by Sally Colby

Brad Edwards and Travis Birdsell, North Carolina Research and Extension, want to help Christmas tree growers create a welcoming environment for guests, and part of that effort involves eliminating weeds from the fields.

Edwards and Birdsell presented an engaging session on weed management at the Pennsylvania Christmas Tree Growers Short Course earlier this year.

Edwards said Christmas tree growers in NC have been relying on glyphosate applications over the tops of trees to manage weeds. “We do spray Roundup over the tops of Fraser firs,” he said. “We’ve sprayed it on growing firs throughout the growing season at low rates.”

White clover, often used between rows, easily tolerates glyphosate at the rate of eight ounces/acre. “Once we get a good ground cover base, clover can cut down on weeds like horseweed and ragweed that come through,” Edwards said. “With any weeds we’re having trouble with, a good stand of white clover is our first line of defense.”

However, using the recommended rates of application for glyphosate will result in dead clover. “More bare ground and soil erosion comes from that, greater loss of soil organic matter (SOM), open ground for summer annual germination, new cycle of explosive weed growth,” said Edwards. “Then when we open the ground, we’ll have a serious weed problem.” Good weed control will result in eliminating 99.9% of weeds, but that small percentage that remain will flower and go to seed.

Christmas tree farms are often situated on steep slopes where mowing is close to impossible. However, it’s important to maintain sufficient vegetation to avoid the loss of soil and nutrients. “If it’s done right, chemical mowing provides total soil protection from rain and sun,” said Edwards. “It adds organic matter to the soil and is capable of supplying Fraser firs’ annual nitrogen requirement. It’s really cheap weed control.” The nitrogen from clover is slow-release, and provides enough nitrogen to grow good, strong trees.

Most white clover is already present from natural stands, and that it just has to be encouraged. Edwards recommends perennial white Dutch clover because it stays short, adding that fields with clover adjacent to high deer populations don’t have any more problems with deer than other areas. “Deer will eat the clover,” he said, “but it doesn’t draw them in to where they’ll eat more Frasers than they eat clover.”

Birdsell explained that clover is established through both sowing and self-seeding. He advised growers to plan to sow white clover in March during ground heaving. “When a field is completely emptied of trees and not replanted immediately is the best time to establish a stand of clover to plant into,” he said. “Clover will spread rapidly and thrive if it’s managed, and chemical mowing encourages clover growth.” Clover doesn’t tolerate heat well and may not look good in summer, but will recover with cooler temperatures.

Growers should be aware that clover seed is tiny, and can be sowed in several ways. Some use a hand seeder; others put clover seed in with granular fertilizer so they’re sowing clover at the same time they’re applying nutrients. Growers shouldn’t expect noticeable results the first year – clover requires a year or two to take off.

Weed emergence is associated with soil temperature, and growers can learn to predict which weeds will emerge throughout the season. “If you start monitoring soil temperature, you can gauge when certain weeds will be germinating,” Birdsell said. “Some growers use a pre-budbreak glyphosate application of 10 to 12 ounces per acre.”

The key to managing weeds is scouting, which should take place throughout the season. “Do you need to kill or simply suppress weeds?” asked Birdsell. “Either way, you need the right combination of herbicide, rate, timing, calibration, application and weather.”

Another consideration for weed management through spraying is the amount of heat on bare ground, which is absorbed by soil and released in the evening. “That can actually create thermals coming from the ground,” Birdsell said. “You’ll get far less coverage with a mist blower with heat releasing from the ground because it lifts the spray up.” Determining the optimum time to apply spray can be challenging.

An important aspect of weed management is preserving SOM. “Water-holding capacity increases significantly and with just 4% of SOM, you can hold one to two more inches of moisture in the top layer of soil, then the moisture is released slowly,” said Birdsell.

Growers are increasingly dealing with horseweed (marestail). Applying low rates of glyphosate will burn the tops of horseweed, but the plant rallies and puts out even more top growth, resulting in more seeds. However, a good cover of white clover shades the ground and prevents weed seeds from germinating. “With partial cover and bare spots, we might see one seedling per square foot,” Edwards said. “That’s 40,000 weeds per acre. But if we have a full stand of clover, we have one seedling per 100 square feet. We’ve reduced the population drastically and we can figure out how to manage that more effectively.”

Edwards said glyphosate at four ounces per acre doesn’t kill weeds. “All it’s doing is stunting them and holding them until July when trees are hardened off and we can turn glyphosate rates back up and knock them out,” he said. “It keeps everything lower to the ground.”

Research has shown that low rates of glyphosate can be used up to and through budbreak, but not into shoot elongation. “When shoots start flattening and needles are laying flat, the V stage, that’s when you can burn with glyphosate,” said Edwards. “If the buds are still tight, we can still apply eight ounces around those shoots and not hurt them.”

Birdsell said growers must become “IPM ninjas,” looking at various chemical modes of action (MOA) and carefully using what’s available. “That’s a key IPM strategy,” he said. “Whatever weed you’re targeting didn’t see it coming because it’s different than what you sprayed before.” Birdsell explained researchers are now working with a concept that’s being widely adopted by row crop growers – using chemicals that are premixed in the jug with attention to two or three ingredients/MOA.

“The key to using herbicides is understanding what goes into tank mixes, especially at the end of a rotation,” said Birdsell. “Watch at the end of a rotation, know the longevity and carryover. If you transplant something back into a field, you may have issues.”

Paying attention to group number on tank mixes should be as important as heeding PPE requirements. “Some you’d be alternating are actually in the same group,” said Birdsell. “Glyphosate isn’t the only one with resistance issues. Weeds are resistant to Stinger, 2,4-D and the list keeps going on. It isn’t enough to know the herbicide kills grass and weeds – where does it fit in, and what is its mode of action?”

Birdsell urged Christmas tree growers to use a variety of weed management options including weed-eating, chopping, cultivation, plowing, weed fabric, mulch and even grazing with trained sheep. “Learn how to effectively rotate,” he said. “We can address problem weeds with a rotation of products and modes of action. Tank mixes are going to become the rule as we see more and more resistant weeds.”