Studies were conducted in the summer and fall of 2001 in North Brunswick, NJ, and Marion County, Oregon, to evaluate the response of glyphosate-resistant and glyphosate-susceptible creeping bentgrass hybrids, colonial bentgrass, redtop, and dryland bentgrass grown as individual plants to postemergence (POST) herbicides. Glyphosate at 1.7 kg ae/ha, glufosinate at 1.7 kg ai/ha, fluazifop-P at 0.3 and 0.4 kg ai/ha, clethodim at 0.3 kg ai/ha, sethoxydim at 0.5 kg ai/ha, and a combination of glyphosate and fluazifop-P were applied 6 wk after planting. Glyphosate provided almost complete control of all susceptible bentgrass species at 4 weeks after treatment (WAT). Glufosinate provided 95% or greater control of all bentgrass species at 4 WAT, but regrowth was observed on all species in the summer experiment in Oregon. Fluazifop-P, clethodim, and sethoxydim provided slower control of bentgrass species, which ranged from 38 to 94% at 4 WAT, depending on species, herbicide, and experimental location. By 8 WAT, fluazifop-P at 0.4 kg/ha applied alone or in combination with glyphosate showed the highest levels of control (>90%) across all bentgrass species. Studies were also conducted in 2002 in the spring and summer in North Carolina to evaluate the response of a mature stand of glyphosate-susceptible ‘Penncross’ creeping bentgrass to POST herbicides. Two applications of glyphosate at 1.7 kg/ha were required to achieve 98% bentgrass control at 8 WAT. Fluazifop-P at 0.4 kg/ha, clethodim at 0.3 kg/ha, and sethoxydim at 0.4 kg/ha exhibited herbicidal activity, but two applications were required to reach (>82%) control of bentgrass at 8 WAT. Two sequential applications of clethodim or the combination of glyphosate and fluazifop-P provided 98% control of bentgrass at 8 WAT. Of the other herbicide treatments evaluated, only atrazine and sulfosulfuron provided (>80%) control at 8 WAT. The results of these studies demonstrate that fluazifop-P, clethodim, and sethoxydim have substantial herbicide activity on bentgrass species and may be viable alternatives to glyphosate for control of glyphosate-resistant creeping bentgrass and related bentgrass species in areas where they are not wanted. Glufosinate, atrazine, and sulfosulfuron also exhibited substantial herbicidal activity on bentgrass, and further research with these herbicides is warranted.

Huisache is a major brush problem on native rangelands and pastures in South Texas. Although herbicide applications to foliage provide very high plant-kill levels, the same herbicides have not proven reliable when applied as broadcast ground or aerial foliar treatments. Aerial and ground broadcast herbicide foliar treatments were applied to 31 huisache sites. Soil temperature and soil moisture were measured at a depth of 30 cm at the time of herbicide application. Cumulative rainfall before herbicide application was recorded. Across all aerial treatments, plant mortality was 69% for plants shorter than 2 m versus 40% for plants taller than 2 m. Across all aerial- and ground-treated sites, plants shorter than 2 m had an average 89% mortality when cumulative 2-wk rainfall was at least 50 mm, versus 72% mortality with cumulative rainfall less than 50 mm. Average plant mortality was 84% when 4-wk cumulative rainfall was at least 76 mm, versus 71% with rainfall less than 76 mm; and 85% when, on a dry-to-wet scale of 0 to 10, soil moisture measured at least 8, versus 71% when soil moisture measured less than 8. In a separate aerial trial, plant-mortality effects of spray droplet size (417, 630, and 800 µm) and spray volume (37.4 L ha−1 and 93.5 L ha−1) were replicated and tested at a single study site in 2014. Plant mortality was lowest for the 93.5 L ha−1 and 800 µm treatment. Plant mortality rates for other treatments were similar, demonstrating a greater importance of droplet size than spray volume. Targeting huisache trees shorter than 2 m, when cumulative rainfall has reached at least 50 mm or at least 76 mm 2 or 4 wk before application, respectively, as well as maintaining spray droplet sizes no larger than 630 µm can increase herbicide efficacy with foliar broadcast applications.

