Growing & Winemaking


Resistant Weeds Threaten Vineyards

September 2014
by Peter Mitham

  • The bare soil between vineyard rows can be enticing for weeds that are resistant to broad-spectrum herbicides.
  • Weeds use water that is needed by grapevines--especially during times of drought.
  • Rather than doubling the amount of a single herbicide used, experts recommend combining herbicides.

Working with wheat farmers on Canada’s prairies, agronomist Ken Sapsford of the University of Saskatchewan is no stranger to herbicide resistance in grains.

The use of broad-spectrum herbicides such as glyphosate—best known as Monsanto’s Roundup, though it exists in 22 other formulations—is so widespread that Monsanto has developed Roundup Ready hybrids of some crops to facilitate its use.

But weeds have followed suit, with 28 species worldwide exhibiting resistance to the chemical. Roundup is simply no longer able to corral them.

    Efficacy of weed killers in Napa


    In late July John Roncoroni, a University of California Cooperative Extension weed science advisor for Napa County, held a field demonstration about the efficacy of herbicides newly registered for vineyard application. He had laid out 25 treatment plots and rated the herbicides for weed control in vineyards.

    The trials demonstrated that some new herbicides are very effective, especially if a mix of more than one is applied. But they also showed how ineffective some herbicides were against troublesome, newly arrived weeds.

    The trial regimen
    The test was held in south Napa Valley near Carneros and focused on pre-emergent herbicides, though each application included Roundup systemic herbicide in case any weeds had germinated unseen, plus AMSPro surfactant to improve penetration.

    The plots had been cleared during the winter with glyphosate (such as Roundup) and Shark (carfentrazone, a contact herbicide that is mixed with glyphosate to help control some of the tolerant weeds). The applications were made in February just before significant rains (8.85 inches within a week).

    The plots were 25 feet long by 6 feet wide under the vines, and they were swept clear of leaves before application.

    The treatments were mixed with 30 gallons of water per acre at various rates and combinations of herbicides. They were applied using a CO2 backpack sprayer with two 8002 nozzles, each delivering 700 ml per minute. Roncoroni said that 30 to 50 gallons of water per acre seemed about optimum for adequate coverage.

    No weeds were visible at the time of treatment.

    All of the herbicides are registered. Roncoroni works exclusively with registered materials in Napa, saying: “The grapes are too expensive to destroy! Those trials are made with lower priced grapes in the Central Valley.”

    Glyphosate is widely used in vineyards, but some weeds have developed resistance to it, and others (some new to the region) are tolerant of it.

    He applies the treatments in January or February, as they require rains to be activated, but they’re only effective for four to six months. Vineyards may require mid-season spraying.

    Rely is a preferred contact herbicide for mid-season and post-harvest spraying after pre-emergent herbicides have become ineffective and can’t be applied again, but it is in short supply because of demand in the southeast for use with cotton and soybeans fighting glyphosate-resistant weeds. Rely won’t kill the vines if it gets on suckers, just the suckers, while glyphosate can get into the vine and cause problems in the spring.

    “In the old days, they used paraquat,” said Roncoroni. Paraquat is still registered for use in grapes but requires special permitting and safety equipment for use.

    Some of the new herbicides have some post-emergent properties, but one of the longest lasting, Alion (indaziflam), has none. In any case, Roncoroni emphasizes that mixes of two or more herbicides are generally more effective than a single product.

    The results
    Many of the herbicides did a pretty good job by themselves, notably Matrix (rimsulfuron), Chateau (flumioxazin), Goaltender (oxyfluorfen), and especially Alion at 5 ounces of product per acre.

    The herbicide Prowl performed nearly twice as well on vineyard weeds when combined with Alion or Matrix.

    However, the mix of Alion plus another herbicide such as Prowl (pendimethalin) and Zeus (sulfentrazone) was the most effective. Zeus plus Goaltender did particularly well, too.

