evaluate preplant incorporated (PPI), preemergence (PRE), and postemergence (POE) herbicide treatments on yellow nutsedge control and on growth and yield of transplanted onions. Most effective yellow nutsedge control (≥80%) was achieved with metolachlor applied PPI plus PRE. Metolachlor and SAN 582H were more effective applied PPI than PRE. Bentazon alone applied POE adequately controlled yellow nutsedge only 1 of 2 yr. Highest onion yields and percent jumbo bulbs were produced when treatments included metolachlor applied PPI.

Field experiments were conducted in 2008 and 2010 to determine crop tolerance and weed control efficacy of the POST herbicides bentazon, flumioxazin, and oxyfluorfen applied to direct-seeded dry bulb onions on organic soil. Postemergence application of oxyfluorfen at 0.071 kg ai ha−1 resulted in less than 20% onion injury when applied at the 2 and 4 onion leaf stages and provided good control of ladysthumb and common lambsquarters. Oxyfluorfen EC caused slightly higher visual injury than oxyfluorfen SC, but there was no difference in onion yield among the treatments. Application of flumioxazin at 0.036 of 0.072 kg ai ha−1 alone or in combination with pendimethalin ACS resulted in minimal onion injury and no yield reduction. Combining flumioxazin in a tank mix with pendimethalin EC, dimethenamid-P EC, or S-metolachlor EC resulted in significant onion injury and yield reduction. Flumioxazin plus S-metolachlor, dimethenamid-P, or pendimethalin improved ladysthumb control in one of two years. Bentazon applied at 0.56 kg ai ha−1 produced moderate onion injury and did not control yellow nutsedge adequately. Bentazon applied at 1.12 kg ai ha−1 provided good control of yellow nutsedge but caused serious onion injury and yield loss.

Some herbicides applied to corn or soybean can carry over and injure vegetable crops grown the following year. Injury and yield effects on vegetable crops from carryover of nicosulfuron and flumetsulam 1 yr after application to corn and from carryover of imazethapyr and imazamox 1 yr after application to soybean were evaluated. Experiments were conducted at Oakes, ND, on sandy loam soils with pH 7.7 in 1994 and pH 7.3 in 1996. Nicosulfuron and flumetsulam were applied to corn, and imazethapyr was applied to soybean at a standard herbicide rate (1×) (35, 56, and 70 g ai/ha, respectively) or 2× rates in 1994 and at 1×, 2×, or 4× rates in 1996. Imazamox was also applied to soybean at 1× (35 g ai/ha), 2×, or 4× rates in 1996. Cabbage, carrot, potato, onion, acorn squash, and tomato were planted in 1995, 1997, and 1998. In 1995, residual imazethapyr delayed tomato maturity but did not reduce tomato yield. Other vegetable crops were not injured by herbicide residues. In 1997, flumetsulam carryover injured plants and reduced yields of cabbage and squash; nicosulfuron carryover injured cabbage and onion plants and reduced onion yield; and imazethapyr carryover injured cabbage, onion, and tomato plants and reduced tomato yield. Most crop injury occurred only at the 2× or 4× herbicide rates. Potato and carrot were not injured by herbicide carryover. Herbicides applied to corn or soybean in 1996 did not carry over to injure any vegetable planted in 1998. The low rate of injury to vegetable crops in this study was mostly due to adequate soil moisture and warm summer soil temperatures, which enhanced microbial degradation of these herbicides and hastened their dissipation. Low clay and moderately low soil organic matter, and slightly alkaline soil pH, also promoted dissipation of some herbicides.

