Russian thistle, also known as the ubiquitous tumbleweed.
A new herbicide called Matrix (rimsulfuron) will be available this spring for use in apple, cherry, and pear orchards.
Tim Smith, Washington State University Extension educator who has been testing the product for four years, said it is a broad-spectrum herbicide that seems to be safe for trees and effective against many weeds, but it doesn’t control all weeds.
"If you use it in your orchard by itself, you’ll find out how much Russian thistle you have, because it doesn’t work at all against Russian thistle," he told growers at the Lake Chelan Horticultural Day this winter. Russian thistle is also known as tumbleweed.
The solution, Smith said, is to use a combination of herbicides that compensate for each other’s weaknesses rather than using Matrix alone. "Tumbleweed is a very nasty weed to get rid of once it’s in your orchard," he warned.
Smith has been testing several different herbicide combinations. A combination of a full rate of Matrix with three quarts of Prowl (pendimethalin) and three quarts of Round-up (glyphosate) might be a winner, he reported, and could be a breakthrough for stone fruits.
"We really haven’t had a good residual product to use in cherries because the traditional products are dangerous to cherries," he said. "There really wasn’t a good combination to use safely in cherries. Now there is."
Prowl was included to control the Russian thistle, knotweed, and summer grassy weeds. Prowl needs to be watered in after application because it degrades rapidly in the sunlight. Most herbicides applied in the spring will work better if irrigation water is applied for an hour to set them in, Smith said.
Until about a year ago, Prowl could only be used in nonbearing orchards. Now, it is registered for use in bearing orchards also. In his trials, a combination of Prowl and Matrix kept most weeds out of the orchard for 85 days, at which point bindweed was starting to grow.
A spring herbicide application will not control bindweed because the weed is small and not vulnerable at that time, Smith said. He recommends targeting the weed in the fall when it has plenty of foliage for the herbicides to act upon.
Probably the best combination in his trials was Matrix, eight ounces of Chateau (flumioxazin), and three quarts of Round-up, he said. It was still providing fairly good weed control after 85 days, except for bindweed. However, the oldest leaves on the trees developed holes, though not leaves that developed after the application was made. Smith said the culprit was Chateau, whose label stated that the product should be used only when the trees were dormant and before they showed any green tissue.
The holes in the leaves were caused by fine particles of the herbicide that drifted onto the trees, Smith said.
"There are far more fine particles of herbicides in the air than you think when you’re spraying," he said. "Just the movement of the tractor through the orchard is enough to get those really fine particles up in the air and on the tree."
Smith said pears are particularly sensitive to Chateau. Any trace of the herbicide on leaves or fruit leaves a mark. Dust tainted with the herbicide that is kicked up into the trees can also cause spotting, he said, so, even if it is applied in the fall, leaf damage can still occur. "It doesn’t seem much of a problem on cherries or apples, but on pears it was deadly," he said.
Smith had tested Chateau for about five years without noticing any spotting problem, but in previous years he had applied it in the fall or when the trees were still dormant. When he made the application in 2007, the trees were leafing out.
As a result of this spotting problem, Chateau is no longer registered for use in orchards.
In a cherry orchard, Smith tested a combination of the herbicide Venue (pyraflufen-ethyl) and Round-up. Venue is supposed to be applied in 90 to 100 gallons per acre, but Smith found it gave much better results when applied as a more concentrated spray in 40 gallons of water. Venue is not a good product for grass weeds, he noted. However, for weed control in a young orchard with no grass, it works well on small broadleaf weeds.
Asked whether adjuvant should be used to improve the efficacy of herbicides, Smith said that in tests he has done with glyphosate and wetting agents, the rate of herbicide proved to be more important than whether an adjuvant was used. "The bottom line is if you just spend the amount you would have spent on the wetting agent on more glyphosate, you’re going to get better results," he said.
Smith was asked how to treat root suckers. "If you don’t have very many, and they’re not very big, glyphosate will disrupt the root suckers and you don’t see the symptoms in the tree. If you have lots, and they’re large, you have to mow them or chop them down before you spray. Usually, if you’re using glyphosate regularly, they don’t become much of a problem."
The biggest threat of tree damage from herbicides is when growers accidentally put glyphosate in their airblast sprayer and apply it to their trees, Smith said. Every year, he hears about someone doing that and damaging or even killing the trees. Last season, a grower applied glyphosate, thinking it was a wetting agent. Some people have mistaken it for oil and applied it to trees at the delayed-dormant stage. "These are really smart people and good growers, and it still happens," he said, suggesting that growers store glyphosate in a separate place from insecticides.