Container experiment

Topramezone was evaluated in an outdoor container experiment for over-the-top (OTT) tolerance of balsam fir, canaan fir, Colorado blue spruce, Douglas fir, Fraser fir, Nordman fir, Norway Spruce, and white pine at the Connecticut Agricultural Experiment Station (CAES) in Windsor, CT during summer 2022 and 2023. In spring 2021, bare-rooted seedlings of the tested Christmas tree species (2 + 1) were transplanted into 11.4 L (3 gal) plastic containers containing a pine bark and composted woodchips (1:1) mixture. The potting media was amended with 4.75 kg^.m⁻³ (8 lb^.yard⁻³) 20N-4P-8K controlled-release fertilizer (Harrells Profertilizer; Harrells LLC, Lakeland, Florida, USA), 0.18 kg^.m⁻³ (0.3 lb^.yard⁻³) micronutrients (Harrells LLC), and 2.97 kg^.m⁻³ (5 lb^.yard⁻³) dolomitic limestone (Plant Products LLC, Findley, OH, USA). Pots were kept on an outdoor gravel pad and received overhead 1.25 cm (0.5 inch) irrigation daily in four cycles of 4 mins each with 3-hour interval between cycles. A controlled-release fertilizer 20N-4P-8K was applied at 28 g (1 oz) per container each spring (2022 and 2023) and all weeds were removed manually. The experimental design was a factorial combination of eight Christmas tree species and three topramezone (Frequency^®; BASF Corporation LLC., Research Triangle Park, NC) rates: 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹), 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹), or 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹). There were six plant replicates at each topramezone rate for each Christmas tree species. A nontreated control (six plants per species) was also included for treatment comparison. Topramezone treatments were prepared in tap water mixed with methylated seed oil (Dyne-Amic, Helena Chemical Co., Collierville, TN) on a 1% v/v basis. Christmas trees, depending upon the species, were 4-yr old or 25 to 53 cm (10 to 21 inches) in height in 2022 and 5-yr old or 29 to 65 cm (12 to 26 inches) in height in 2023 at the treatment application time. Treatments were applied over-the-top (OTT), allowing full contact with the new growth (∼4-week-old) on May 27, in 2022, and on June 7, in 2023. A compressed CO₂ research plot sprayer equipped with a single flat-fan Teejet 8002 nozzle (TeeJet Technologies, Springfield, IL) calibrated to deliver 187 L·ha⁻¹ (20 gal·A⁻¹) at 207 kPa and 4.8 kmph (3.0 mph) was used. Plants were allowed to dry by withholding overhead irrigation for about 4 hours after treatment application. Christmas tree injury was assessed visually at weekly intervals using a scale ranging from 0 (no injury) to 10 (dead plant). Christmas tree injury estimates were based on chlorosis, necrosis, and stunting of the new growth of the treated plants compared with the nontreated control plants.

Field experiment

A 2-year field experiment was conducted at a commercial Christmas tree farm in Enfield, CT (41°57.37N, 72°31.33W) in 2022 and 2023. The soil at the experiment site was an Agawam fine sandy loam with 21% silt, 62% sand, 17% clay, 2.7% organic matter, and 5.4 pH. The bare-rooted Fraser × balsam hybrid fir seedlings (2 + 1) were transplanted at a spacing of 1.8 m (6 feet) between rows and 1.5 m (5 feet) between plants in the spring of 2021. The experiment was established in a randomized complete block design with four replications. Each experimental plot consisted of two rows of four plants each. During both 2022 and 2023 springs, no PRE herbicide treatment was made before POST application of topramezone. Treatments comprised of a factorial combination of three topramezone rates: 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹), 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹), or 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) and two application methods: OTT or semi-directed (SD). A nontreated control was included for treatment comparison. On June 3, in 2022, and June 9, in 2023, topramezone was applied OTT of Fraser × balsam hybrid transplants with a single flat-fan Teejet 8002 nozzle or semi-directed (SD) with an OC-2 nozzle (TeeJet Technologies, Springfield, IL) using a compressed CO₂ research plot sprayer calibrated to deliver 187 L·ha⁻¹ (20 gal·A⁻¹) at 207 kPa and 4.8 kmph (3.0 mph). Topramezone treatments were prepared in tap water mixed with methylated seed oil on a 1% v/v basis (Dyne-Amic, Helena Chemical Co., Collierville, TN). The OTT treatments were applied as a single pass over each tree row. The SD treatments were applied on both sides of each tree row with spray contacting about 10 cm (4 inches) basal tree growth. Weeds were treated OTT both between and within the rows. Common ragweed in the 4- to 6-leaf stage, horseweed 15 to 20 cm (6 to 8 inch) tall, fall panicum in the 3- to 4-leaf stage, large crabgrass in the 4- to 5-leaf stage, and yellow foxtail in the 4- to 5-leaf stage were the predominant weed species during both experiment years. Weed control and Christmas tree injury were assessed visually at 4, 8, and 12 weeks after treatment (WAT) using a scale ranging from 0 (no control) to 100% (complete control) for weed control and a scale of 0 (no injury) to 10 (dead plant) for injury. Christmas tree leader length (cm) in the field experiment was recorded from all plants in a plot at 12 WAT. Visual weed control assessments were based on chlorosis, necrosis, and stunting of treated weeds compared with the weeds in the nontreated control plots. Christmas tree injury estimates were based on chlorosis, necrosis, and stunting of the new growth of the treated trees compared with the trees in the nontreated control plot.

