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Christmas Tree Weed Control – Developing New Herbicides

Posted by Marcelo Moretti, Oregon State University | September 8, 2022

Oregon leads the United States in the production of Christmas trees, with almost 8.5 million trees sold in 2015. Weed control is essential in Christmas trees to reduce competition for moisture and nutrients, allow fast and robust tree growth, and ensure growers top prices for high-quality trees. Research is needed on weed management in Christmas trees in the West to ensure they remain economically competitive. Herbicides are the primary weed control option for Christmas tree growers, but options are limited, and herbicide-resistant weeds challenge weed control. Herbicide resistance has increased weed management costs in Christmas trees; growers must now turn to mixtures of herbicides or planned rotations. Increased diversity of herbicide modes of action is needed to manage herbicide-resistant weeds and to maintain the competitiveness of Oregon Christmas tree production.

A series of studies were initiated in 2022 and funded by the Christmas Tree Promotion Board to evaluate new herbicides (Figure 1). Here I will share results of fluridone (BrakeOn!) applied to Christmas trees. Fluridone belongs to the group 12 herbicide mode of action, the phytolen desaturase inhibitors. In simple terms, group 12 herbicides inhibit the synthesis of pigments, leading to the bleaching of foliage on susceptible plants and death. Fluridone  is a preemergence herbicide. Fluridone is not a new molecule, but it is now expanding its label to include uses in perennial crops like stone fruits. If approved, it would be a new mode of action in Christmas trees, and would address resistance management. This study is the first to evaluate Christmas tree tolerance to fluridone in the Pacific Northwest and perhaps elsewhere.

Figure 1. Overview of a Christmas tree planting where study was installed in the Winter of 2022.

Four field studies were initiated between January and February of 2022 in commercial Christmas tree farms in the Willamette Valley. The trees were Douglas fir and Nordman fir in the second year after transplanting. A single application was made over the top of the young trees mimicking growers’ practices with other herbicides.

Initial Results

The study is ongoing, but I will share some data from 12 weeks after the treatment. To date, no crop injury has been observed in any of the studies in Douglas Fir or Nordman fir treated with Fluridone at the expected field rate and twice that, equivalent to a Brake On! rates 43 and 83 fl oz/A. These encouraging results include growth measurements unaffected to date by these treatments. Growth measurement data are not shown.

Most of the research sites had low weed pressure, making it difficult to assess weed control. Postemergence herbicides were not included in the treatments. One of the fields planted with Nordman firs was infested with false dandelion (Hypochaeris radicata) and rattail fescue (Vulpia myuros). Most of the treatments did not provide good control of false dandelion that was present at the time of the application. Fluridone did not control false dandelion (<28%) regardless of the rate but did control rattail fescue (58 – 85%) (Table 1). When Brake on was used in mixtures, the mixture of simazine + Goal Tender + Brake On resulted in 73% control of false dandelion and 91% control of rattail fescue. Cleantraxx controlled 86% of false dandelion and 92% of rattail fescue.

Table 1. False dandelion and rattail fescue control 12 weeks after treatment with different preemergence herbicides in Nordman Fir field in the Willamette Valley, OR in 2022.
Trt Trade name Rate (product/A) False dandelion Rattail fescue
Control (%)
1 Nontreated control 0 c 0
2 Simazine + GoalTender 4 qt + 4 qt 48 abc 0 b
3 Cleantraxx 4.5 pt 86 a 92 a
4 Sureguard 12 oz 45 abc 99 a
5 Marengo SC 15 fl oz 15 c 73 ab
6 Mission 2.85 oz 10 c 99 a
7 Brake On! (Fluridone) 43 fl oz 20 c 58 ab
8 Brake On! (Fluridone) 83 fl oz 28 bc 85 ab
9 Simazine + GoalTender +Brake On! 4 qt + 4 qt + 43 fl oz 73 ab 91 ab
10 Cleantraxx +Brake On! 4.5 pt + 43 fl oz 68 ab 91 ab
11 Sureguard +Brake On! 12 oz + 43 fl oz 35 bc 94 a
12 Marengo SC +Brake On! 15 fl oz + 43 fl oz 10 c 70 ab
13 Mission +Brake On! 2.85 oz + 43 fl oz 20 c 81 ab

Mixing fluridone with other preemergence herbicides will expand the spectrum of weeds controlled in Christmas tree plantations. Simazine and Goal Tender were included in this study as the grower standard treatment. Fluridone improved weed control compared to that mixture. The results are becoming more apparent as the season progresses (Figure 2), and we will continue assessments during the 2023 season.

Figure 2. Overview of a nontreated plot (left) and Simazine + GoalTender + Brake On! on the right 12 weeks after treatment in a Nordman Christmas tree field. 

Fluridone is currently not registered in Christmas tree, but the initial results are promising and will be used to support a future registration of Brake On! in Christmas tree.


I would like to thank the many Christmas tree growers that hosted studies, and the support from the Christmas Tree Promotion Board.

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