Vineyard Weed Control After a Wet Winter
Karl Lund Viticulture Advisor Madera, Merced, and Mariposa Counties
The California Department of Water Resources maintains 5 precipitation monitors in the Sierra Mountain foothills within the headwaters of the San Joaquin River. These monitors are used to track the amount of rainfall in the region that supplies the San Joaquin Valley with the bulk of its water supply. These monitors can also be used to compare current trends in rainfall with historic high and low rainfall events. The average rainfall is 39.9 inches per rainfall year. The two driest rainfall years on record are 1923-1924 (14.6 inches), and 1976-1977 (15.4 inches). While the two driest years occurred some time ago, the next three driest years on record were much more recent. The 3rd driest is 2020-2021 (18.8 inches), the 4th driest is 2014-2015 (19.0 inches), and the 5th driest is 2013-2014 (20.4 inches). On the other extreme, the wettest two rainfall years on record were 1982-1983 (77.4 inches) and 2016-2017 (72.7 inches). 2018-2019 (50.0 inches) was also on the wetter side, being the 21st wettest rainfall year on record. The 2022-2023 rainfall year is now on track to be one of the wettest with over 58 inches of rain. With more rain in the forecast before the end of March, and all of April left to accumulate more rain. This already large total amount of precipitation will affect how the 2023 growing season in several different ways.
Image 1. Rainfall totals in the portion of Sierra Nevada Mountain foothills that are the headwaters of the San Joaquin River. Current rainfall through February 14, 2023 (dark blue) is compared to 1982-1983 (green), 2016-2017 (light brown), 2018-2019 (blue), average rainfall across 1991-2020 (light blue), 1976-1977 (black), and 1923-1924 (dark brown). Image downloaded from: https://cdec.water.ca.gov/precipapp/get5SIPrecipIndex.action on February 14, 2023
One of those effects, and the focus of this article, will be on weed management. The emergence of weeds is closely related to several factors including precipitation, land disturbance (tillage), and temperature (Roberts and Potter 1980, Roberts 1984, Werth et al. 2017). Specifically, it was found that rainfall totals above 10mm of precipitation would initiate weed emergence (Werth et al. 2017). Thus far this rainfall year we have had several events that would trigger weed emergence. November 7th and 8th combined crossed the 10 mm threshold, and December 1st, 3rd, 10th, 27th, and 31st each individually crossed the 10mm threshold, as did January 5th, 9th, 14th, and February 5th (table 1). Not only is this a large number of events, but they are also well spread out to trigger several separate weed emergence events. In addition, as there have been so many events, the soil has remained saturated for much of the winter allowing weeds to continuously germinate. This will make the weed pressure high in your vineyard to start the season. And if the rainfall continues, we could see even more weed pressure going forward. This will make weed management an important item to plan and get started.
Table 1. Rainfall totals near 10mm (0.4 inches) for CIMIS station 80 on the Campus of Fresno State for October 1, 2022, through February 14, 2023.
The first question to ask is when to start trying to control the weeds. While cover crops are often something that is planted, they can just as easily be the plants that emerged naturally. So, you can think of the plants (weeds) currently growing in your vineyard as a native cover crop. Cover crops can help prevent soil erosion and water runoff while increasing water infiltration and soil organic matter (Novara et al. 2021). These are all positive benefits to help prevent issues from heavy rains while saving more water in your soil profile for the growing season. Cover crops can also increase the biodiversity of the soil microbiome (Novara et al. 2021), improving soil health. And lastly, cover crops can increase arthropod diversity (Geldenhuys et al. 2021) leading to the possibility of more natural predators in your vineyard.
These are just a few reasons to consider allowing winter weeds to continue growing as long as the rains continue. However, weeds can lead to an increased risk of spring frost damage. By shading the soil surface weeds will reduce solar heating of the vineyard soil during the day. The soil releases this heat into the vineyard overnight. Thus, by shading the soil, weeds can lower the effectiveness of the soil to warm your vineyard and lead to a higher chance of frost damage. In addition, once the rains end the weeds will continue to use water and start to affect the grapevines in your vineyard. If water becomes limited in your vineyard’s soil profile the weeds can devigorate the grapevines (Monteiro and Lopes 2007, Novara et al. 2021). Whether you are worried about water use, or frost damage, once the growing season starts the extra weed growth a wet winter has brought us should be dealt with.