Yellow nutsedge infests a large number of hectares in the Treasure Valley of eastern Oregon and western Idaho. Much of its continued expansion appears to be related to onion production in the valley. Fall applications of metham often produce inconsistent results when used to control yellow nutsedge before planting an onion crop. Trials were conducted in the laboratory to determine the influence on yellow nutsedge control of metham dose, duration of exposure, temperature during exposure, and tuber conditioning by washing and chilling at 3 C. All factors influenced metham efficacy against yellow nutsedge tubers. The dose causing 50% reduction in sprouting tubers (I 50) for metham ranged from 22 to 76 mg kg−1 of soil and was lower for conditioned tubers than nonconditioned tubers across all conditions, except when tubers were exposed at 25 C for 3 d. Nonconditioned tubers were unaffected by metham after 1 d exposure at 5 C. Increasing exposure temperature or increasing exposure duration decreased sprouting for nonconditioned tubers. As exposure duration and exposure temperature increased, differences among conditioned and nonconditioned tubers were less. Temperature and exposure duration affects metham efficacy against yellow nutsedge, and the condition of the tubers at the time of treatment also has a significant effect. Applications of metham at a time when yellow nutsedge tubers are not dormant may improve yellow nutsedge control.

Increasing concerns about pesticide use and a steadily increasing conversion to organic farming have been major factors driving research in physical and cultural weed control methods in Europe. This article reviews some of the major results achieved with nonchemical methods and strategies, especially adapted for row crops (e.g., corn, sugar beet, onion, leek, and carrot) and small-grain cereals (e.g., barley and wheat). In row crops, intrarow weeds constitute a major challenge, and research has mainly aimed at replacing laborious hand-weeding with mechanization. A number of investigations have focused on optimizing the use of thermal and mechanical weeding methods against intrarow weeds, such as flaming, harrowing, brush weeding, hoeing, torsion weeding, and finger weeding. And new methods are now under investigation such as robotic weeding for row crops with abundant spacing between individual plants and band-steaming for row crops developing dense crop stands. The strategic use of mechanical weed control methods in small-grain cereals has been another area of considerable interest. Weed harrowing and interrow hoeing provide promising results when they are part of a strategy that also involves cultural methods such as fertilizer placement, seed vigor, seed rate, and competitive varieties. Although research in preventive, cultural, and physical methods have improved weed control in row crops and small-grain cereals, effective long-term weed management in low external input and organic systems can only be achieved by tackling the problem in a wider context, i.e., at the cropping system level. Basic principles of this approach, examples of cover crop and intercropping use for weed suppression, and an application in a 2-yr rotation are presented and discussed.

Field experiments were carried out during two growing seasons at Adnan Menderes University, Faculty of Agriculture, in Aydın-Turkey to evaluate the weed control efficacy of olive processing waste (OPW) in okra, faba bean, and onion. OPW was incorporated into the soil prior to seeding at 10, 20, 30, and 40 tons (t)/ha. Non-treated plots and plots treated with trifluralin in okra and pendimethalin in faba bean and onion were used for comparison. OPW suppressed common purslane, redroot pigweed, and junglerice in okra; littleseed canarygrass, annual bluegrass, wild chamomile, and shepherd's-purse in faba bean and onion. OPW was in most cases equally as effective as soil herbicides; however, 10 t/ha provided sometimes lower efficacy than herbicides. OPW had no negative effects on okra and faba bean, while onion was negatively affected by doses over 30 t/ha. Overall, OPW can be applied at 10 to 20 t/ha doses for weed control with adequate crop safety.

Field trials were conducted in 2003/2004 and 2005/2006 at Reidsville, Georgia, to evaluate the effects of previously applied residual herbicides on onion growth and bulb production. Before transplanting onion, preplant applications of imazapic at 18 and 36 g ai/ha, diclosulam at 7 and 14 g ai/ha, pyrithiobac at 27 and 54 g ai/ha, trifloxysulfuron at 6.6 and 13.2 g ai/ha, diuron at 224 and 448 g ai/ha, and cloransulam at 22 g ai/ha were made. An untreated control was included for comparison. Trifloxysulfuron at 13.2 g/ha, diclosulam at 14 g/ha, pyrithiobac at 54 g/ha, and cloransulam at 22 g/ha injured onion 26, 73, 86, and 86% in 2003/2004, respectively, and 13 to 44% injury in 2005/2006. These same herbicides also reduced yield. Imazapic and diuron injured transplanted onion 6% during both seasons but did not reduce yield. This research suggests imazapic and diuron restrictions could possibly be reduced. However, the onion rotational restrictions for diclosulam, pyrithiobac, trifloxysulfuron, and cloransulam are accurate.

