Improving nursery weed control by choosing herbicides based on application timing flexibility and formulation

New research evaluates active ingredients, shows growers the importance of formulation and application timing.

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Research trials were conducted in 2018 and 2019 to evaluate five different herbicides including indaziflam, flumioxazin, prodiamine, prodiamine + isoxaben, and dimethenamid-P + pendimethalin for control of spotted spurge (Euphorbia maculata) and bittercress (Cardamine flexuosa) based on their formulation (granular vs. spray-applied) and seeding date relative to the herbicide treatment. Overall results showed formulation was the most significant factor in control of spurge or bittercress. In most cases, dimethenamid-P + pendimethalin, prodiamine, and prodiamine + isoxaben tended to perform similarly regardless of formulation. However, significantly better control was observed with spray-applied formulations of flumioxazin or indaziflam.

This research is part of a multi-year project focused on developing improved herbicide rotation programs and schedules for Florida container growers. Funds from FNGLA allowed us to conduct a study to investigate both current herbicide efficacy protocols utilized by weed scientists and evaluate different herbicide active ingredients in terms of how efficacy is influenced by formulation (granular or spray-applied) and seeding date.

In recent years, container growers have reported different results with many granular preemergence herbicides compared to results developed from small-plot research trials. These differences could potentially be from inherent differences in research trials and real-world nurseries relating to seeds present in the container substrate at the time of treatment or the use of different carriers/formulations for herbicide active ingredients.

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While poor herbicide control in nurseries could be a result of application/calibration errors, growers may still report less than satisfactory control of certain weed species after correcting these issues (and other factors such as resistance are determined to not be a factor). Weed species in which poor control have been observed are often fast-germinating species, such as bittercress, which germinates in three or four days in ideal environments. Herbicides applied on new/different carriers have been shown to provide effective control of bittercress in research trials, but this could be due to differences in nursery environments and in standard herbicide efficacy protocols. Researchers following general procedures will typically place weed seeds onto clean and sterile substrates one to three days after herbicide application to evaluate efficacy. In this scenario, excellent weed control is often observed. However, in nurseries, growers may only have weed-free substrates immediately after potting. During production, weeds will inevitably escape control and go to seed before they can be hand-pulled. Consequently, later in production cycles, growers will have substrates containing weed seeds at various stages of germination and development. If a herbicide, specifically those applied with these new/different carriers, are only effective on seeds that are introduced to the substrate 1 to 3+ days after application, they would not be as effective at certain times and other products could be chosen at different stages of production.

The objective of these experiments was to compare efficacy of common nursery preemergence herbicides based on their formulation and application timing relative to seeding spotted spurge (Euphorbia maculata) or bittercress (Cardamine flexuosa).

Methods

All trials were conducted at the Mid-Florida Research and Education Center in 2018 and 2019. On November 21, 2018, 1.68 L nursery pots were filled with a pinebark:peat:sand (8:2:1 v:v:v) substrate containing incorporated fertilizer and amendments, and placed inside a greenhouse, and received 0.3 inches of overhead irrigation per day. On November 27, herbicides were applied (Table 1). Granular herbicides were applied using a hand-shaker while spray-applied herbicides were applied using a CO2 backpack sprayer calibrated to deliver 50 gallons per acre. All herbicides were applied at manufacturer label rates and the amount of active ingredient applied with granular and spray-applied herbicides was the same. A non-treated control group was also included for comparison.

Bittercress seeds were surface sown onto pots on five different dates relative to the herbicide application including two days before the application (2DBT), and 0, 2, 4, and 7 days after treatment (DAT). Evaluating herbicide efficacy across different seeding dates allowed us to compare which herbicide active ingredient and/or formulation provided the greatest control across multiple seeding dates and to determine which herbicides are most affected by seeding date and formulation. This trial was repeated for bittercress following the same experimental protocols in 2019 with treatments being applied on February 21. Trials with spotted spurge were also conducted in 2019 with the first experimental run being treated on May 6, and the second experimental run on June 24. All trials contained eight single pot replications per treatment for each weed species and experimental run. After seeding, weed counts were taken in each pot at 4 and 10 weeks after treatment. At 10 weeks, shoots were clipped at the soil line and dried in a forced air oven until reaching a constant weight. All data were converted to percent control relative to the non-treated by using the formula [(non-treated – treated)/ non-treated)] × 100. Data were arcsine transformed and then subjected to analysis of variance (ANOVA) using JMP software (SAS Institute). Means and standard errors are reported for significant main effects and interactions. For the sake of brevity, only dry weight data are discussed for each species separately.