Methyl bromide is a common fumigant for effective weed control in polyethylene-mulched vegetable crops. However, the ban on methyl bromide in the United States has created a need to find a suitable alternative. This study investigated the herbicidal efficacy of phenyl isothiocyanate (ITC) as a methyl bromide alternative for weed control in polyethylene-mulched bell pepper during 2006 and 2007. Six rates of phenyl ITC (0, 15, 75, 150, 750, 1,500 kg ha−1) under low-density polyethylene (LDPE) or virtually impermeable film (VIF) mulch were tested against yellow nutsedge, Palmer amaranth, and large crabgrass. Additionally, a standard treatment of methyl bromide/chloropicrin (67 : 33%) at 390 kg ha−1 under LDPE mulch was included for comparison. VIF mulch provided no advantage over LDPE mulch in either improving weed control or marketable yield in bell pepper. Unacceptable pepper injury (≥ 60%) occurred at the highest phenyl ITC rate of 1,500 kg ha−1 at 2 WATP in both years, regardless of mulch type. Higher bell pepper injury was observed in 2006 (≥ 36%) than in 2007 (≤ 11%) at 750 kg ha−1 of phenyl ITC. The lower injury in 2007 could be attributed to aeration of beds 48 h prior to transplanting. Regardless of mulch type, phenyl ITC at 2,071 (± 197) and 1,655 (± 309) kg ha−1 was required to control yellow nutsedge, Palmer amaranth, and large crabgrass equivalent to methyl bromide in 2006 and 2007, respectively. Bell pepper marketable yield at all rates of phenyl ITC was lower than methyl bromide in 2006. In contrast, marketable yield in phenyl ITC at 750–kg ha−1 was equivalent to methyl bromide in 2007. It is concluded that phenyl ITC should be applied at least 4.2 times higher rate than methyl bromide for effective weed control, and bed aeration is required to minimize crop injury and yield loss. Additional research is needed to test phenyl ITC in combination with other weed control strategies to obtain effective weed control with acceptable crop safety.

Field experiments were conducted at the North Central Agricultural Research Station in Fremont, OH, in 2006 and 2007, to evaluate tolerance of banana pepper to S-metolachlor and clomazone, and the efficacy of these herbicides on green and giant foxtail, common lambsquarters, and common purslane. The crop was machine-transplanted in late spring of each year. Pretransplant (PRETP) herbicide treatments included two S-metolachlor rates (534 and 1,070 g ai ha−1), two clomazone rates (560 and 1,120 g ai ha−1), and four tank mixes of S-metolachlor plus clomazone (534 + 560 g ha−1, 1,070 + 560 g ha−1, 534 + 1,120 g ha−1, and 1,070 + 1,120 g ha−1). Crop injury and weed control data were collected at 2 and 4 wk after treatment (WAT). The crop was harvested two times from August to September. Minor crop injury was observed at 2 WAT only in 2006 and in plots treated with S-metolachlor, alone or in combination with clomazone. In 2007, slight crop injury at 6 WAT in most herbicide-treated plots was mostly related to weeds that grew regardless of herbicide treatment. In general, S-metolachlor provided less weed control than did clomazone or tank mixes of S-metolachlor plus clomazone. Clomazone did not reduce yield of banana pepper. Registration of clomazone would provide banana pepper growers an opportunity to control weeds caused by late emergence or poor initial control following a burndown herbicide application.

Atrazine applied at planting is commonly used for weed control in corn. With global climate change causing an increase in river flooding in the United States over the past decade, producers need information to determine the best course of action in flooded fields treated with atrazine into which they wish to immediately plant soybean. Studies were designed to understand the effect of flooding on atrazine residual activity including atrazine concentration, soybean injury, and soybean yield. In 2012, soybean yield in flooded treatments was reduced by prior atrazine application. In 2014, soybean injury was <10% in all plots, and nonflooded, atrazine-treated soils had yields equal to the nontreated. Findings from this research indicated that it is possible for producers to consider replanting soybean after atrazine application, with appropriate changes to product labeling.

Experiments were conducted throughout central Florida from 2010 to 2014 to determine the effectiveness of fluroxypyr or aminopyralid for control of spreading pricklypear. Aminopyralid + 2,4-D (0.09 + 0.75 kg ae ha−1) was not effective and provided only 15% control by 18 mo after application (MAT). However, fluroxypyr at 0.55 kg ae ha−1 or sequential applications of 0.27 kg ha−1 provided greater than 82% control at 18 MAT. Reducing fluroxypyr rates to 0.32 kg ha−1 reduced control to 40 and 71% for spring versus fall applications, respectively. However, the addition of aminopyralid + 2,4-D to fluroxypyr at 0.32 kg ha−1 improved pricklypear control to 92%, regardless of application timing.

