Weed control considerations for 2022

By Brent Holtz, UC Farm Advisor, and Mick Canevari, UC Emeritus Farm Advisor

This article is part of a Pacific Nut Producer December tasks newsletter

The 2021/2022 rainy season started with early and significant rain in November throughout the state. Weed germination is well underway and now is a perfect time to apply winter pre-emergent herbicides ahead of a rain event and before weeds get too large.

Pre-emergent Weed Control in Orchards

The advantage of an early “dormant” post-harvest, pre-emergent herbicide program, is that it will control most winter germinating weed seeds. Using a pre-emergent ahead of a rain, or by activating with sprinklers, will control millions of weed seeds for 3-6 months while preventing continuous emergence throughout the winter into spring. Applying a pre-emergent early ahead of leaf drop eliminates an additional operation of sweeping or blowing leaves after drop has occurred. Weed control is enhanced and herbicide efficacy is extended by a month or two when a barrier of leaves covers the herbicide treated berms. In contrast to having to make additional herbicide applications to remove excessive weed growth before a later pre-emergent herbicide can be applied. Applying soil herbicides over large weeds inhibits the herbicide from having good contact with the soil. A weed control program of applying soil active herbicides in early fall (November/December), and repeated again in early spring (March/April), will extend weed control well into summer. This is the best strategy to manage herbicide resistance, which has been observed to occur in glyphosate, paraquat, sethoxyodim, and clethodim. Herbicide resistant weeds continue to spread across the state in different areas and cropping systems, with the greatest occurrence in perennial cropping systems of orchards and vineyards. Pre-emergent herbicides are the preferred method of weed control to reduce weed resistance. There are several herbicide programs that will control resistant ryegrass, junglerice, goosegrass, fleabane, and horseweed. Attacking resistant weed seeds at the point of germination gives us the greatest opportunity to control weeds using both chemical and non-chemical cultural practices. Using a timely balanced approach of tillage, herbicides, and other cultural practices, can save time and money with the best weed control.

Weed Control Options

There are numerous herbicide options and combinations available. Not one size fits all but there are many ways to achieve good results for almost every situation. Before herbicide recommendations are made the following key questions should be answered with your pest control advisor (PCA). Tree age, root stock, soil composition (sand, clay, gravel), irrigation design and irrigation practices, water source and quality, cultural weed practices used, total weed species, the predominant target species, if perennials are present, sprayer type, spray volume, and adjacent crops. Discussing these questions provides a basis for the PCA and applicators recommendations.

New Herbicides for 2022

Craze (orthosulfamuron) HRAC Group 2 is a new broad-spectrum soil residual herbicide from Nichino America. The active ingredient in CRAZE is orthosulfamuron. This herbicide, an acetolactate synthase (ALS) inhibitor, that works by inhibiting the enzyme responsible for the production of branched chain amino acids (valine, leucine, and isoleucine) essential for plant growth. It has a similar weed control profile as Matrix and Mission. CRAZE has demonstrated broad spectrum residual control of many broadleaf and grassy weeds including marestail and fleabane and suppression of yellow nutsedge if applied in a timely pre-emerged manner. Several years of testing has demonstrated that it will fit into our Tree and Vine weed control program with flexibility and safety. In our testing, tank mix combinations with Alion, Chateau and Prowl provided additional long-term control. Figure 1.

Embed Extra (2,4-D) HRAC group 4 post emergent herbicide registered in almonds, pistachios, walnuts, pecans and certain tree fruit. It controls a long list of broadleaf weeds, both winter and summer annuals, and broadleaf perennials including marestail, bindweed, malva. Embed Extra is a new post-emergent herbicide, containing a choline formulation of the active ingredient 2,4-D which reduces the risk of volatility (soil lift off) while minimizing drift. The systemic action of Embed Extra provides a more thorough kill down to the roots of larger broadleaf weeds, that many contact herbicides struggle with. Embed Extra translocates at a slower pace in weeds than contact herbicides. A grower won’t see immediate visible burn, since its mode of action is systemic, mimicking the action of the plant growth hormone affecting cell division that is often referred to as “a plant growing itself to death”. Embed Extra can be tank mixed with other post-emergent herbicides, such as glyphosate and glufosinate. It can also be tank mixed with many “residual”-type herbicides.” Our trials on spring treated larger fleabane, marestail and willow weed showed excellent results. Figure 2

Overview of Pre-Emergent Herbicides (before weeds germinate)

Prowl H2O (pendimethalin) HRAC group 3 herbicide has excellent annual grass control along with many broadleaves, especially those that germinate in the spring and summer. Surflan (Oryzalin) and Prowl are of the same mode of action and similar in their weed spectrum and residual properties. Prowl H2O and Surflan remain stable on the soil without moisture for up to 21 days. Prowl is often a herbicide of choice on newly planted orchards because of its soil binding properties and safety to young trees. A common strategy is to use a different mode of action herbicide such as Chateau, Matrix, Alion, Pindar GT or Broadworks, and then substituting with the DNA chemistry of Prowl in the spring (March to May) to control summer annual weeds and grasses.

