Like Debalina Saha, Ph.D., in the Department of Horticulture at Michigan State University says, successful Christmas tree production requires a good and effective weed management program.

Christmas trees are grown on 350,000 acres across 15,000 farms in the U.S., resulting in $250 million in sales annually. The practice takes patience, though, as it takes eight to 12 years for a tree to reach a harvestable size.

Saha notes that weed competition can interfere directly with the growth rate of Christmas trees during their establishment phase (the first three years post-transplant in the field). These weeds can steal limited moisture from sandy soils and in dry summers, which can cause up to an 80% mortality rate of very young trees.

In addition, young trees can be shorter than fast-growing weeds, causing shading, which leads to photosynthesis reduction and reduced leaf area in the trees.

The most common weed control practices in Christmas tree production involve mechanical mowing and the application of chemical herbicides. While herbicides remain the main weed management strategy, frequent applications of the same herbicides can lead to resistance issues. Herbicides can also cause phytotoxic damage to Christmas trees when not applied properly.

Presenting her research, “Evaluating newer herbicide combinations for weed control efficacy and phytotoxicity in Christmas tree production,” at the most recent American Society for Horticultural Science conference, Saha said it’s important to test newer herbicides – with different modes of action – to expand the list of options for growers.

Saha and her team conducted an experiment at a commercial operation, Wahmhoff Christmas Tree Farm, in Gobles, MI, in summer and autumn 2024. Weeds seen in the Christmas tree fields included horseweed, ragweed, white clover and hoary alyssum.

They wanted to evaluate the labeled (1x) and doubled (2x) rates of Python®, Classic®, Envoke®, FirstRate® and Hornet® herbicides vs. the commercial standards for weed control in Christmas tree production. They also wanted to evaluate the labeled and doubled rates of the same herbicides vs. the industry standards for phytotoxicity affecting Fraser fir.

(For background, Envoke is often used in cotton, sugarcane and transplanted tomatoes; Python, in corn and soybeans; Hornet in field corn; Classic in soybeans, peanuts and non-crop areas; and FirstRate, soybeans.)

The herbicides were applied before bud break (May 3) at 1x and 2x rates. The treatments were applied toward the lower 18 inches of the trees with a carbon dioxide backpack sprayer calibrated at 27 gallons/acre output.

Weed Management

The industry standards for Christmas tree herbicides are GlyStar®, Impact® and SureGuard®. Treatments tested for weeds included GlyStar by itself; GlyStar + Impact and GlyStar + SureGuard, as compared to the aforementioned herbicides + GlyStar, as well as GlyStar + Python + Impact.

According to Saha, this was mostly adding post-emergents to pre-emergents.

The researchers looked at tree height at the beginning of the experiment and four months after treatment. They also conducted visual estimates of weed control and phytotoxicity. The general trend meant seeing increased weed pressure three months after treatment, with newer weeds sprouting up with increased summer temperatures.

How did the different herbicides rate? At the 1x rate, GlyStar + Classic, GlyStar + Envoke and GlyStar + Python + Impact provided good control (70% to 99%) until four months after treatment and are comparable to industry standards, according to Saha.

At the 2x rate, except for GlyStar + Python, all the other treatments provided excellent control (85% to 100%).

Phytotoxicity

However, those successful weed treatments did not come without casualties. GlyStar + Python, GlyStar + Hornet and GlyStar + Python + Impact showed significant (35% to 56%) phytotoxicity injuries.

At the 2x rate, GlyStar + Python, GlyStar + Hornet, GlyStar + Python + Impact and GlyStar + FirstRate showed significant injuries to the lower portions of trees as well (22% to 75%).

Based on this experiment, it appears GlyStar + Classic and GlyStar + Envoke are the best out of all the treatments tests, resulting in 70% to 99% weed control, very little phytotoxic damage and no reduction in tree height.

Saha cautioned, though, that these results came from only one year of data. Research is ongoing.

by Courtney Llewellyn