Statistical analyses

Data were subjected to ANOVA using the PROC GLIMMIX procedure in SAS (Version 9.4, SAS Institute, Cary, NC). Year, topramezone rate, and year × topramezone rate were treated as fixed effects whereas the replication and its interaction with the fixed effects were treated as random effects. Before the ANOVA test, data were tested for normality using PROC UNIVARIATE and homogeneity of variance with the modified Levene test. When the topramezone rate by year effect was nonsignificant, data were combined over years. Weed control data were arcsine square root–transformed to improve the normality and homogeneity of variance assumptions, but the non-transformed means are presented in the tables. The topramezone rate main effect was significant (p < 0.05) for the weed control data, indicating an improvement as the topramezone rate increased from 49-196 g^.ha⁻¹ (0.04 to 0.17 lb^.A⁻¹. Multiple means comparisons of significant effects were made using the “Adj = simulate” option in SAS PROC GLIMMIX at the 5% significance level.

Fraser × balsam hybrid tolerance

No chlorotic, necrotic, or stunting injury was reported during both experimental years. Topramezone did not cause injury to Fraser × balsam hybrid regardless of the application method and rate used (data not shown). Analysis of the final leader length data at 12 WAT indicated that only the year effect was significant (p = 0.031). There were no differences in leader length in response to topramezone rates averaged across application methods and rates (Table 1).

Table 1.Fraser × balsam hybrid leader length at 12 WAT^z in Enfield, CT in 2022 and 2023 following applications of topramezone.

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Annual grass control

The topramezone rate main effect was significant (p < 0.05), indicating an improvement in weed control as the topramezone rate increased from 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹) to 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹). In general, the topramezone rate labelled for use in Christmas trees was highly effective in controlling fall panicum and large crabgrass. For yellow foxtail control, the highest tested rate (196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹) was relatively more effective. This suggests that sequential POST topramezone applications at labelled rates (98 g ai·ha⁻¹ or 0.09 lb ai·A⁻¹) may be needed for effective control of yellow foxtail.

Topramezone was highly effective on fall panicum, with control ranging from 88 to 93% at 4 WAT as the topramezone rate increased from 49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹ to 196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹ (Table 2). All topramezone rates tested in this study were similar, with 92 to 98% control at 8 WAT and 93 to 98% control at 12 WAT. Fall panicum density data at 12 WAT showed > 94% reduction (0 to 1 plants per m²) with topramezone rates of ≥ 49 g ai·ha⁻¹ (≥0.04 lb ai·A⁻¹) when compared with the nontreated weedy check (17 plants per m²) (Table 2). These results are consistent with Rodriguez et al. (2023), who previously reported 93, 89, and 91% fall panicum control in sweet corn at 14, 28, and 42 d after POST application of topramezone at 25 g ai·ha⁻¹ (0.02 lb ai·A⁻¹). Negrisoli et al. (2020) observed 86 and 91% fall panicum control at 2 WAT which declined to 23 and 44% by 10 WAT with topramezone rates of 25 g ai·ha⁻¹ (0.02 lb ai·A⁻¹) and 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹), respectively.