Horseweed, also known as marestail, is a problematic weed for no-till soybean producers that can emerge from late summer through the following spring. Overwintering cover crops can reduce both the density and size of fall-emerged weeds such as horseweed and reduce further spring emergence, although typically cover crops do not provide complete control. Cover crops may be integrated with additional spring herbicide applications to control emerged horseweed, and selective herbicides such as 2,4-D may be used to target horseweed while maintaining small grain cover crop growth. However, cover crops may affect herbicide deposition, which could reduce their efficacy to control weeds. The objective of this study was to determine how the amount and variability of 2,4-D ester spray solution deposition, measured with water-sensitive paper, was affected by a cereal rye cover crop and fall-applied saflufenacil. We also examined deposition at the soil surface relative to the cereal rye row position. In a year with greater cereal rye biomass accumulation, there was 44% less coverage and average deposit size was 45% smaller immediately adjacent to cereal rye rows compared with between rows and areas without cereal rye. Greater variability in these measurements was also noted in this position. Percent spray solution coverage was also 22% greater in plots that received saflufenacil in the fall, and deposits were 28% larger. In a year with less cover crop and winter weed biomass, no differences in spray deposition were observed. This suggests that small horseweed plants and other weeds immediately adjacent to cereal rye cover crop rows may be more likely to survive early spring herbicide applications, though the suppressive effects of cover crops may mitigate this concern.

Winter cover crops (CCs) provide soil conservation benefits for strip-tillage tobacco producers, but soil-residual herbicides may interfere with their establishment and growth. Tobacco is planted later than many agronomic crops, but growers often terminate CCs early to minimize CC residue at planting, and this may reduce weed suppression potential. We examined residual herbicide effects on CCs across two seasons and the potential for CC-based weed suppression within strip-tilled tobacco. Mixtures of wheat plus crimson clover and cereal rye plus crimson clover were examined, with a no-CC control. Herbicides included two rates of PRE sulfentrazone (177 or 354 g ai ha–1) plus carfentrazone (20 or 40 g ai ha–1); the higher rate was also followed by POST clomazone (840 g ai ha–1) or mixed with PRE pendimethalin (1,400 g ai ha–1). Controls with no weed management and hand weeding were also included. CC density and biomass were not reduced by weed management (WM) treatments with residual herbicides. However, CCs did not reduce density of annual grasses, small-seeded broadleaves, or perennials in the tilled in-row or untilled between-row zones. Cereal rye plus crimson clover resulted in lower weed biomass at tobacco harvest in the untilled between-row zone in 2017. WM effects were variable between the years, weed groups, and zones. Adding clomazone or pendimethalin was more consistent for reducing weed density and biomass compared to the low rate of sulfentrazone plus carfentrazone. Tobacco yield was unaffected by CCs in 2017 but lower in some WM treatments in 2018. In this study, tobacco herbicides did not interfere with wheat, cereal rye, or crimson clover establishment, but additional research should determine if these results apply to other environments and soil types. However, when these CC species were terminated 5 to 6 wk before transplanting, they did not consistently contribute to weed control.

Methiozolin is commonly used for the safe and selective removal of annual bluegrass from creeping bentgrass golf greens. Studies were conducted in 2013 and 2014 with the objective of assessing fertility programs consisting of synthetic fertilizers and biostimulants, with and without the plant growth regulator trinexapac-ethyl, to aid putting green canopy recovery following annual bluegrass removal via methiozolin. Additional studies were conducted to compare recovery of creeping bentgrass following an aggressive core aerification event with fertility programs with and without methiozolin. In all cases, the addition of 7 kg ha−1 of N-P-K from fertilizer or biostimulant biweekly to greens increased turfgrass recovery time by 1 to 3 wk compared to a standard green’s fertility program alone. Creeping bentgrass treated with biostimulants recovered equivalent to or quicker than creeping bentgrass treated with synthetic fertilizer (SF) in all cases. In the presence of methiozolin treatments, trinexapac-ethyl reduced time to 90% recovery (T90) by 0.25 to 0.5 wk at two locations, and increased T90 recovery time by 0.1 wk at one location. Otherwise, plots treated with SF plus trinexapac-ethyl were equivalent to plots treated with SF only. Methiozolin slowed turfgrass recovery time at one location where severe drought stress occurred but not at the other location that did not experience drought stress. These results suggest that turf managers should increase fertilizer treatments but will not need to discontinue trinexapac-ethyl use to maximize creeping bentgrass recovery following annual bluegrass control with methiozolin. These data also suggest that methiozolin has the potential to negatively affect creeping bentgrass recovery when drought stress is experienced.

