Weed efficacy

Weed species response to dimethenamid–p (0.75%) + pendimethalin (1%) granular herbicide was evaluated at the Valley Laboratory of the Connecticut Agricultural Experiment Station in Windsor, CT. The experiment was initiated on June 22, 2017 and was repeated on May 25, 2018. Twenty-four flat trays (52 × 26 × 6 cm) were filled with Pro-Mix Premium All Purpose planting media (200 Kelly Rd, Quakertown, PA 18951). Pro-Mix Premium All Purpose contains Canadian sphagnum peat moss (80-90%), peat humus, perlite, limestone, and mycorrhizae PTB297 technology. Trays were watered using 1.2 cm (0.5 in) overhead mist irrigation and the substrate was allowed to settle for two days. On June 24, 2017 and May 27, 2018, dimethenamid–p + pendimethalin (Freehand^®; BASF Corp., Research Triangle Park, NC) granular herbicide was applied at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) with a hand-held shaker bottle. Each dimethenamid–p + pendimethalin treatment was applied to six flat trays; three for the broadleaf weed species mixture and three for the grassy weed species mixture. After herbicide application, trays were placed back in a greenhouse under the overhead mist irrigation system and 1.2 cm irrigation was applied. Approximately 4-hr after overhead irrigation, 50 seeds each of fringed willowherb (Epilobium ciliatum Raf. ssp. ciliatum), hairy bittercress (Cardamine hirsuta L.), redroot pigweed (Amaranthus retroflexus L.), and yellow woodsorrel (Oxalis stricta L.) were mixed together and applied to the surface of twelve flat trays with a shaker bottle. Similarly, 50 seeds of each of barnyardgrass [Echinochloa crus-galli (L.) Beauv.], giant foxtail (Setaria faberi Hermm.), goosegrass [Elusine indica (L.) Gaertn.], and large crabgrass [Digitaria sangunalis (L.) Scop.] were applied to the surface of a different set of twelve flat trays. A nontreated control (three trays) was also included, separately for broadleaf and grassy weeds, for treatment comparison. The experiment was established in a completely randomized design with three replications per treatment. Jacks Professional General Purpose (JR Peters Inc., Allentown, PA) soluble fertilizer (20N– 20P₂O₅ –20K₂O) was applied weekly at 300 ppm beginning two weeks after weed emergence until harvested for fresh biomass. An overhead mist irrigation of 1.2 cm was applied daily in four cycles of 4 min each with a 3 h gap between cycles. Weed control was evaluated by separately counting the weed species at 4 and 8 wk after seeding (WAS) and data were converted to percent control compared to the nontreated control. At 8 WAS, all weeds, where present, were harvested from each flat tray and the combined shoot fresh biomass was recorded.

Ornamental plant tolerance

The granular herbicide dimethenamid–p (0.75%) + pendimethalin (1%) was evaluated for crop safety on a variety of ornamental species. The experiments were conducted at the Valley Laboratory of the Connecticut Agricultural Experiment Station in Windsor, CT in 2017 and 2018. Three ornamental plant species evaluated for tolerance to dimethenamid–p + pendimethalin were: chocolate flower (Berlandiara lyrata Benth.), leucothoe [Leucothe fontanesiana (Steudel) Sleumer ‘Rainbow'], and pygmyweed [Crassula radicans (Haw.) D. Dietr. ‘Red Carpet']. Chocolate flower is an herbaceous perennial prized for its sweet, chocolate-scented flowers. It is commonly grown as a border plant or as a ground cover. Leucothoe is an evergreen, low-growing, perennial shrub which is valued for its variegated foliage and pollinator attraction. Leucothoe is planted as a groundcover or a border plant in the landscape areas. Pygmyweed is a perennial, mat-forming succulent that is commonly grown as a ground cover in landscapes.

Plugs of leucothoe [15-cm tall (6 in)], chocolate flower [10-cm tall (4 in)], and pygmyweed [6-cm tall 2.4 in)] were planted. All ornamental plant species were transplanted on June 9, 2017 into 2.9 L (3 qt) (C350; Nursery Supplies Inc., Chambersburg, PA) containers filled with composted pine bark, leaf compost, and woodchips (2:1:1) mixture by volume. The potting substrate was amended (per 0.76 m³ or 1 yd³) with 3.74 kg (8.24 lb) 20N-4P-8K controlled-release fertilizer (Harrells Profertilizer; Harrells LLC, Lakeland, FL), 0.15 kg (0.33 lb) booster micronutrients (Harrells LLC, Lakeland, FL), and 2.27 kg (5.0 lb) dolomitic limestone (Plant Products LLC, Findley, OH). Containers were kept on an outdoor gravel pad. In 2018, the experiments were repeated and the ornamental species were transplanted on May 23, 2018 into the same sized containers and similar substrates as previously described.