    However, the quantities obviously matter. Alion, for example, was significantly more effective at 5 ounces than at 3 ounces per acre. Some materials required much larger quantities per acre than others.

    A note on specific weeds
    Panicle willowherb and horseweed are new to the Napa area and represent some of the biggest challenges at present.

    Roncoroni considers horseweed (Conyza canadensis) (often called mare’s tail) and panicle willowherb (Ebilobium bracycarpum), both relatively new to the area and his biggest challenges at present.

    Willowherb can grow to 5 feet tall, getting into the clusters and even interfering with mechanical harvesting. It isn’t resistant to glyphosate but tolerates it. Glyphosate can kill the top of the plant, but the weed will simply grow out from lateral buds. “It’s really become a problem,” he said. It is killed by some pre-emergent treatments.

    Horseweed (the preferred name) has gone from tolerance to resistance to glyphosate in the San Joaquin Valley. “It germinates almost year-round, so it can crop up if you water after harvest,” Roncoroni said.

    He said that it was relatively new in Napa Valley, and it’s a real problem. “It can be 0.5 inches in diameter and 6 to 8 feet tall. It uses a lot of water and can interfere wit h harvesting.”

    Fortunately, Matrix, Chateau and Alion are very effective in killing it, and the first two have some post-emergent effectiveness. “If it germinates in the fall, you need post-emergent herbicide to kill it.”

    Glyphosate will only kill small plants (4-5 inches tall).

    Rely can control both horseweed and willowherb, but again only when they are relatively small.

    Roncoroni added. “The only thing that will kill large willowherb or horseweed is iron—hardened on the end of a handle.’

    Roncoroni noted that the pre-emergent herbicides aren’t effective against field bindweed (Convolvulus arvensis), which has very deep rhizomes. It is best controlled with spot application when the morning-glory relative is blooming. This may take several years once established.

    Turkey mullein (Croton setigerus) is a common weed, but Roncoroni says it isn’t a big water user and doesn’t grow very big so it isn’t a real problem. It prefers uncultivated soil, which is common in vineyards.

    Roncoroni’s trials showed that some of the new herbicides are very effective against weeds including tenacious types, but he noted that he didn’t have information about price. Some like Alion are so new that other weeds may prove resistant to them in the future, but most resistance has been found recently in the overuse of post-emergent herbicides by themselves.

    Organic herbicides
    Roncoroni didn’t include any organic herbicides in the trials since no new ones have been approved recently, though he expects some soon and will conduct trials with them. He fears, however, that they may be expensive and not effective against tough weeds.

    —Paul Franson

Some of those species are common in British Columbia’s Okanagan Valley, where Sapsford and his wife bought a 2.5-acre vineyard in 2013.

Kochia (Kochia scoparia), Canada fleabane (Conyza canadensis), and both common and giant ragweed (Ambrosia artemisiifolia and Ambrosia trifida, respectively) are four resistant species resident in Canada; and of these, kochia and Canada fleabane are present in the Okanagan.

While glyphosate resistance hasn’t yet been observed in local populations of the weeds, Sapsford said that’s no reason for complacence; in fact, the weeds are ideal candidates for developing glyphosate resistance.

Where there’s a problem
Typically found on open, bare ground, these resistant species adapt well to hardy conditions with low competitive pressure. Roundup weakens them, but there are no competitors in these environments that can leverage that weakness to overtake the weeds. And so the weeds thrive.

“Where glyphosate is applied as a pre-seed burndown and a crop is planted, the crop provides competition to any weeds that may survive the treatment and helps to control the weed,” Sapsford explained. But “when glyphosate is applied to an area to control all weeds and keep the ground bare, there is nothing to compete with any weed that may survive the treatment.”

This makes kochia and Canada fleabane especially problematic in vineyards, where they’re often found on bare soil within the rows. The canopy doesn’t shade them, giving them the light needed for photosynthesis in an environment with little competitive pressure.

This is where Roundup comes into play, keeping the vineyard floor clean and reducing competition for resources such as water.