Chateau (flumioxazin) HRAC group 14 is a long-lasting pre-emergent herbicide available for tree, nut, and vine crops. Applied between 6-12 ounces per treated acre, Chateau enhances burndown of small broadleaf weeds and provides long residual control of difficult to control weeds such as fleabane and horseweed (marestail), malva, willow weed, filaree and a host of other winter weeds. Its broad-spectrum control of winter annuals makes it an excellent herbicide for use in the fall and early winter timing during the dormant period. Chateau at lower rates (6-8 ounces) has become a popular tank mix with other pre-emergent herbicides to extend control of more weed species.

Matrix (rimsulfuron) HRAC group 2 is an ALS inhibitor pre-emergent herbicide active on many winter and summer weeds including fleabane, willow weed, and marestail, barnyard grass, jungle rice, crabgrass. Its broad-spectrum activity on grasses and broadleaf weeds, makes it a good fit for an early fall application timing of November / December. It has become a popular herbicide to manage glyphosate resistant ryegrass pre-emergent and is effective post emergent on small emerged ryegrass. We have observed nice broad spectrum weed control when other pre-emergent herbicides are combined with Matrix. A post emergent herbicide should be added when weeds are emerged. Matrix is safe on young trees and a popular tank mix with Prowl or Trellis.

Mission (Flazasulfuron) is a HRAC group 2, an ALS inhibitor in the same family as rimsulfuron, orthosulfamuron (Matrix, Craze) with a similar weed spectrum. Our research has shown that Mission has shown an increase in post emergent activity on certain weeds at a rate of 2.14 ounces per acre. Research from other Farm Advisors suggests improved yellow nutsedge activity when applied ahead of emergence with adequate water for incorporation. Use post tank mix partners if weeds are up.

Alion (indaziflam) is a broad spectrum preemergent soil active herbicide registered in tree nuts and vines. It is a long-lasting soil residual herbicide exceptional in controlling many grasses and broadleaf weeds. It is effective on both winter and summer annual weeds, including fleabane, marestail, sowthistle, and willow weed, shepherds purse, chickweed, and most grasses. At least 1/4 inch of water is needed to set and activate soil residual. Since it is strictly a pre-emergent herbicide, it requires a tank mix with a post herbicide for emerging weeds, typically Rely, Matrix, Shark, Goal, Roundup, Venue, Treevix and Gramoxone are all compatible. Alion has shown to be an excellent tank mix partner with other pre-emergent herbicides to increase weed spectrum, especially for malva and knotweed. Since it does not control weeds already germinated and just under the soil (called white phase) it should be tank mixed with herbicides that will control weeds just emerging (Matrix, Pindar Gt or Goal are popular choices).

Pindar GT (oxyfluorfen and penoxsulam) HRAC group 14 & 2 is a premix with two active ingredient herbicides in one bottle. It combines both a pre- and post-emergence punch for many broadleaf weeds used in tree nuts and fruits but not registered in vineyards. An excellent herbicide for winter applications beginning in November to January, that can provide residual control lasting into spring and early summer. It is especially effective on fillaree, malva, henbit, willow weed, sowthistle, fleabane and marestail. If weeds have emerged, it is still recommended to combine it with a post emergent herbicide such as Roundup, Rely, or Gramoxone. If heavy grass pressure is anticipated in the orchard, an addition of Prowl, Matrix or Alion will extend long term grass control into spring. Within 14 days of application, an half-inch of water is needed to set and activate the herbicide.

Trellis (isoxaben) HRAC group 21 is registered for use in tree nut and fruit crops and for non- bearing newly planted orchards (check label for specific timing). It is a pre-emergent herbicide controlling only broadleaf weeds, therefore consideration of using another herbicide for grass control. Applied in the fall winter time frame, Trellis will provide 4-5 months of broadleaf control. It has no post-emergent activity; therefore, it must be tank mixed with the commonly used post emergent herbicides Trellis’ mode of action is unique; it inhibits cellulose development making it a good rotational herbicide to manage weed resistance. If grass weeds are an issue, the addition of grass herbicides such as Matrix, Alion, Prowl or Surflan will be needed.

