Choosing a herbicide program for an orchard is not so simple as choosing which herbicide kills what weeds and when. A careful reading of the label of any herbicide reveals a host of warnings, most of which deal with tree safety.

Wayne Mitchem, a weed control specialist from North Carolina State University who serves growers in South Carolina and Georgia as well, says growers should know what those label warnings are and take them seriously. When an herbicide becomes cheap, broadly effective, and widely used—as happened with glyphosate—­growers can let down their guard.

Mitchem spoke last winter to growers at the Great Lakes Fruit, Vegetable, and Farm Market Expo as part of a session addressing new concerns about glyphosate. He told how to avoid problems with herbicides generally.

Many herbicides labeled for tree fruits came with warnings about using them on young trees. Sometimes that means the year of planting. Sometimes it means only a few weeks after planting. Sometimes it can mean three or four years of age.

In general, newly planted fruit trees with immature green bark on trunks are very sensitive to some herbicides. Peaches, regardless of age, are very sensitive to glyphosate. Glyphosate has specific restrictions regarding its application in peach orchards in the Southeast and other states; hooded or shield application equipment must be used, or glyphosate may not be applied later than 90 days past bloom, and low hanging limbs and suckers should be removed ten days prior to application. In a number of states, glyphosate may only be used in peach orchards with wiper application equipment.

Milk cartons

Growers who use milk cartons to protect young trees from herbicide sprays or want to gain some protection using exterior white latex paint should be careful. The cartons should be firmly in place, not sitting too high, Mitchem said. If trees need protection in the second year, more paint may be needed because the paint cracks as trees get larger, exposing tissue to potential damage.

When spraying, growers need to direct the spray to the vegetation and be wary of drift, volatilization, and the upward movement of spray particles caused by an inversion.  An inversion occurs during calm conditions when warm air is aloft  and air is cooler near the surface. As air warms and rises, it can carry small spray particles from the ground up into the tree canopy.

To keep spray where it’s supposed to be, he suggests using nozzles that produce large droplets, with sprayer pressures under 30 pounds per square inch, and avoiding spraying when winds are above ten miles per hour. Research has shown that reducing spray pressure to less than 30 psi and using spray nozzles that produce large droplets are far more effective at reducing drift than drift retardants, he said.

Mitchem also advises keeping spray volume low, at about 20 gallons per acre.  A spray volume of 20 gpa works well for nearly all herbicides, he said. It provides adequate coverage for contact postemergence herbicides like paraquat and is adequate for uniform coverage of the soil surface with preemergence herbicides while not diluting the spray concentration of systemic herbicides, like glyphosate, to the point where their activity is reduced.

Label warnings

Besides choosing the right herbicide for the age of the tree, Mitchem advises paying close attention to label warnings about soil texture and organic matter content.

“Grower response to those warnings has been to cut application rates,” he said. “When a herbicide has a label warning against applying them to sandy or low organic matter soil, some growers use the herbicide but cut the rate—and that is not a recommended practice, and doing so is risky.”

Widely used materials such as Chateau (flumioxazin), Casoron (dichlobenil), Karmex (diuron), Princep (simazine), and Sinbar (terbacil) have restrictions relating to soil texture or organic matter.  In orchards with mixed soils, growers have to match the rate to the most sensitive sites—and know the sensitive sites and adjust accordingly. “Use the lowest rate dictated by the soil,” Mitchem said.

Don’t double up surfactants, he warned. Some herbicides are formulated with surfactants to improve the rate of uptake. Adding more surfactant can make them more easily taken up by trees.

The surfactant issue, and the issue of inert ingredients, have emerged as key ones with glyphosate, as the number of formulations has proliferated.

It’s always wise to read labels, even when you think you know what they say. Companies will sometimes change formulations that change the amount of active ingredient or the rate of uptake, Mitchem said.  Directions on herbicide labels are intended to provide guidance that results in maximum herbicide performance without compromising crop safety.