Table 2.Common ragweed, horseweed, fall panicum, large crabgrass, and yellow foxtail control at 4, 8, and 12 WAT^z in Enfield, CT following postemergence applications of topramezone.

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For large crabgrass control, topramezone rates of 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹) and 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) were similar with approximately 88% control at 4 WAT and ≥ 96% control at 8 and 12 WAT. For topramezone applied at 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹), large crabgrass control was comparatively less, ranging from 73 to 84% between 4 and 12 WAT. (Table 2). Large crabgrass density data at 12 WAT was consistent with the control data. Compared to the nontreated weedy check (43 plants per m²), 76 to 99% reduction in density of large crabgrass (0 to 11 plant per m²) was observed as topramezone rate increased from 49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹ to 196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹ (Table 3). In a previous study, fall panicum, large crabgrass, and yellow foxtail were controlled 77, 85, and 96% at the 2- to 3-leaf stage and 62, 84, and 88% at the 5- to 6-leaf stage with topramezone at 49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹ (Soltani at al. 2012).

Table 3.Common ragweed, horseweed, fall panicum, large crabgrass, and yellow foxtail densities at 12 WAT^z in Enfield, CT following postemergence applications of topramezone.

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Among all weed species tested, topramezone was relatively less effective on yellow foxtail. Control of yellow foxtail ranged from 61 to 68% at 4 WAT, 66 to 89% at 8 WAT, and 45 to 85% at 12 WAT, respectively, with an increase in topramezone rate from 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹) to 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹). It appears that an additional topramezone application at the labelled rate (98 g ai·ha⁻¹ or 0.09 lb ai·A⁻¹) will be required around 8 weeks after initial treatment (WAIT) for excellent season-long control of yellow foxtail. Consistent with visual control ratings, topramezone applied at 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹) to 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) significantly reduced yellow foxtail density (7 to 13 plants per m²) at 12 WAT as compared to nontreated weedy check (31 plants per m²) (Table 3). Zollinger and Ries (2006) reported variable control of yellow foxtail with different HPPD-inhibitor herbicides. In that study, topramezone (92%) and tembotrione (88%) were found more effective in yellow foxtail control than mesotrione (65%).

Broadleaf weed control

Horseweed response to topramezone differed with the application rate (Table 2). At 4 WAT, the maximum control (79%) occurred with the highest tested rate (196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹), which was higher than the 66% control with the labelled rate (98 g ai·ha⁻¹ or 0.09 lb ai·A⁻¹). As the season progressed, horseweed was controlled > 95% with topramezone applied at 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹) and 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) rates. Whereas with the lowest tested rate (49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹), control increased to 83% at 8 WAT and declined again to 79% at 12 WAT. Improvement in horseweed control by 8 WAT with topramezone at 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹) or 49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹ may be attributed to deterioration in injury symptoms from mainly chlorotic injury and stunting at 4 WAT to necrotic injury at 8 WAT. Consistent with visual control ratings, topramezone at 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹) to 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) lowered horseweed density 83 to 100% at 12 WAT as compared to the nontreated weedy check (69 plants per m²) (Table 3). Horseweed control may vary with the topramezone rate and horseweed size at the time of POST treatment. For instance, Fluttert et al. (2022) reported 85, 87, and 89% control of 4-inch-tall glyphosate-resistant horseweed with 12.5 g ai·ha⁻¹ (0.01 lb ai·A⁻¹) topramezone at 2, 4, and 8 WAT, respectively. Our results are also consistent with Ganie and Jhala (2017), who previously reported 87 and 96% control of 7 to 12 cm (3 to 5 inches) tall or 6- to 8-leaf glyphosate resistant and glyphosate sensitive biotypes of horseweed, respectively, in a greenhouse study 3 weeks after topramezone treatment at 18.8 g ai·ha⁻¹ (0.015 lb ai·A⁻¹). Bollman et al. (2008) reported 90% reduction in horseweed density at 8 WAT with topramezone at 12.5 g ai·ha⁻¹ (0.01 lb ai·A⁻¹) when compared with the nontreated control. They also observed 14 to 19% improvement in horseweed control when atrazine (560 g ai·ha⁻¹ or 0.5 lb ai·A⁻¹) and topramezone (12.5 g ai·ha⁻¹ or 0.01 lb ai·A⁻¹) were combined. However, the topramezone and atrazine combination was not always synergistic. Koepke-Hill et al. (2010) observed no improvement in POST control of volunteer potato (Solanum tuberosum L.) when topramezone was tank mixed with atrazine. Previously, Mahoney et al. (2017) reported only 67% control of 15 cm (6 inches) horseweed at 8 WAT with a topramezone (12.5 g ai·ha⁻¹ or 0.01 lb ai·A⁻¹) and atrazine (505 g ai·ha⁻¹ or 0.45 lb ai·A⁻¹) tank mixture. In the same study, horseweed control with atrazine at 910 g ai·ha⁻¹ (0.90 lb ai·A⁻¹) was only 37%.