Proso millet is a short-season summer annual grass that is well adapted to the central Great Plains. Proso millet is commonly planted as a summer crop when winter wheat stands are lost due to adverse conditions. Sulfonylurea herbicides labeled for use in winter wheat prohibit planting proso millet for intervals up to 10 mo following application. A series of greenhouse and field studies determined proso millet tolerance to CGA-152005, metsulfuron, and triasulfuron soil residue. In the greenhouse, proso millet was not affected by soil-applied CGA-152005 at doses up to 160 g ai/ha, while metsulfuron and triasulfuron doses of 4 and 15 g ai/ha, respectively, inhibited proso millet biomass accumulation. In the field, metsulfuron and triasulfuron caused early season stunting and chlorosis at doses two to four times those recommended; however, grain yields were not affected. Organic matter and clay content were highly correlated with proso millet growth response to the herbicides under greenhouse conditions, but in the field, soil pH may have influenced herbicide bioavailability.

Field studies were conducted to determine the effects of synthetic auxin herbicides at simulated exposure rates applied to ‘Covington’ sweetpotato propagation beds on the quality of nonrooted stem cuttings (slips). Treatments included diglycolamine salt of dicamba, 2,4-D choline plus nonionic surfactant (NIS), and 2,4-D choline plus glyphosate at 1/10, 1/33, or 1/66 of a 1X application rate (560 g ae ha−1 dicamba, 1,065 g ae ha−1 2,4-D choline, 1,130 g ae ha−1 glyphosate) applied at 2 or 4 wk after first slip harvest (WASH). Injury to sweetpotato 2 wk after treatment was greatest when herbicides were applied 2 WASH (21%) compared to 4 WASH (16%). More slip injury was caused by 2,4-D choline than by dicamba, and the addition of glyphosate did not increase injury over 2,4-D choline alone. Two weeks after the second application, sweetpotato slips were cut 2 cm above the soil surface and transplanted into production fields. In 2019, sweetpotato ground coverage 8 wk after transplanting was reduced 37% and 26% by the 1/10X rates of dicamba and 2,4-D choline plus NIS, respectively. Though dicamba caused less injury to propagation beds than 2,4-D choline with or without glyphosate, after transplanting, slips treated with 1/10X dicamba did not recover as quickly as those treated with 2,4-D choline. In 2020, sweetpotato ground coverage was 90% or greater for all treatments. Dicamba applied 2 WASH decreased marketable sweetpotato storage root yield by 59% compared to the nontreated check, whereas treatments including 2,4-D choline reduced marketable yield 22% to 29%. All herbicides applied at 4 WASH reduced marketable yield 31% to 36%. The addition of glyphosate to 2,4-D choline did not increase sweetpotato yield. Results indicate that caution should be taken when deciding whether to transplant sweetpotato slips that are suspected to have been exposed to dicamba or 2,4-D choline.

Students studying weed science would expand their knowledge by conducting targeted experiments on invasive weeds. Tamarix spp., some of the most problematic weeds known in the United States, have value for weed science education. Tamarix was used in an undergraduate laboratory course to demonstrate weed science principles for a minimal cost and with great potential for academic enhancement. The laboratory exercise was designed to teach weed science students about the difficulty associated with controlling invasive weeds even at a relatively young age in a region where plants have been detected but large-scale invasion has not occurred to emphasize the importance of early detection and rapid response. The successful execution of this exercise and the positive student response suggests that Tamarix and other weeds with similar reproductive capacities could be valuable additions to weed science curricula. Innovative approaches to teaching weed science facilitate greater learning of this complex subject by students from diverse backgrounds and academic disciplines.

Glyphosate and paraquat are effective preplant burndown herbicide options for multicrop vegetable production that uses plastic mulch, but problematic weeds such as wild radish, cutleaf evening primrose, annual morningglory, or horseweed may not be adequately controlled with these herbicides alone. The herbicides 2,4-D and dicamba could help control these troublesome weeds prior to planting if they can be removed from plastic mulch and thus avoid crop damage. Treatments included 2,4-D (1,065 and 2,130 g ae ha−1) and dicamba (560 and 1,120 g ae ha−1) applied broadcast over plastic mulch a day before transplanting. Just before transplanting, treatments received either 0.76 cm of water via overhead irrigation or no irrigation. Plastic mulch samples were collected at application and planting to determine herbicide presence using analytical techniques, and cantaloupe and zucchini squash were subsequently transplanted on the plastic beds. Analytical ultra-high performance liquid chromatography revealed that 88% to 99% of the initial herbicide concentration was present at crop planting when irrigation was not implemented. At most, a 1/50 rate of dicamba and a 1/500 rate of 2,4-D was present at planting when overhead irrigation was applied prior to transplanting. Maximum cantaloupe and squash injury from 2,4-D with irrigation was 10% and did not influence plant growth, biomass, or yield. For dicamba with overhead irrigation, cantaloupe injury was 35%, vine lengths were reduced by 24%, and maturity was delayed, whereas squash injury ranged from 9% to 12%, with no influence on growth or yield. Without irrigation to wash herbicides from the mulch prior to planting, 60% to 100% injury of both crops occurred with both herbicides. Zucchini squash was more tolerant to dicamba than cantaloupe. Results demonstrated that 2,4-D can be adequately removed from the surface of plastic mulch with irrigation, whereas a single irrigation event was not sufficient to remove dicamba.

