Winter rains complicate weed control for vineyards

By Vicky Boyd

While winter rains have delivered much-needed drought relief, they have also brought challenges for grape vineyard weed control by creating wet conditions and reducing management options.

Bud break—where vines awaken from dormancy and leaves emerge—is beginning across the state, further limiting weed management choices, said John Roncoroni, University of California Cooperative Extension weed science advisor emeritus for Napa County.

Joe Valente, a vineyard manager in the Lodi area, said he felt fortunate because he was able to apply herbicides to most of the vineyard ground between storm breaks.

“We put ours on in mid-February after all of that rain in January,” he said. “We pretty much got it in. But this year, for whatever reason, it just seems like the weeds are slower growing because of the rain and the cold weather. On one ranch that was flooded, there’s virtually no weeds on the berms.”

Vineyard weed management is typically focused on a band under vines to reduce competition, potential pest habitat and interference with air movement and harvest. Meanwhile, growers frequently plant cover crops or allow native vegetation to grow in the middle of rows during the winter and spring.

A popular weed-management trend is to graze sheep in vineyards during the winter. The vineyard owner receives vegetation management and nutrient recycling while the flock owner has pasture for the animals.

But Roncoroni said running sheep through some vineyards this winter may cause soil compaction because of unusually wet conditions.

In addition, sheep like to graze on tender grapevine vegetation. With bud break imminent, a “180-pound ewe could wipe out this year’s harvest,” he said.

Mechanical cultivation, which involves running a modified plow, flexible fingers or sets of blades in the soil around grapevine trunks, also carries compaction concerns.

“The last thing you want to do is get in with a cultivator when the soil is wet,” Roncoroni said. “Not only does it mess up the soil structure, but it doesn’t do a very good job on the weeds.”

His comments came as part of the San Joaquin Valley Winegrowers Association’s 2023 grower education series.

If growers opt for chemical weed control, Roncoroni recommended always reading herbicide labels beforehand since some have application restrictions after bud break. Others may allow use only on bearing or nonbearing vines or require hooded sprayers after bud break.

Mick Canevari, a UCCE farm advisor emeritus, said ideally he’d like to see growers apply pre-emergent herbicides in November or early December so they can take advantage of rains to incorporate products into the soil.

Pre-emergent herbicides kill weed seeds before they germinate or seedling weeds just after germination. The late fall timing also allows growers to apply products before leaf fall.

“When the plants start defoliating, that barrier helps because it prevents sunlight and germination for a month and helps extend the weed control,” said Canevari, who conducts weed control trials in San Joaquin County.

A properly timed fall pre-emerge program may provide weed control through most of the winter, he said.

Growers may follow up in late winter or early spring with post-emergent herbicides to manage already germinated weeds or a tankmix of pre- and post-emergent products.

As mostly contact materials, post-emergent herbicides work best on small weeds, Canevari said. That may be a challenge this season, because rains provided ample moisture and weeds grew “pretty much uncontested” for months, he said. Standing water in some vineyards also has prevented growers from making timely herbicide applications.

“If you think an herbicide is going to control weeds that are 2 to 3 feet tall, it’s not going to happen,” Canevari said.

What some growers may end up doing is making several post-emergent herbicide applications this season to try to knock down overgrown weeds, he said.

Among the weeds Roncoroni sees as potentially more troublesome this year are cheeseweed, also known as malva; ryegrass; nutsedge and horseweed.

“Cheeseweed can be a problem every year, but it could be a bigger problem this year,” Roncoroni said. “It’s one of the first weeds to germinate during the fall, and we’ve had good growing conditions. By the time you’re getting out into your vineyards, these things are going to be huge.”

If cheeseweed becomes too large, Roncoroni said growers may have to incur additional expenses to hire hand crews to hoe out the tough, woody-stemmed plants.

Italian ryegrass is a “huge” problem in North Coast vineyards, particularly those biotypes resistant to multiple herbicide modes of action. That includes glyphosate, which at one time was the go-to herbicide for ryegrass.

Horseweed, also known as marestail, germinates in the fall as well as in the spring, he said. As with most other weeds, growers obtain the best control when they target small plants.

“In light soil, when it’s given a chance, it can grow to be 6, 7 or 8 feet tall, which is a problem,” Roncoroni said.

Many populations of horseweed and the related hairy fleabane are resistant to both glyphosate and paraquat herbicides.

Yellow nutsedge flourishes in well-watered fields and vineyards. With standing water a problem in many areas this year, Roncoroni said he expected the weed to be a larger problem for some growers.

Nozzle selection also is important to help herbicides hit weed targets, Roncoroni said. Some labels specify nozzle types or droplet sizes. Aside from those, he said, flat fan or other low-pressure nozzles that produce larger, courser droplet sizes help reduce drift and minimize potential off-target damage to young, tender plant tissue.

With the possibility of larger weeds this season, Roncoroni said additional surfactants, which enhance herbicidal activity, may be needed. Ammonium sulfate, a common plant nutrient, also can boost glyphosate activity. In hard water, it ties up positively charged calcium, sodium and iron atoms and prevents them from binding with glyphosate molecules.

In addition, he said growers should check product labels for optimum water pH. Most herbicides work best when water is somewhat acidic in the 4-6 pH range. If water pH is above 7, or neutral, Roncoroni recommended adding a buffer to acidify it unless otherwise specified on the label.

(Vicky Boyd is a reporter in Modesto. She may be contacted at vlboyd@att.net.)