Oct 18, 2013
California growers fighting tomato bindweed

Most of California’s processing tomato growers have had their fair share of weeds to contend with. But in the past couple of years, field bindweed has become an increasingly difficult weed to deal with using existing control methods. This creates problems, not only with fruit quality and yield, but also during harvest when this noxious weed can foul up picking equipment and slow down overall production.

To try to combat this weed, Scott Stoddard, a University of California (UC) farm adviser in Merced, Calif., and other researchers have been experimenting with different timings and chemicals to reduce bindweed.

“It just seems that in tomato commercial production, in recent times, bindweed has become more of a problem,” Stoddard said. “Since we’ve been changing our production and irrigation methods, this weed seems to be getting worse.”

Stoddard strongly suspects that the lack of aggressive tillage in fields that now have subsurface drip has hastened the spread of bindweed, making it more difficult to kill.

“Depending on the thickness of the drip tape and other factors, you have to be real ginger when you’re doing your cultivation,” Stoddard said.

After harvest, when growers prepare for the next crop, they have to shape their beds very lightly so as not to rip up the drip tape.

But even in fields that don’t have drip tape, deep tillage doesn’t always eradicate the problem, since the cut-up bindweed roots can regenerate and grow back again for next year’s tomato crop.

The reason bindweed is such a problem and in many ways worse than other weeds, such as nightshade, is that it literally crawls on top of the tomatoes. With nightshade, growers can go through and mow it off of their fields. But, once bindweed gets tangled up in the tomato plants, it’s hard to physically get rid of it. At harvest time, bindweed has a tendency to wrap itself around the shakers and belts of the harvesting machines, thereby slowing down the overall harvest.

“I’ve actually seen harvesting machines stop in the field and watched guys climb out and have to rip up the wrapped-up bindweed around the pulleys,” Stoddard said.

One good way to deal with bindweed is right after harvest, Stoddard said. It helps to irrigate the post-harvested fields, wait for the bindweed to come up again and then knock it back down with Roundup.

“That can be a very effective postharvest treatment,” Stoddard said. “The main problem, though, is water. You don’t always have the water to spend on re-irrigating your fields after harvest, for no other reason than to bring up weeds just so you can kill them. Some people can do that and some people can’t. It all depends on your water situation.”

In his own trial in Fresno County, Stoddard started with a field that already had bindweed in it. He did his irrigation in the spring and used Roundup to burn the weeds back before putting on any pre-emergents.

“Bindweed is a perennial, so we know it will emerge again and keep coming up from the roots, and that it will continue to emerge all throughout the season,” Stoddard said.

At the moment, there are no herbicides that can completely control bindweed. In very high-pressure bindweed areas, some growers simply have to rip out their drip tape and rotate their tomatoes out with another crop, such as wheat, to try to clean up the fields, Stoddard said.

To try to find a solution to bindweed in tomatoes, Stoddard and his colleague, Tom Lanini, another UC researcher, have experimented with using combinations of pre- and post-emergent herbicides.

They looked at Prowl as well as Zeus as pre-emergents, but found that Treflan is the most effective pre-emergent treatment for suppressing bindweed, and that the most effective post-emergent treatments were Shark or Matrix.

The best strategy for growers, in general, is to start off cultivating their fields and getting rid of as many weeds as they can before planting, so they can apply their pre-emergents on clean beds.

While some growers use one pre-emergent or one post-emergent, when it comes to the bindweed problem it helps to use both, Stoddard said.

Out of the more than 20 treatment combinations that he tried, Stoddard said four of them worked out relatively well. One possibility, he said, would be to use Treflan at pre-planting, incorporate it into the soil, then follow it by a second shot of Treflan within two or three weeks of transplanting.

“Some guys would be concerned about this because too much of this herbicide could cause injury to the plants,” Stoddard said.

Another scenario would be to use Treflan as a pre-plant, incorporate it, followed by Matrix at transplanting, incorporated by sprinklers. The problem with this, though, is that many growers don’t use sprinklers anymore, which means this would not be an option, Stoddard said.

When incorporating Treflan into the soil, try to use a cultivator to get it into the top 2 or 3 inches of the soil, since that’s where it’s most effective, Stoddard said.

A third option for growers would be to use Treflan, incorporated as a pre-plant, and then to use Matrix two or three weeks after transplanting. At this point, sprinklers wouldn’t be necessary.

Another alternative that works fairly well is to use 2 ounces of Matrix applied by sprinklers at transplant, and then to apply Matrix without sprinklers two or three weeks later. The problem with using Matrix twice, however, is plant back, since some crops are too sensitive to residual Matrix left in the soil.

“Crops that are too sensitive to Matrix can have stand emergence problems,” Stoddard said. “For instance, if you make a Matrix application in June, and the following year go into cotton in early April, you may have an effect from the Matrix.”

At this point, growers would need to rotate out of tomatoes and try something like melons that wouldn’t be affected by plant-back restrictions.

The bottom line is that there is no magic bullet for eradicating bindweed right now, Stoddard said.

“Even with the better treatments I’ve seen 50 percent control of bindweed and in other years I’ve seen 90 percent control, but this is the best we’ve got,” he said. “We’ve still got a lot of work to do.”

Lisa Lieberman

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