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Glyphosate has been such a cheap and effective herbicide that there’s a risk of overusing it.

It’s tempting to attach the weed sprayer any time the orchard is being mowed, because it costs very little extra to do the two tasks together, says Tim Smith, Washington State University extension educator in Wenatchee.

Some orchardists are spraying four to six times a year.

“Every year, almost every orchard gets glyphosate on a regular basis,” Smith said. “It’s very cheap, and you don’t need to put much of it on for it to work.”

The problem is that weeds are developing resistance to glyphosate where it’s been frequently used, warns Smith, who would like to see it used no more than once a year, and only if necessary. And by ­“necessary” he means when perennial weeds are blowing up.

Perennial grasses and broadleaf weeds are susceptible to glyphosate between midsummer and fall. So there is little use in applying it in the spring before many of the perennial weeds are growing.

That’s why Smith recommends applying glyphosate in October to control the perennial weeds. Follow that with a tank-mix of contact and residual herbicides in the spring (April or May) to catch the new weeds.  Residual (preemergence) herbicides leave residues in the ground that prevent weeds from growing. Any weeds that do grow will be controlled with contact herbicides that burn the plants on contact.

“If you depend on one kind of weed control, then the weeds that make it through that weed control will become dominant in the orchard, so we like to change up the products we use and have both ­residual products and contact products in the orchard,” Smith explained.

In trials, Smith has been able to achieve season-long weed control with this program.

The following year, a combination of two different herbicides should be used in the spring, to avoid resistance development.

“There are weeds that have become resistant to every herbicide class we have had in the past,” Smith said. “A lot of it had to do with reliance on a single herbicide every year.”

The newer herbicides are safer to trees and the environment, Smith said, though they are more expensive. With the older herbicides, such as Simazine, Diuron, and Terbacil, growers had to take care when applying them; the older herbicides can be taken up by tree roots. High rates can injure trees.

Newer pesticides are not picked up by tree roots, he said. Another advantage is that they can be used in all tree fruits, including stone fruits, which Simazine could not.

Spring herbicides need to be applied before the weeds grow out of control, so the grower needs to be vigilant and have the weed sprayer ready to go, Smith said. Conyza canadensis (horseweed or marestail) is a particularly difficult species to control because it is resistant to many herbicides and it can grow six feet tall. “If it gets six inches tall, you’ve lost it,” Smith said. “It’s best to get it when it’s smaller than a 50-cent piece. You have to get ahead of it.”

Test the weed sprayer

Often, the weed sprayer is the least-maintained piece of equipment on the entire orchard, because it can look like it’s doing a good job, even if it isn’t, Smith has found.

If the herbicide is not applied evenly, you’ll have strips in the orchard where more herbicide than necessary is applied, which is a waste of money. There will also be strips with insufficient herbicide where the weeds break through.

“Your weed control is no better than the least rate you put on,” Smith warned.

The fastest way to test a weed sprayer is to fill it with water and spray a 20-foot section of pavement or driveway. Then, watch the water dry. If it dries fairly evenly, the sprayer is probably doing a good job, but if some parts dry earlier or later, it is not applying the spray evenly.  Check the height of the boom and make sure the nozzles are all putting out the same rate.

One of the reasons weed control has become more important in mature as well as young orchards over the past couple of decades is the switch from large impact sprinklers to microsprinklers for irrigation, Smith said.  Most of the new sprinklers are only 12 to 18 inches above the ground and don’t put out a strong stream of water that can force its way through the weeds.

In a high-density orchard, there can be enough interference by weeds and tree trunks to create dry spots on the ground. Incomplete coverage can be a problem when the trees are on dwarfing rootstocks and don’t have large root systems to explore the soil.