Changing Weeds Require Changing Weed Control Practices

Dead leaves and weeds under vines reduce the efficacy of herbicides and can be a source of future disease and insect problems. Photo: John Roncoroni
Dead leaves and weeds under vines reduce the efficacy of herbicides and can be a source of future disease and insect problems. Photo: John Roncoroni

Weed control in vineyards certainly isn’t getting any easier. Drought and increased resistance to herbicides complicate an already tough issue. As fall approaches, American Fruit Grower talked with John Roncoroni, Weed Science Farm Advisor for the University of California Cooperative Extension, about the major weed control issues facing grape growers and the one thing he hopes growers will do that will make a difference.

Q. What would you say is the most difficult issue in the area of weed control in vineyards right now?

Roncoroni: The most difficult issue is resistance. There are two categories: resistance and tolerance. There are weeds that have always been tolerant — that glyphosate has never really killed. In California, those are panicled willowherb (Epilobium brachycarpum), little mallow, which is also known as cheeseweed (Malva spp.), and filaree (Erodium spp.). Then there are weeds that are tolerant but have become even more resistant, such as horseweed, or mare’s tail (Conyza canadensis), and hairy fleabane (Conyza bonariensis). Another weed that has become an increasing problem is annual or perennial ryegrass, which geneticists now tell us are pretty much the same. And now with Festuca (ryegrass was Lolium and is now Festuca) we have multiple resistance, starting with glyphosate, some glufosinate resistance, and now we are seeing resistance to some of the grass herbicides as well.

Then there’s another category that I call “avoidant.” There are some weeds that have become well-established because of our farming practices  — getting away from cultivation and trying to reduce herbicide applications to just one or two per season starting in the late fall or winter, and then  a follow up in early  summer. We have weeds that fit into that niche. A prime example of the one of those weeds is sharppoint fluvellin (Kickxia elatine). It germinates all year long except for in the dead of winter. [In vineyards] it starts to germinate after growers stop spraying herbicides in the summer, yet are still irrigating. Many growers that use glyphosate have gotten away from pre-emergence and/or post-emergence applications in the fall due to the increased risk of damage due to low-hanging vines. This weed has fit in and continues to grow through the fall.

Q. Should growers reduce their use of glyphosate because of resistance?

Roncoroni: Some growers are already looking into reducing glyphosate use because a few of the wineries are pushing for it. But what I’m concerned about is the use of a ‘glyphosate only’ program and with proper timing of applications. Because of glyphosate’s systemic activity, we have to wait until there are no green leaves, which takes a while here, to make applications. It has actually kept us from doing fall weed control; it’s pushing back weed control that should be done soon after harvest. Growers have to wait, and while they are waiting, many conclude that they might as well wait for the rains and more weeds to germinate before making the application, which pushes it back even more. So yes, I do think growers need to reduce their dependence on glyphosate for a number of agronomic reasons.

Q. How important are cultural practices?

Roncoroni: They are quite important. I would like to see growers do a light cultivation — maybe not every year — where feasible that would work, but at a minimum every three years. I do know there are some places that just can’t be cultivated due to erosion or rocks,  but growers do need to get away from first of all, a post-emergent-only routine. Secondly, I think they need to do something other than herbicides at the least every few years.

I am also a big proponent of removing all fallen leaves before applying an application, whether post-emergent or pre-emergence. If you use a pre-emergent and do a good job of keeping weeds out at the beginning, when the leaves do fall, they blow through, and you can start clean with a post-emergent.

It doesn’t have to be a special machine or operation. One of our largest growers simply secured two backpack blowers to the front of an ATV to blow the leaves away from the undervine area. Right behind the ATV was the sprayer, so the herbicide hit clean ground. And it made a big difference. Read any pre-emergence herbicide label, and they all say “apply to clean beds.”

Q. What do you suggest for growers who are dealing with resistant or tolerant weeds?

Roncoroni: First, call your local Farm Advisor or Extension Agent. You will definitely need to change up your strategy; it may be a different herbicide, or it may involve cultivation or other non-chemical methods. If you have several hard-to-control weeds, you need to determine the worst of these, and then concentrate practices on that one or two, and then move on from there. You can’t always control all of them all the time. And it may change the next year because you’ve changed your practices.

Q. If you could suggest one thing that growers could do that would make a real difference in their weed control programs, what would it be?

Roncoroni: I would love to see growers do their weed control budgets on a three-year cycle, so they can sneak in a cultivation every few years. A cultivation will likely be more expensive, but when you average it over three years, it’s not so bad. It would be one cultivation instead of one herbicide application, and it would likely change the weed spectrum. A light cultivation would break up the soil a little bit and will also make herbicides more effective, especially in a dry year. Just doing this once every two or three years would help quite a bit.