Glyphosate doesn’t impact the vine as long as the spray doesn’t touch the leaves, Sapsford said, leading grapegrowers to apply it at rates of as much as 5 to 7 liters (1,800 to 2,500 grams acid equivalent) per acre.

By comparison, grain growers apply glyphosate at rates closer to 1 liter (360 grams acid equivalent of glyphosate) per acre.

Bigger isn’t always better
But just because a regular dose of glyphosate does a good job doesn’t mean a stronger dose will do a better job, Sapsford said. Rather, it ultimately inures the weed to the herbicide.

“The higher the rate of application, and the more frequent it is, the sooner resistant weed strains are likely to emerge,” he said. “I’ve seen it on the prairies, and it’s only a matter of time before grapegrowers are facing resistant strains of these weeds.”

Kochia responds by producing more of the protein that Roundup attacks, frustrating the action of the herbicide, while Canada fleabane increases efforts to isolate the glyphosate, preventing it from moving to where it can take effect.

“Plants have developed different resistance mechanisms such as reduced translocation, isolation of (the) herbicide and gene amplification to develop resistance to glyphosate,” Sapsford explained.

A smarter approach to weed control is combining herbicides to broaden the challenges weeds face. Rather than battling one kind of poison, a species will face two herbicides with different modes of action. The double-headed challenge makes it harder for the weed to adapt to either one successfully.

“Glyphosate has made it very easy to control the weeds under the vine. But easy is not always the answer, good management is required to avoid resistance,” Sapsford said.

Anecdotal evidence
Sapsford’s vineyard manager, Pierre Levesque of Earlco Vineyards Ltd. in Penticton, B.C., says his work for Sapsford has heightened his attention to signs of emerging glyphosate resistance among weeds in the approximately 130 acres of vineyard he oversees in the southern Okanagan, primarily along the Naramata Bench.

“I have a feeling that people are using too high a dosage of glyphosate,” he said, acknowledging that even he has been using too much—2.5% versus a recommended rate of 1.5%.

“But I know my neighbors are talking about 5% and 7%. That’s deadly.”

Levesque is now combining herbicides from different groups.

Glyphosate, a Group 9 herbicide, inhibits amino acid synthesis, but Levesque combines it with Chateau, a moisture-activated herbicide belonging to Group 14, which inhibits a chlorophyll enzyme. It kills the seeds of weeds, preventing them from returning after mowing.

“It’s a different mechanism and a different active ingredient,” he said. “That’s definitely made a difference. It&rsqu o;s minimized the amount of weeds on your second round.”

Earlco has also invested in a Fischer GmbH mower that mows within the rows, between the vines, and is able to apply a focussed spray to the area around the trunks to ensure thorough, and targeted, suppression of weeds.

But if glyphosate resistance is becoming an issue of greater concern, Brad Hanson, cooperative extension weed specialist for vineyards and orchards in the Department of Plant Sciences at the University of California, Davis, told Wines & Vines that it hasn’t become one that’s taking an economic toll on West Coast growers.

Rather, it stands to increase management headaches and costs more than anything else.

“Is this a huge problem in the vineyard system? Probably not,” he said. “It’s an inconvenience—and certainly a greater expense, but for the most part it’s not a crop failure situation like it could be in cotton or corn or wheat.”

Fighting for water
But with competition for water a significant issue in California right now, weed control is attracting greater interest.

“Any weeds on the vineyard floor are using water that’s not going to be available to the crop,” Hanson said. “Growers who are very, very concerned about water are saying, ‘Y’know what, maybe I should kill all of that with tillage or herbicide applications because I can’t afford to have that weed using water.”

Knowing how to control weeds without making them stronger makes it important that growers follow good management practices when they go about conserving water.

Back in the Okanagan, Sapsford will continue to work toward that, discussing his findings with local agronomists. He spoke at a professional development event this past March and will also address a workshop at the University of British Columbia’s campus in the Okanagan this month.

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