Results

Bittercress: Herbicide was the only significant main effect, but interactions for herbicide and formulation, as well as the interaction between herbicide and seeding date (Table 2).

Bittercress control differed by formulation for pendimethalin + dimethenamid-P with the granular (FreeHand) providing better control than the spray-applied formulation (Tower + Pendulum), but both formulations provided approximately 90% control (Figure 1). The spray-applied formulation of flumioxazin (SureGuard) provided better control than the granular (Broadstar), but similar to dimethenamid-P + pendimethalin, both formulations provided over 90% control. Similar results were observed with indaziflam, with 83% control was observed with the granular formulation (Marengo G) compared with 99% control with the spray-applied formulation (Marengo SC). No difference in formulation was observed for prodiamine + isoxaben (Gemini G and Gemini SC) or prodiamine alone (RegalKade or Barricade), but only the isoxaben + prodiamine combination provided acceptable control (>80%).

Averaged over both formulations, there was no difference in seeding time for dimethenamid-P + pendimethalin. Flumioxazin and indaziflam both provided better control of bittercress when seeds were sown on the day of (0DAT) or after treatments were applied (2 to 7 DAT) (Figure 2). Prodiamine + isoxaben and prodiamine results were variable depending on seeding date, but overall, the combination tended to provide at least 80% control regardless of seeding date while 77% or less control was observed with prodiamine alone.

Spurge: Herbicide, formulation, and the interaction between herbicide and formulation were significant (Table 2). There was no difference in formulation for dimethenamid-P + pendimethalin and both formulations provided over 90% spurge control. For both indaziflam and flumioxazin, better control was observed with the spray applied formulations (96% and 97% control with flumioxazin and indaziflam, respectively) compared with the granular products (84% and 81% control for flumioxazin and indaziflam, respectively). No difference in formulation was observed with prodimaine + isoxaben and prodiamine alone.

Conclusions

Results from these trials indicate that for the five preemergence herbicides evaluated in these experiments, formulation is more important than seeding date with respect to spotted spurge or bittercress control. Some differences in seeding times were observed with bittercress, notably with flumioxazin and indaziflam, but acceptable (>80% control) was achieved with these herbicides. For both bittercress and spurge, there was no difference in formulation for any herbicide that can be applied over-the-top of ornamental plants including dimethenamid-P + pendimethalin, prodiamine + isoxaben, or prodiamine alone. If proper application procedures are followed, growers should achieve similar control of bittercress or spurge with either granular or spray-applied formulations of these herbicides. Seeding date also had little influence on control with these herbicides, so they should continue to perform well in nursery environments if timely applications are made during regular application schedules. It should be noted that these experiments were small-scale in nature and performed in a very controlled environment, and growers may have more success with one formulation or the other depending upon available equipment and labor, nursery layout/infrastructure, and environmental conditions. For example, spray-applied formulations typically have a lower chemical cost and require less labor to apply, and can be applied to plants with wet foliage, providing more timing flexibility. On the other hand, granulars formulations are often safer in terms of phytotoxicity and allow application in difficult to access areas with large spray equipment such as shadehouses or other covered areas inaccessible by large booms.

Greater spurge and bittercress control was observed with the spray-applied formulations of indaziflam and flumioxazin compared with the granular formulation applied at the same rate. However, neither of these two herbicides can be applied over-the-top of ornamentals due to phytotoxicity concerns, and both formulations provided acceptable control. In non-crop areas, growers would likely see better control from a spray-applied formulation compared with a granular formulation applied at a similar rate.

Overall, results suggest that seeding dates ranging from two days before application to up to seven days after application should have little to no influence on control of these herbicides. Growers should expect similar control of bittercress or spurge regardless of formulation for dimethenamid-p + pendimethalin, isoxaben + prodiamine, or prodiamine alone, but greater control may be achieved with spray-applied formulations of flumioxazin or indaziflam. As these herbicides could not be applied over-the-top of ornamentals, granular formulations could be used as part of an overall rotation program. Further, growers could still expect to observe acceptable to excellent control of bittercress and spurge with flumioxazin and indaziflam regardless of formulation. Results could possibly differ depending upon production practices, seed pressure, weather, and other environmental conditions.

The author is associate professor of ornamental and landscape weed management in the Department of Environmental Horticulture UF/IFAS Mid-Florida Research and Education Center, Apopka, FL

August 2022
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