Scientists are finding there is a critical time during the year when weeds can impact tree fruit yields. But after the critical period, a weedy orchard floor may be more of an eyesore than an economic problem.

Dr. Leslie Huffman, weed management specialist for the Ontario Ministry of Agriculture in Harrow, Ontario, Canada, said data from a trial she conducted with other researchers in weed management indicates that the critical weed-free period for young fruit trees is May to July. For bearing fruit trees, the critical weed-free period is from bud break to 30 days after bloom.

“It’s a hard idea to totally believe in because we all like to see weed-free orchards,” Huffman said during a presentation at the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan.

This concept focuses on when weed control is most important in regard to yield reduction from weeds, she explained. However, there may be other times during the year when weeds are desirable. By keeping the orchard weed-free during the critical time period, yield reductions should not occur. Weeds emerging after the critical period will not affect yield. Research data showed that with new trees, trunk diameter growth was affected by weeds from May to July.

“If you plant new trees, don’t let the weeds get away from you or you’ll have less growth,” she said. “Bearing trees were more difficult to figure out, but the data suggests that if you control the weeds early, from bud break to 30 days postbloom, you won’t lose yields.”

Integrated approach

“Most growers would like a herbicide that will eliminate weeds for the season in one application,” she said. “But complete weed control is just not feasible.” Weed management is always a challenge to tree fruit growers because one herbicide application rarely gives season-long control.

As part of an integrated weed management program, researchers are using critical weed-free periods to help manage weeds, along with a variety of management tools. They are also considering the environmental impact of weed management techniques to help guide grower decisions.

Integrated weed management strategies usually combine several tools, including herbicides, cultivation, mulching, mowing, and flaming. Growers should be aware of the pros and cons of each tool.

“None of the tools are totally perfect,” Huffman said, adding that they all have advantages and disadvantages.


She encouraged growers to consider the total environmental impacts of controlling weeds. Most weed management practices have some good and some bad effects.

Many of the mechanical means of weed control, like mowing and mulching, are considered environmentally friendly, Huffman said, yet they, too, have negative aspects. They require several passes with a tractor that uses nonrenewable fossil fuels and emits engine exhaust.

She advocated that growers use the Environmental Impact Quotient developed by Dr. Joe Kovachs at Cornell University to calculate the broad environmental impacts of certain herbicides and pesticides.

Huffman explained that the quotient measures impact in ten areas, ranging from applicator and picker to groundwater and fish to beneficial insects and ecological components. The formula takes into account different rates of the pesticide used, number of applications, and active ingredient.


Cornell scientists have calculated a field use rating (Environmental Impact Quotient) for most fruit pesticides. Growers can look up the EIQ on Cornell’s Web site and multiply it by the percent of active ingredient, rate, and number of applications to compare the environmental impact of soil residual pre-emergent herbicides to postemergent herbicides.

For example, Huffman said that the environmental impact from growers using soil residual herbicides with low EIQs can be comparable to postemergent-only programs. She noted that if growers calculate the environmental impact of their herbicide program, they can compare alternative materials and strategies and choose the program with the least impact that still works.

Additional information about EIQ can be found at: publications/eiq/.

Things to remember about critical weed-free period:

—Yield reduction should not occur if the crop is kept weed-free during the critical period.
—Weeds emerging after the critical period will not affect yield.