The experiment was arranged in a completely randomized design with 12 replications per treatment. Dimethenamid–p + pendimethalin granular herbicide was applied within 7 d after transplanting at 0, 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), with a hand-held shaker bottle and a second application was made approximately 6 wk after the initial application. These rates were adopted from a previous IR-4 crop safety research protocol. The maximum single application rate for dimethenamid–p + pendimethalin granular herbicide is 3.92 kg ai^.ha^–1 (3.50 lb ai^.A^–1). The 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) rates are above the maximum labeled rates. All plants received 1.2-cm irrigation within an hour of treatment application and daily afterwards. Phyto-toxicity ratings for chlorosis, necrosis, and stunting injury were recorded at 4 wk after each application on a 0 to 10 scale with 0 = no damage, 1 = minor (10%), 2 = moderate (20%), 2–4 = severe (20% to 40%), 5–9 = extreme (50% to 90%), and 10 = dead plant. Plant height and average width (width at the widest point + perpendicular width / 2) were recorded at 6 wk after the second application.

Statistical analyses

Data on various response variables were analyzed with a generalized linear mixed model methodology using the GLIMMIX procedure in SAS (Version 9.3; SAS Institute, Inc., Cary, NC). Before the ANOVA test, data were tested for normality using PROC UNIVARIATE and homogeneity of variance with the modified Levene test. The weed efficacy and fresh biomass data were analyzed separately by wk after treatment (WAT). The weed efficacy data were arcsine-transformed and the fresh biomass data were square-root transformed for correcting non-normality and heterogeneity of variance. However for simplicity, the back transformed means are discussed and presented in the tables. Ornamental plant injury data from the first and the second application were analyzed separately. Year and dimethenamid–p + pendimethalin rate were treated as fixed effects whereas replication and its interactions with fixed effect factors were considered as random effects. Means were separated with Fisher's protected least square difference at α = 0.05.

Broadleaf weed control

The herbicide rate x broadleaf weed species interaction was significant (p=0.011), which suggested that control varied with the weed species and dimethenamid–p + pendimethalin rate. At 4 WAT, hairy bittercress and redroot pigweed were controlled 97% to 99% without a significant dimethenamid–p + pendimethalin rate effect (Table 1). However, fringed willowherb and yellow woodsorrel control increased significantly from 93% at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), to 99% at ≥5.88 kg ai^.ha^–1 (≥5.25lb ai^.A^–1). At 8 WAT, all tested weed species were still controlled ≥87% with dimethenamid–p + pendimethalin at 2.94 kg ai.ha^–1 (2.62 lb ai.A^–1). At 5.88 kg ai^.ha^–1 (5.25lb ai^.A^–1) or higher rates, control was in the 94% to 99% range.

Table 1 Percent broadleaf weed control 4 and 8 wk after application of dimethenamid–p (0.75%) + pendimethalin (1%) granular herbicide^z.

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Dimethenamid–p + pendimethalin efficacy may vary with the substrate type (Robertson and Derr 2017). Stewart et al. (2019) reported differential control of yellow woodsorrel with dimethenamid–p in different substrates. In one experiment they achieved excellent yellow woodsorrel control regardless of substrate type. However, in another similar experiment they reported differences in ED₈₀ values for dimethenamid–p across substrates (peat, 0-month aged pine bark, and standard pine bark) for yellow woodsorrel control. The ED₈₀ was significantly higher in both peat (ED80= 2.82 kg ai^.ha^–1 or 2.51 lb ai.A^–1) and 0-month (ED80= 3.06 kg ai^.ha^–1 or 2.72 lb ai^.A^–1) substrates compared with the standard (ED80= 1.86 kg^.ai^.ha^–1 or 1.66 lb ai^.A^–1), indicating that a higher rate of dimethenamid–p was needed in peat and the 0-month pine bark substrate. Wallace and Hodges (2007) observed variable control of Palmer amaranth [Amaranthus palmeri (L.) Swat], a similar pigweed species in the Amaranthaceae family, with dimethenamid–p in field-grown ‘Red President' cannas. Palmer amaranth control decreased from 90% at 6 WAT to 43% at 10 WAT in 2003 but increased from 58% to 95% in 2004. Greater than 90% control of problematic broadleaf weed species such as kochia (Kochia scoparia L.) and Palmer amaranth has been reported with dimethenamid–p or dimethenamid–p + pendimethalin (Kumar and Jha 2015a; Kouame et al. 2022).