Mesotrione is an effective herbicide for volunteer potato control but is not selective in several crops, including onion and carrot. Studies were conducted in 2006 to evaluate the effectiveness of wiper-applied mesotrione for control of volunteer potato in a potato crop. Surprisingly, nontreated potatoes growing adjacent to mesotrione-treated plants exhibited bleaching symptoms resembling mesotrione injury. Additional field trials confirmed injury to nontreated plants growing within 26 cm of potatoes treated with mesotrione applied with a wiper at 0.25, 0.5, and 1% (v/v) solutions. Greenhouse bioassays confirmed that mesotrione applied to potato leaves moved down through the plant and was exuded into perlite potting medium in sufficient quantities to injure potato plants that were exposed to the leachate from the perlite pots. In tracer studies, 52% of 14C-labeled mesotrione applied to potato leaves was absorbed into the potato plant by 15 d and 15% of the absorbed 14C-mesotrione was exuded into the soil and soil leachate. Mesotrione applied to potato by the wick application method has potential to injure neighboring susceptible plants from root uptake of exuded mesotrione.

Trials were established in 2007, 2008, and 2009 in Ontario, Canada, to determine the effect of soil residues of saflufenacil on growth, yield, and quality of eight rotational crops planted 1 yr after application. In the year of establishment, saflufenacil was applied PRE to field corn at rates of 75, 100, and 200 g ai ha−1. Cabbage, carrot, cucumber, onion, pea, pepper, potato, and sugar beet were planted 1 yr later, maintained weed-free, and plant dry weight, yield, and quality measures of interest to processors for each crop were determined. Reductions in dry weight and yield of all grades of cucumber were determined at both the 100 and 200 g ha−1 rates of saflufenacil. Plant dry weight, bulb number, and size and yield of onion were also reduced by saflufenacil at 100 and 200 g ha−1. Sugar beet plant dry weight and yield, but not sucrose content, were decreased by saflufenacil at 100 and 200 g ha−1. Cabbage plant dry weight, head size, and yield; carrot root weight and yield; and pepper dry weight, fruit number and size, and yield were only reduced in those treatments in which twice the field corn rate had been applied to simulate the effect of spray overlap in the previous year. Pea and potato were not negatively impacted by applications of saflufenacil in the year prior to planting. It is recommended that cabbage, carrot, cucumber, onion, pepper, and sugar beet not be planted the year after saflufenacil application at rates up to 200 g ha−1. Pea and potato can be safely planted the year following application of saflufenacil up to rates of 200 g ha−1.

Natural products might provide an organic means of weed control. Our objective was to evaluate the potential use of vinegar and a clove-oil product with regard to how volume, concentration, and application timing affect weed control and crop response. Treatments included broadcast applications of 200- and 300-grain vinegar at 318 liters per hectare (L/ha), 150- and 200-grain vinegar at 636 L/ha, a 3.4% v/v clove oil mixture in water (318 L/ha), and a 1.7% clove oil mixture in 200-grain vinegar (318 L/ha). Field trials were conducted in sweet corn, onion, and potato. Weed control, weed biomass, crop injury, and yield data were collected. Corn treated at 15 and at 30 to 45 cm was initially burned and stunted by these products. By 4 wk after application much of the initial injury was outgrown. Late applications significantly reduced yields of early-maturing sweet corn ‘Trinity’. With the exception of the 200-grain vinegar (318 L/ha) treatment, early applications to sweet corn ‘Avalon’ did not reduce marketable yield. Two hundred-grain vinegar (636 L/ha) applied to pre-emergence–flag stage onion reduced the duration of the first handweeding by 59 to 67%. All treatments reduced onion yields when treated at the 2-leaf stage. Potato treated early (2 to 10 cm) and late (30 cm) were injured by all vinegar treatments 59 to 83%, 1 d after treatment (DAT). Potato yield losses were insignificant with applications of 3.4% clove oil and with some low-volume (318 L/ha) vinegar treatments. Product efficacy was dependent on the weed species and their size at the time of application. Weed control was greatest (83%, 1 DAT) with 200-grain vinegar (636 L/ha). Broadcast applications of vinegar and clove oil have potential for use on young, actively growing sweet corn, onion, and potato.