Regarding common ragweed, control ranged from 72 to 84% at 4 WAT, 83 to 98% at 8 WAT, and 82 to 96% at 12 WAT with the topramezone rates ranging from 49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹ to 196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹ (Table 2). The topramezone rate of 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) provided higher control than the labelled (98 g ai·ha⁻¹ or 0.09 lb ai·A⁻¹) and lower (49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹) rates at 8 and 12 WAT. Similar reduction (1 to 3 plants per m²) was observed in common ragweed density at 12 WAT with topramezone rates tested in this study when compared with the nontreated check (18 plants per m²) (Table 3). In North Dakota, Zollinger and Ries (2006) also found topramezone (0.75x and 1x) very effective in controlling common ragweed (95%) in corn (Zea mays). In another study, topramezone POST at 12.5 g ai·ha⁻¹ (0.01 lb ai·A⁻¹) provided 95% or higher control of common ragweed (Bollman et al. 2008).

In comparison to previous studies (Bollman et al. 2008, Ganie and Jhala 2017, Mahoney et al. 2017, Negrisoli et al. 2020, Rodriguez et al. 2023, Soltani at al. 2012, Zollinger and Ries 2006), higher season-long control of common ragweed (4- to 6-leaf), fall panicum (3- to 4-leaf), horseweed (6- to 8-inch), large crabgrass (4- to 5-leaf), and yellow foxtail (4- to 5-leaf) in the current study was mainly due to a four (49 g ai·ha⁻¹ or 0.04 lb ai·A⁻¹) to sixteen- (196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹) times higher topramezone rates used. The maximum labelled rate of topramezone for PRE or POST weed control in Christmas trees is 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹) per application (Anonymous 2025). The highest rate of 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) in the field experiment was included to evaluate the level of Fraser × balsam hybrid tolerance to topramezone. In separate container experiments in Connecticut, several species of true firs, spruces, and white pine tolerated OTT applications of topramezone at rates up to 196 g ai·ha⁻¹ or 0.17 lb ai·A⁻¹ (J.S. Aulakh, unpublished data).

In conclusion, results from this study indicated that POST-applied topramezone at 49 g ai·ha⁻¹ (0.04 lb ai·A⁻¹) to 196 g ai·ha⁻¹ (0.17 lb ai·A⁻¹) rate did not cause injury to balsam fir, canaan fir, Fraser fir, Nordman fir, Norway spruce, white pine, and Fraser × balsam hybrid, whereas it showed temporary chlorosis on Colorado blue spruce and Douglas fir at 3 WAT. Furthermore, topramezone at the labelled rate (98 g ai·ha⁻¹ or 0.09 lb ai·A⁻¹) effectively controlled common ragweed, horseweed, fall panicum, and large crabgrass and significantly reduced their densities. In contrast, topramezone at 98 g ai·ha⁻¹ (0.09 lb ai·A⁻¹) only provided moderate control of yellow foxtail. These results highlight the importance of using an effective PRE herbicide program along with the POST-applied topramezone for season-long weed control in Christmas trees. It is important to note that overreliance on POST-applied topramezone should be avoided to prevent the evolution of HPPD herbicides-resistant weed populations. Therefore, Christmas producers should also integrate other weed control tactics, including sanitation, alternate herbicide modes-of-action, and physical methods for weed control. Future studies should assess the efficacy of POST-applied topramezone in conjunction with PRE and other POST herbicides for enhanced safety and weed control in Christmas trees.