Broadworks (mesotrione) group 27 is registered in almond, walnut, pistachio and certain fruit crops. It is primarily a pre-emergent herbicide to control only broadleaf weeds hence the name Broadworks. We have observed some post activity on small broadleaf weeds (fleabane marsetail) when actively growing but will still require a post herbicide partner for complete burn down of other weeds. Broadworks controls some of the more difficult broadleaf weeds in orchards but will need a tank mix partner that controls grasses. We have observed a general reduction in bindweed populations with spring applications of Broadworks.

Post Emergent Herbicides (when weeds are up)

Rely (glufosinate) a HRAC group 10 herbicide has become a mainstay for growers needing a broad spectrum burn down herbicide to control tough broadleaf weeds and grasses like fillare, willowweed, barnyardgrass, crabgrass or glyphosate resistant fleabane and marestail. In recent years, the development and spread of glyphosate resistant weeds has shifted our industries to use more Rely. Rely does not completely translocate like glyphosate, so the key to success is good coverage and smaller weeds. It is a popular tank mix with any of the pre-emergent herbicides. We have found a Rely + Roundup combination to be very effective across most weed species including malva.

Treevix (saflufenacil) HRAC group 14 is a PPO (Protoporphyrinogen oxidase) post-emergent broadleaf herbicide with a short soil residual of approximately 30-45 days, registered in tree nut and fruit crops. It is recognized as an excellent post emergent herbicide for some of our toughest emerged broadleaf weeds. However, mostly effective on broadleaf weeds, it should be mixed with another post-emergent herbicide to control emerged grasses. Like all post contact herbicides, treating small weeds, 1- 6 inches tall, with thorough spray coverage is important using a 25-40 gallon per acre (GPA) rate. Treevix is excellent in burning down young fleabane, marestail, malva, willowweed, sowthistle, and prickly lettuce, especially in cool conditions from fall through spring. It is not considered a soil residual herbicide, so it should be mixed with a soil active herbicide for long term residual control.

Similar group 14 PPO herbicides in this category are Shark, Venue, ET, and Goal. They are also post emergent herbicides with little or no soil residual (except Goal at higher rates). They provide a nice tank mix addition to pre-emergent herbicides to control emerged broadleaf weeds. They also work well with other post herbicides glyphosate and glufosinate (Roundup & Rely). A rule of thumb for all post-emergent herbicides, “Spray when weeds are small!”

Some growers and weed pressures may need multiple post-emergent treatments for specific weed issues or escapes. Roundup, Rely, Gramoxone, PPO group 14 and certain 2,4-D formulations are registered for multiple use in nut and fruit orchards. Glyphosate is moderately effective on purple nutsedge with repeated applications prior to the six-leaf growth stage. Yellow nutsedge can be controlled by using 3-4 quarts per acre of glyphosate at two sequential application timings. Sandia® has shown very good results to control emerged nutsedge but is not registered on almond. Sandia is registered for use in pistachio and walnut orchards. The key to nutsedge and other perennial weeds is repeated applications as it emerges and before it can regenerate new underground nutlets or rhizomes. Problem broadleaf’s such as fleabane, marestail that germinate almost year around should be targeted with a post herbicide application when weeds are small before seed heads develop. Mutiple post emergent herbicide applications may be necessary until orchards mature and a full tree canopy begins to shade out perennial weeds.

NOTE: Before using any herbicide, always check the label for any use restrictions applicable to your area, crop, or soil type. Our comments and suggestions are from years of testing herbicides in our research plots and field trials. We appreciate our grower cooperators, who let us have access to their orchards. Again, always review and follow label instructions.

The UC Weed Science blog (http://ucanr.edu/blogs/UCDWeedScience/index.cfm) and the Weed Research and Information Center (http://wric.ucdavis.edu/) is an excellent place to find answers to determine the mode of action of different herbicides, what weeds they control, recommended rates, and in what crops they are registered to be used. The UCIPM web site has charts that show which weeds are controlled by what herbicides, and an excellent weed photo gallery that includes many weed species commonly found in California for easy identification and reference http://www.ipm.ucdavis.edu/. Consider buying the handy shirt-pocket sized Weed ID cards produced by UC ANR which can be ordered at: (http://anrcatalog.ucdavis.edu/Details.aspx?itemNo=3541). A free online weed ID tool is also available at http://weedid.wisc.edu/ca/

Good Luck!