Knotroot foxtail has become more prevalent and problematic in pastures and hayfields in the southeastern United States. Gaps exist in our knowledge of which herbicide practices are best for managing this species in bermudagrass forage production. This study was conducted to determine the efficacy of various ways to control knotroot foxtail in bermudagrass with herbicide applications in autumn, postemergence (POST), with and without also applying a herbicide in preemergence (PRE), in spring. The study was a randomized complete block with a factorial arrangement of treatments and included a nontreated control for both fall and spring timings. Glyphosate at two rates (0.35 or 0.7 kg ae ha−1), nicosulfuron (0.07 kg ai ha−1) + metsulfuron (0.012 kg ai ha−1), and hexazinone (1.3 kg ai ha−1) were applied alone in the fall or followed by indaziflam (0.067 kg ai ha−1) or pendimethalin (4.46 kg ai ha−1) in the spring. Three harvests were conducted throughout the growing season to evaluate weed species (knotroot foxtail, large crabgrass, and horsenettle) and bermudagrass biomass as well as overall species composition. The combination of fall and spring treatments did not affect weed species or bermudagrass biomass. Therefore, treatment main effects were analyzed by fall or spring application timing. A spring application of either pendimethalin or indaziflam increased bermudagrass biomass compared with that of the nontreated control. However, neither PRE herbicide effectively reduced knotroot foxtail biomass compared with the nontreated control, although pendimethalin did reduce season-long knotroot foxtail composition. Spring PRE herbicides are an effective tool for forage producers, but further research is needed to identify effective herbicides and additional approaches for the control of knotroot foxtail.

Southern crabgrass is a common turfgrass weed throughout the United States, and in Florida a troublesome problem in St. Augustinegrass lawns. Because of herbicide label changes and lack of herbicide tolerance, no POST herbicides are currently registered for homeowner use for crabgrass control in residential and commercial St. Augustinegrass lawns. Alternative weed control methods, including cultural practices and unconventional herbicides, have been investigated to a limited extent for postemergence crabgrass control. In this study, alternative herbicides evaluated included 30% acetic acid, borax, sodium bicarbonate, and Garden Weasel AG Crabgrass Killer (sodium bicarbonate formulation including cinnamon, wheat and corn flour, and cumin). Treatments were applied to southern crabgrass at three growth stages (one to two leaf, three to four leaf, or one to two tiller). In the greenhouse, 30% acetic acid applied twice at 280 L ha−1 and two rates of AG Crabgrass Killer at 976 or 1,465 kg ha−1 provided ≥ 70% control of one to two–leaf southern crabgrass when evaluated 7 d after initial application (DAIA). No treatment provided > 70% control of three to four–leaf or one to two–tiller southern crabgrass or any size crabgrass beyond 7 DAIA. In field trials, no treatment provided acceptable (≥ 70%) southern crabgrass control at any weed stage. Initial turf injury was unacceptable for most rates of 30% acetic acid, sodium bicarbonate, and AG Crabgrass Killer, causing > 20% St. Augustinegrass injury 7 DAIA. By 21 DAIA, turf injury levels had fallen to ≤ 20% for most treatments. Because of high turfgrass injury and little residual control, alternative herbicides tested are not an effective substitute for using preemergence herbicides for southern crabgrass control. If other options are not available, they may have a role as a limited spot treatment in St. Augustinegrass.

