by Tamara Scully
Is there a better way to grow tree fruit than the traditional orchard management practice — an herbicide strip in the tree row and mowed grass alleys? David Granatstein, Sustainable Agriculture Specialist, Washington State University (WSU) Tree Fruit Research and Extension Center, has researched a variety of methods of orchard floor weed control, some of which may hold promise for enhancing tree health, fruit quality and economic returns.
“Our tree fruit plants have wimpy root systems,” Granatstein said, and keeping weeds away is a priority.
The primary concern with weed competition is during tree establishment. For mature trees, weed control is most important during the critical months of May through July, when nutrient uptake is vital. Twenty square feet of weed-free space per tree is needed for optimal growth. At other times of the year, there is more tolerance for weeds.
“Weed control doesn’t have to be the same for the entire life of the orchard,” he said.
Speaking during a webinar, Granatstein outlined the four primary reasons why orchard weed suppression is needed: weeds compete for nutrients, particularly with young trees; controlling weeds can reduce rodent habitat; keeping weeds suppressed prevents interference with sprinkler systems; and weeds can serve as hosts for pest and diseases.
Herbicides are the control method of choice for most conventional growers. Although there are a few herbicides available for organic tree fruit production, organic growers utilize other strategies, primarily tillage, to control orchard weeds.
With a wide array of herbicides labeled for tree fruit use, growers have many choices. The best control is often achieved by combining herbicide strategies using two residuals in the fall to inhibit annual weed germination and applying a systemic herbicide in the mid-late summer to control perennial weeds.
“We’ve got residual herbicides. We’ve got contact herbicides…and we’ve got an array of modes of action,” he said. “There’s a lot of opportunity to rotate materials and avoid resistance. There is no reason for us to have herbicide resistant weeds in tree fruit.”
Young trees and even some mature stone fruit trees can be readily injured by contact with herbicides. There is evidence that fruit trees can absorb some herbicides, such as glyphosate, with sub-lethal effects on the trees. Herbicides can also be costly, while adequate weed suppression is not always achieved. And the bare ground that results from herbicide use is not conducive to soil health.
“The interface of the tree and the soil is the root system, and that root system is lined with all sorts of microorganisms. Those microorganisms need something to eat,” Granatstein explained. “What we do to the soil effects their ability to interact with that tree root.”
Mowing down weeds in the tree row can be labor intensive, and it has to be done repeatedly. This method tends to promote a population of weeds that are prone to regrowth, while eliminating those that can’t tolerate the stress. The roots left in the soil might still be too much competition for young trees.
Tillage, often used in organic orchards, has the potential to negatively impact surface feeder roots on the trees. One WSU three-year research trial in a Gala apple orchard demonstrated that tillage had a detrimental effect on tree growth and fruit production over time when compared to other methods of control.
Flame weeding can be dangerous, with fires having been started on numerous occasions. Perennial weeds are able to regrow after flaming, so shifts in weed population are seen in orchards relying on flame weeding.
Mulches come in many forms, and “they all have other impacts on the fruit system,” Granatstein said.
Some mulches, such as fabric mulches, will have different effects on the trees, depending on their color. Colors reflect light in various ways onto the tree canopy, and some can be more effective than others in weed suppression. The same color fabric mulches can even have different results depending on the microclimate. Colors that work well in colder climate may cause problems in a hotter climate.
Wood chips have been shown to have positive effects on tree growth and fruit size and to suppress weeds for several years. They don’t attract voles. Although the initial cost tends to be more expensive on a per acre basis than other options, the positive impact on orchard health outweighs the expense. In a two-year study in apples, the use of wood chip mulch achieved better weed suppression compared to mowing or four time per season tillage. Fruit size was enhanced, as was the total dollar value of the crop, in the wood chip system.
In the three-year WSU study comparing wood chips, tillage and flame weeding in Gala apples, wood chips had a positive economic benefit of almost $5,000 per acre when compared to tillage.
Another approach involves throwing mowed cuttings from the alley into the tree rows, for use as a mulch. In order to achieve good levels of weed suppression, the thickness of the required amount of mulch attracts rodents. A thinner mulch layer used in combination with herbicides can provide the trees with nutritional benefits from the organic mulch, keep rodents away, and offer adequate weed control.
One mulch that works well is shredded paper. It absorbs all the nitrogen and weed seeds cannot germinate on it. There are also sprayed-on paper options, which are less labor intensive and may hold promise.
Living mulches using ground covers or cover crops have the potential to “keep out the weed species we don’t want, and perform functions we do want,” said Granatstein.
A living mulch can provide a source of nitrogen, enhancing soil health and increasing biodiversity and pollinator habitat. One problem: nitrogen can be too plentiful at certain times, and negatively impact the fruit crop. A white clover living mulch was shown to attract voles, but the voles ate the clover and left the trees alone. However, after two years, the clover was decimated and needed replanting. Orchards with living mulches retain water better than those with herbicide strips.
Some cover crops can actually promote weed growth. Oriental mustard actually causes more weeds to grow the next season. Other cover crops, such as winter rye, have an allelopathic effect, and are very good at suppressing weeds.
“It’s more than the cost of the weed control itself. It’s the impact of the economics on the system that is probably much more important to look at,” Granatstein said.
As you prepare for another growing season, explore alternative methods of orchard weed control. Trying something new has the potential to enhance orchard health, impact tree growth, fruit size and quality, and have a positive effect on orchard economics.