The combined broadleaf weed fresh biomass data at 8 WAT conformed to the percent weed control results. The average combined broadleaf weed fresh biomass in the nontreated control was 156 g per tray. The average fresh weed biomass was 24.6, 6.2, and 1.2 g per tray with dimethenamid–p + pendimethalin at 2.94 kg ai^.ha^–1 (2.62 lb ai.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1). This was 84%, 96%, and 99% less compared to the nontreated control. Excellent (>81%) control of hairy bittercress has earlier been reported 90 d after pendimethalin application at 1.1 kg ai^.ha^–1 (1.0 lb ai^.A^–1) in a pot-in-pot production in Mississippi (Fare et al. 2005). Comparatively in the current study, there were 1.68 kg ai^.ha^–1 (1.5 lb ai^.A^–1) of pendimethalin in 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) of dimethenamid–p + pendimethalin.

Grassy weed control

No differences were observed in the level of control for any grassy weed species with dimethenamid–p + pendimethalin rate at 4 WAT (Table 2). All grassy weeds were controlled 98% or higher with dimethenamid–p + pendimethalin at ≥2.94 kg ai^.ha^–1 (≥2.62 lb ai^.A^–1). However at 8 WAT, an herbicide rate effect was significant (p=0.036). Control decreased to 82% for goosegrass, 86% for barnyardgrass, 89% for large crabgrass, and 91% for the giant foxtail with 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1). All grassy weeds were still in the 94% to 99% control range without differences between the 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1) and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) rates.

Table 2 Percent grassy weed control 4 and 8 wk after application of dimethenamid–p (0.75%) + pendimethalin (1%) granular herbicide^z.

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Previously, Robertson and Derr (2017) reported excellent control of southern crabgrass [Digitaria ciliaris (Retz.) Koel.] with dimethenamid–p applied at 1.7 kg ai^.ha^–1 (1.52 lb ai^.A^–1), pendimethalin at 3.7 kg ai^.ha^–1 (3.3 lb ai^.A^–1), and dimethenamid–p + pendimethalin at 3.9 kg ai^.ha^–1(3.47 lb ai^.A^–1). They also reported higher smooth crabgrass control with dimethenamid–p at 1.7 kg ai^.ha^–1 (1.52 lb ai^.A^–1) in a pine bark substrate than in the field soil. In contrast, pendimethalin at 3.7 kg ai^.ha^–1 (3.3 lb ai^.A^–1) or dimethenamid–p + pendimethalin at 3.9 kg ai.ha^–1 (3.47 lb ai^.A^–1) were more effective on smooth crabgrass in field soil. This was attributed to increased potential for dimethenamid–p leaching in field soil than pine bark (Robertson and Derr 2017) while pendimethalin leached more in pine bark than field soil. Patrick et al. (2017) reported differences in goosegrass control with dimethenamid–p applied at 1.68 kg ai^.ha^–1 (1.50 lb ai^.A^–1). At 7 months after treatment, goosegrass control was <50% in 2011 but ≥90% in 2012. In the current study, dimethenamid–p rates were 1.277 kg ai^.ha^–1 (1.14 lb ai^.A^–1), and 5.107 kg ai^.ha^–1 (4.56 lb ai^.A^–1), in 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), and 11.77 kg ai^.ha^–1(10.5 lb ai^.A^–1) of dimethenamid–p + pendimethalin, respectively.

The fresh biomass data at 8 WAT showed similar herbicide rate differences (p=0.042). The average combined fresh biomass at 8 WAT was 327 g per tray for the nontreated control. The fresh biomass was 54.8, 9.6, and 2.1 g per tray with dimethenamid–p + pendimethalin at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively. This was 83%, 97%, and 99% less compared to the nontreated control.

Ornamental plant tolerance

For chocolate flower, a year by dimethenamid–p + pendimethalin rate effect was highly significant (p<0.0153). Injury occurred only in 2017 and was rated the highest at 4 wk after the first and second application (Table 3). Following the first application, the injury symptoms included necrotic spots on leaves, leaf malformation (crinkling of the leaves), and growth stunting. Injury form dimethenamid–p + pendimethalin applied at 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) was rated 2.5 (25%) which was higher than 0.7 (7%) and 1.5 (15%) with 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) and 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), respectively (Table 3). Following the second application, only stunting injury was observed and was rated 1.4 (14%), 1.6 (16%), and 2.0 (20%) with 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively (Table 3). These results showed that dimethenamid–p + pendimethalin at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) or 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1) caused little to no injury to newly transplanted chocolate flower. The highest rate of 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) was marginally injurious. An injury rating of 2.0 (20%) on a 0 to 10 rating scale is considered to be mild and is commercially acceptable.