The invasion of waterhemp into northern sugarbeet growing regions has prompted producers to re-integrate inter-row cultivation into weed management programs, as no currently registered herbicides can control glyphosate-resistant waterhemp POST in crop. Inter-row cultivation was a common weed control practice in sugarbeet until the release of glyphosate-resistant sugarbeet cultivars in 2008 made the use of inter-row cultivation unnecessary. In the late 2010s, producers began again to use inter-row cultivation to remove weeds that glyphosate did not control, but producers need information on the effectiveness and safety of inter-row cultivation when used with soil-residual herbicide programs. Efficacy and tolerance field experiments were conducted in Minnesota and North Dakota from 2017 to 2019. Results from the efficacy experiment demonstrated that cultivation improved waterhemp control 11% and 12%, 14 and 28 d after treatment, respectively. Waterhemp response to cultivation was dependent on crop canopy and precipitation after cultivation. Cultivation had minimal effect on waterhemp density in three environments, but at one environment, near Galchutt, ND in 2019, waterhemp density increased 600% and 196%, 14 and 28 d after treatment, respectively. Climate data indicated that in 2019 Galchutt, ND received 105 mm of precipitation in the 14 d following cultivation and had an open crop canopy that probably contributed to further weed emergence. Results from the tolerance experiment demonstrated that root yield and recoverable sucrose were not affected by cultivation timing or number of cultivations. In one environment, cultivating reduced sucrose content by 0.8% regardless of date or cultivation number, but no differences were found in four environments. Damage/destruction of leaf tissue from in-season cultivation is probably responsible for the reduction in sucrose content. Results indicate that cultivation can be a valuable tool to control weeds that herbicide cannot, but excessive rainfall and open crop canopy following cultivation can create an environment conducive to further weed emergence.

Alligatorweed is subject to an eradication program in Victoria, Australia. In aquatic situations, the herbicides glyphosate and metsulfuron are used. Alligatorweed has been shown to break up soon after the application of these herbicides, resulting in the production of many stem fragments that are viable and capable of downstream colonization, compromising the effectiveness of the eradication program. This paper reports on an experiment to investigate the usefulness of commercially available plant growth regulators (PGRs) in reducing the number of viable propagules produced post-herbicide application. Three herbicide treatments (no herbicide, glyphosate, and metsulfuron) and four PGR treatments (no PGR, aviglycine [AVG], naphthalene acetic acid [NAA], and 2,4-D) were investigated in a factorial experiment. Chemicals were applied to alligatorweed growing in separate aquaria, the resulting stem fragments were collected and counted, and a subset was tested for viability. There was no evidence of PGRs having any effect on the total number of viable stem fragments produced. However, AVG reduced the total number of fragments produced. PGRs in combination with herbicide treatment had an antagonistic effect on the efficacy of the herbicides. PGRs increased belowground biomass of alligatorweed, as well as the number of apical growing tips present. Results indicate that although PGRs, particularly AVG, may be of benefit in reducing the number of alligatorweed propagules produced post-herbicide application, at the application rates tested here there would be no benefit from incorporating them into herbicide control programs for alligatorweed.

Ammonium nonanoate is registered for weed control in certified organic cropping systems and may be useful to control cool-season weeds in organic Vidalia® sweet onion production. Ammonium nonanoate combined with tine-weeder cultivation was evaluated for weed control in organic onion in Georgia. There were no statistical interactions between main effects of herbicides and cultivation with a tine weeder for cool-season weed control and onion yield, indicating that ammonium nonanoate does not improve weed control compared with cultivation. Ammonium nonanoate at 4% and 6% did not adequately control weeds and onion yields were reduced. Ammonium nonanoate at 8% and 10% controlled cutleaf evening-primrose and lesser swinecress equal to the standard of d-limonene (14%), but the degree of control did not consistently protect onion yields from losses due to weeds. These results are in agreement with previous studies using clove oil and pelargonic acid. There is no advantage to using ammonium nonanoate for cool-season weed control in organic Vidalia® sweet onion production.

Yellow mignonette (Reseda lutea (L.) # RESLU) is a broadleaf perennial weed that is native to the Mediterranean Basin and Asia Minor. It has been spread around the world and is widely recognized in Australia as a troublesome noxious weed in croplands. It is a problem in cropland and pastures because of its high seed output, extensive reproductive root system, ability to thrive under disturbed conditions, and high nitrate levels. While nitrate levels of 2.5 to 3.1% have been detected in plants during the rosette to early flower growth stages, when yellow mignonette is most palatable, no livestock injury or deaths have been reported. Yellow mignonette was first reported in Montana in 1958 but was not recognized as a weed problem until 1990. Since 1988 it has invaded several hundred hectares of small grains and alfalfa (Medicago sativa L.)-grass pastures in Central Montana and is spreading rapidly along gravel road shoulders.