To begin controlling the weeds in the vineyard means putting equipment into the field. However, putting heavy equipment into the field while the soil remains saturated can cause soil compaction. As your equipment passes over saturated soils it creates compacted soil layers leading to hardpans within the soil profile. These compacted soils and hardpans make it harder for the vine’s roots to easily spread throughout the soil profile, limiting the growth of your vine’s roots. Compacted soils and hardpans also make it hard for water to penetrate deep into the soil profile. This can make your irrigation less effective at wetting deep soil layers, and leave your vineyard open to worse runoff and erosion when the next heavy rainfall year happens. Overall, it may be best to wait until after the soil profile begins to dry before putting heavy equipment into the vineyard.
Tillage of the soil adds another soil layer to the compaction discussion. As we just discussed bringing tillage equipment into the vineyard while the soil is still saturated runs the risk of creating deep compaction layers in the soil. But tillage before a rain event can also lead to surface layer crusting of the soil. When rains interact with the unprotected soil surface, it can form a crust on top of the soil. This crust prevents water penetration leading to more issues with runoff in your vineyard. Preventing surface soil crusting can be done with cover crops (another advantage of holding off on removing weeds) or other residual plant material left on top of the soil surface. Soil surface crusting can be broken up afterward with a ring roller. However, this is another activity that you will need to work into an already busy spring growing season.
Table 2. Water Solubility and Soil Adsorption of preemergent herbicides registered for use in California vineyards.
The use of preemergence herbicide is another control method that may not work as well as during a wet year. Again, applying preemergent herbicide to your vineyard does carry a risk of soil compaction. However, if your vineyard soil dries out quickly you may be able to get in an application between rain events. Preemergent herbicides require some rain or irrigation to properly incorporate into the soil profile, but too much rain or irrigation can dilute preemergent herbicides making them less effective. The likelihood of a preemergent herbicide being diluted by rain event depends on solubility in water and soil adsorption. Herbicides with greater solubility in water are more easily washed away. And the lower soil adsorption a preemergent herbicide has the easier it will be to wash away. Table 2 shows a list of water solubility and soil adsorption for preemergent herbicides labeled for vineyard use. If you use a preemergent herbicide with high water solubility and/or low soil adsorption prior to a major rain event there is a possibility that its efficacy will be lowered.
Wet winters also make it difficult to make effective use of postemergence herbicides. Considering that the first rain event that could have triggered weed growth was back in early November, the weeds growing in San Joaquin Valley vineyards are going to be fairly large by now. For postemergence herbicides the larger the weed the lower the efficacy the herbicide will have. Work done on Hairy Fleabane found that glyphosate, glufosinate, paraquat, and saflufenacil could all achieve 100% weed control when sprayed at the 4-5 leaf stage. However, when sprayed on larger bolting plants glyphosate still managed 93% weed control, while glufosinate and paraquat fell to 64% and 60% weed control respectively, and Saflufenacil only managed 37% weed control (Sosnoskie 2017). While hairy fleabane is a summer weed, the same holds true for winter weeds. Meaning that the larger weeds will need either a second pass or additional methods to get them under control. In addition, larger winter weeds can also physically block efficient herbicide application on young (small) summer weeds. Giving the summer weeds a chance to outgrow easy control and giving you a second reason to get your weeds under control as soon as the soil allows you into the vineyard.
Mechanical control methods can complement postemergence herbicides. Once the soil has dried down and the rains have slowed tillage will again be a viable option to downsize the weeds growing in your vineyard. This would allow postemergence herbicides to again be efficient in controlling weed growth and regrowth. However, tillage does have negative effects even once the risk of soil compaction has passed. Tillage lowers soil health (Lazcano et al. 2020) and has higher levels of greenhouse gas emissions (Zumkeller et al. 2022). Mowing is another option for reducing weed pressure and potentially improving postemergence herbicide performance. Mowing will also leave residual plant material on the soil surface to help prevent soil crusting if rains continue into the growing season. Cover crops terminated by mowing also saw a 45% higher water infiltration when compared to tillage termination (Zumkeller et al. 2022). This would indicate that mowing instead of tillage would help to allow spring rains to better infiltrate into your vineyard soil which would be beneficial should any further rainfall occur as it would help save more water in the vineyard soil, thereby reducing irrigation demand in the first part of the growing season. Overall, this makes mowing weeds a better option for regaining control over weed growth in combination with herbicides.
Conclusion
The 2022-2023 rainfall year has already been especially wet with no end in sight. This extra precipitation will lead to an excess in weed growth to start off the 2023 growing season. Wet soils can also prevent timely access to the vineyard delaying the ability to start dealing with this elevated weed pressure. The excess precipitation and excessive weed growth can lower the efficiency of both pre and postemergence herbicides. Therefore, it is best to use mowing, mixed with some tillage after the threat of major rain has passed, to bring weed growth under control. Postemergence herbicide can then be used to maintain the level of control you need for your vineyard operations.