Weeds can thrive in young orchards. A young orchard has everything weeds need. It has lots of water and fertilizer, because the newly planted trees are irrigated and fertilized frequently. “You have to be extra careful in a young orchard,” warns Tim Smith, Washington State University Extension educator for north central Washington. The best time to control weeds is in the fall while the weeds are still active.
They can also be controlled in the spring, but by the middle of April, the orchard floor in a nonbearing orchard where no residual herbicides have been applied will have a lot of growth, Smith warned. And bigger weeds are more difficult to control than small weeds. “Timing is important,” Smith said. “If you don’t get the herbicides on late in the fall or early in the spring, weeds get in the way.”
In many cases, growers are not using the best equipment, Smith said. They invest many thousands of dollars on sprayers, but hesitate to spend $50 on a boom. Many booms are old and homemade. The spacing of the nozzles is related to the height the boom operates at. Usually, nozzles are about one foot apart and about 18 inches off the ground.
Smith recommends checking charts that are in manuals available from suppliers to find out the appropriate nozzle spacing and distance from the target to ensure the proper overlap in coverage. It’s difficult to tell just from looking while applying the herbicide, he noted. Flat-fan, 80-degree nozzles are the industry standard in Washington, and still work fine, Smith said, but there is new spray-nozzle technology that the industry has not investigated.
“Maybe you could be looking at different kinds of nozzles that are available today that weren’t available even a few years ago. There are some real advances that have taken place. We may be able to run our booms better.” He cautioned growers not to use flood-jet nozzles to apply residual pesticides because the placement of the product is not very precise.
Sloppy coverage is not a good idea with residual products because the weeds will come through the poorly treated areas first, and soon it will look as if they were never treated, even though the product might be working in spots that were thoroughly treated. In a young orchard, a swath at least three feet out from each side of the tree should be treated. In a mature orchard, the weed-free strip should be at least four feet wide on each side to avoid weeds competing with the trees.
The choice of product has a lot to do with the soil texture and type, because that affects how easily the herbicide moves down through the soil before it binds up, Smith said. Some of the older herbicides are quite mobile, and if they get into the root zone, the tree roots might take them up.
Herbicides are most likely to move in the sandier soils. Typically, the amount of organic matter in eastern Washington’s soils is very low. Many of the desert soils have only 0.5 percent organic matter. Soil texture can vary tremendously within an orchard.
Smith recommended that growers apply herbicide in relation to the weaker soils in the orchard, not the average or best. “Sometimes, it’s easiest just to plan the whole orchard as if it was the weaker soil and treat the whole thing the same,” he said. “You just choose not to use products that would be dangerous to trees in sandy soils.”
Smith has tested various treatments for weed control, including the new broad-spectrum herbicide Chateau (flumioxazin) from Valent. Chateau is safe to trees when applied to the ground, but it does have foliar activity, as many herbicides do, he said. The product is also registered for use in vineyards. However, it’s usually necessary to tank mix two or three different products so that they each make up for the weaknesses of the others.
“There aren’t many products that are perfect, so we try to blend them,” he said. In trials, Smith has tested: —The residual herbicides Chateau and Solicam (norflurazon), and the knock-down herbicide Round-up (glyphosate); —The residual herbicides Surflan (oryzalin) and Solicam, with glyphosate; —The residual herbicides Chateau and simazine, with glyphosate. He thinks the product mix that’s likely to become the industry favorite for apples or pears is Chateau plus a light rate of simazine and glyphosate.
However, Smith said the grower would need to be certain that the trees were old enough and the soil good enough that the simazine would not affect the trees. Simazine should not be applied to stone fruits because it would likely get into the root zone and be taken up by the tree roots. Smith said he believes in using residual products on young orchards. With knock-down products, there’s the risk of not applying them frequently enough and allowing the weeds to grow out of control.
Horseweed (marestail) can grow as tall as the trees sometimes and comes in fast in summer. On nonbearing trees, Chateau seems to control it at low rates. “This is the first product that’s come along that seems to do the job,” Smith said.
Buckwheat vine will climb trees and plug up irrigation equipment. It is not controlled by Chateau, but is controlled by older residual products.
Buttonweed (mallow). Contact materials just burn back this weed, and then it becomes well established. The secret to controlling mallow is to use a high rate of glyphosate (three quarts in 20 gallons of water), but be sure not to get it on the trunks of small trees. Once the mallow is killed, the rest of the weeds can be controlled with a much milder program. For root suckers in cherry orchards, Smith said applications of glyphosate on a regular basis tend to keep them under control.
Usually, the glyphosate is not taken up by the tree. However, if the suckers get out of control and grow big and bushy, that might happen, Smith warned. “Get them before they get real large. If they’re large, mow them off before you spray them.”