Early-season weed competition may cause substantial yield losses in onion. Oxyfluorfen and bromoxynil are POST herbicide options for weed control once onion has developed two leaves, which often takes 4 to 6 wk. Multiple applications of oxyfluorfen at 35 and 18 g ai ha−1 and bromoxynil at 35 and 18 g ae ha−1 with adjuvants were evaluated for onion safety and weed control under controlled greenhouse conditions. Oxyfluorfen at 35 g ha−1 plus organosilicone surfactant caused 42% onion injury at 12 d after three sequential applications. Onion treated with bromoxynil at 18 g ha−1 plus high surfactant oil concentrate had lower fresh weight (0.7 g) compared to methylated seed oil (MSO) (1.2 g) or petroleum oil concentrate (POC) (1.3 g) at the same bromoxynil rate. The addition of nonionic surfactant to bromoxynil, averaged across bromoxynil rates, provided 17 and 39% control of redroot pigweed and common lambsquarters, respectively. Redroot pigweed control with oxyfluorfen at 35 or 18 g ha−1 plus any tested adjuvant was excellent (≥ 93%). Results suggested the use of POC or MSO with either oxyfluorfen or bromoxynil for subsequent field trials because of similar common lambsquarters and redroot pigweed control and onion safety.

Isoxaflutole, a preemergence herbicide for use in corn, causes bleaching of plant tissue and plant death at low rates. A concern regarding widespread use of isoxaflutole is the unintentional exposure of high-value, minor hectareage crops that may be sensitive. Unintentional exposure could occur because of carryover from a previous application, spray drift, or contamination of irrigation water. The objective of this study was to determine the potential for injury to nine minor hectareage Michigan crops. Crops evaluated were: adzuki bean, alfalfa, carrot, cucumber, dry bean (navy and black beans), onion, sugar beet, and tomato. Experiments were conducted in the greenhouse to evaluate injury from low rates of isoxaflutole applied to soil to simulate carryover as well as low concentrations of isoxaflutole in 2.54 cm of irrigation water applied over the course of 1 h to 15-cm-tall plants. Isoxaflutole rates and concentrations that cause 20% injury (I20) were calculated using Seefeldt's log-logistic dose–response model. Regardless of application type, onion was always the least sensitive plant to isoxaflutole (I20 = 37 g/ha applied to soil and 194 μg/L in irrigation water), whereas navy bean and black bean were the most sensitive (I20 = 9 g/ha applied to soil and 5 μg/ L in irrigation water). The remaining plants exhibited intermediate sensitivity. All of the rates that resulted in injury were substantially less than the rates used for weed control in corn. Carryover from isoxaflutole applications in corn production may require plant back restrictions for certain sensitive crops.

Trials were established in 2003, 2004, and 2005 in Ontario to determine the effects of residues of mesotrione, atrazine, and mesotrione plus atrazine 1 and 2 yr after application on broccoli, carrot, cucumber, onion, and potato. One yr after mesotrione application, injury was 43, 37, 18, 24, and 0% in broccoli, carrot, cucumber, onion, and potato, respectively. The addition of atrazine to mesotrione in the year before planting increased injury to 55, 53, 30, 42, and 3% in broccoli, carrot, cucumber, onion, and potato, respectively. Plant dry weight and yield were also decreased by mesotrione residues the year after application in all crops except potato. The addition of atrazine to mesotrione accentuated the reduction in dry weight and yield in broccoli, carrot, cucumber, and onion. There was no injury, or reductions in dry weight or yield in any crop planted 2 yr after application of mesotrione alone or in tank mix with atrazine. A recropping interval of 2 yr is recommended following applications of mesotrione or mesotrione plus atrazine for broccoli, carrot, cucumber, and onion. Potato can be safely planted the year following application of mesotrione plus atrazine.