Table 3 Ornamental plant injury 4 wk after the first and second application of dimethenamid–p (0.75%) + pendimethalin (1%) granular herbicide^z.

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Final plant height and width data also revealed a similar response of chocolate flower to different rates of dimethenamid–p + pendimethalin. Averaged over years, the final plant height measurements were: 36 cm, 35 cm, 33 cm, and 29 cm with dimethenamid–p + pendimethalin rates of 0 kg ai^.ha^–1 (nontreated control), 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively (Table 4). Similarly, the final width measurements were: 23 cm, 25 cm, 22 cm, and 20 cm with dimethenamid–p + pendimethalin rates of 0 kg ai^.ha^–1 (nontreated control), 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively (Table 4).

Table 4 Final plant height and width of ornamental plant species 6 wk after the second application of dimethenamid–p (0.75%) + pendimethalin (1%) granular herbicide^z.

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As for leucothoe, no chlorotic, necrotic, or stunting injury was observed in either experimental year. This showed that dimethenamid–p + pendimethalin granular herbicide did not injure leucothoe at rates up to 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) when two sequential applications were made at 6 wk intervals. Final plant height and width data (Table 4) also conformed to the visual injury ratings. Averaged over years and dimethenamid–p + pendimethalin rates, final plant height and width measurements for leucothoe were 27 cm and 24 cm, respectively.

Injury to pygmyweed from dimethenamid–p + pendimethalin granular herbicide was significant (p=0.0017). Injury symptoms comprised necrotic lesions on leaves, malformation of terminal buds, reddening of leaves and stems, and stunted growth. Injury was the highest at 4 wk after each application (Table 3). Dimethenamid–p + pendimethalin applied at 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1) was the most injurious treatment with the severe injury ratings of 4.2 (42%) and 4.7 (47%) following the first and second application, respectively. This was significantly higher than injury ratings of 0.5 (5%) and 0.8 (8%) with 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) and injury ratings of 2.7 (27%) and 3.1 (31%) with 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1) following the first and the second application, respectively. Similarly, dimethenamid–p + pendimethalin at 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1) was more injurious to pygmyweed than at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1). In this study, two applications of dimethenamid–p + pendimethalin at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) made at 6 wk interval caused little to no injury to pigmyweed. This rate was lower than the maximum single application rate of 3.92 kg ai^.ha^–1 (3.50 lb ai^.A^–1). For an ornamental plant to be considered tolerant it must tolerate at least two-time the maximum single application rate of an herbicide (to allow for application overlaps). Therefore, dimethenamid–p + pendimethalin granular herbicide failed in meeting the minimum crop tolerance requirement to be considered safe on pygmyweed.

Similar trends were observed in pygmyweed final plant height and width measurements with different rates of dimethenamid–p + pendimethalin granular herbicide (Table 4). Averaged over years, final plant height measurements were: 13 cm, 13 cm, 9 cm, and 8 cm with dimethenamid–p + pendimethalin rates of 0 kg ai^.ha^–1 (nontreated control), 2.94 kg ai.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively. Similarly, the final width measurements were: 12 cm, 12 cm, 9 cm, and 9 cm with dimethenamid–p + pendimethalin rates of 0 kg ai^.ha^–1 (nontreated control), 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1), 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1), and 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1), respectively.

This study showed that the granular herbicide dimethenamid–p + pendimethalin at 2.94 kg ai^.ha^–1 (2.62 lb ai^.A^–1) can effectively control tested grassy and broadleaf weeds without risk for significant injury to chocolate flower and ‘Rainbow' leucothoe. This rate was 0.98 kg ai^.ha^–1 (0.88 lb ai^.A^–1), lower than the maximum single application rate of 3.92 kg ai^.ha^–1 (3.50 lb ai^.A^–1). The control of the tested weed species improved with dimethenamid–p + pendimethalin rates ≥5.88 kg ai^.ha^–1 (≥5.25 lb ai^.A^–1) but higher rates caused unacceptable injury to some of the ornamental species tested. For example, leucothoe var. ‘Rainbow' was extremely tolerant to dimethenamid–p + pendimethalin rates up to 11.77 kg ai^.ha^–1 (10.5 lb ai^.A^–1). In chocolate flower, the injury levels were within the commercially acceptable limits with dimethenamid–p + pendimethalin rates up to 5.88 kg ai^.ha^–1 (5.25 lb ai^.A^–1). For pygmyweed var. ‘Red carpet', the dimethenamid–p + pendimethalin was highly injurious and therefore, should be avoided.