Written by Bob Beede, UC Farm Advisor Kings County
In order to properly address your weed problem, you first have to identify the weeds! Many of the new materials are very effective, but they are expensive, because they control noxious weeds such as flaxleaf fleabane and mare’s tail (horse weed), which have developed resistance to the older, less expensive preemergent herbicides. Kurt Hembree, Vegetation Management Farm Advisor, Fresno County, has his 2012 weed susceptibility and herbicide registration information at: 2012 Almond Weed Susceptibility; 2012 Pistachio Weed Susceptibility; 2012 Walnut Weed Susceptibility. Kurt has suggested rates of application for each product, along with suggested adjuvants. This is an excellent resource to help in effective material selection Great job, Kurt!
Prowl H2O, grass herbicide is very similar to Surflan AS (Oryzalin) in its weed spectrum and residual. Prowl remains stable on the soil without rainfall for 21 days. Apply it at the higher label rates (4-6 quarts per sprayed acre) for extended weed control. Surflan also controls annual grasses and a select number of broadleafs such as chickweed, lambquarters, purslane and the pigweeds. It is also is stable on the soil prior to a rain. One gallon per treated acre of these two products in the fall usually runs out before the end of the season, especially under drip irrigation. Hence, many growers elect to treat early season “winter weeds” with a low rate of glyphosate (Roundup, Touchdown) and Goal (each at about one quart per sprayed acre) and then wait to apply the Surflan or Prowl later in January or February to achieve season-long grass control. Devrinol is no longer registered for use on pistachio. Cost and residual are important factors to consider in herbicide selection. Read the label, and discuss your program with a qualified crop consultant for best results.
Chateau, is a relatively new preemergent herbicide (Valent) available for bearing and non-bearing pistachios that are at least one year old. Applied at 12 oz. per treated acre, Chateau enhances burndown of existing weeds (similar to Goal) and controls difficult weeds such as fleabane and horseweed (mare’s tail). Because of its postemergent characteristics, be careful using it in young trees. Avoid injury with tree wraps and use a shielded sprayer to reduce drift. Apply Chateau only during the dormant period to avoid phytotoxicity to emerging bud tissue in the early spring. Kurt Hembree, UC Farm Advisor, Fresno County, for weeds, also advises split applications in November and January for heavy fleabane control, since this noxious weed germinates early in the fall. A single application in January may result in “escapes” which make one think the product is ineffective. The addition of Rely or Gramoxone helps in controlling emerged fleabane and marestail. NOTE: Before using Chateau, check with your Valent representative for any use restrictions applicable to your area or soil type.
Matrix FNV is another relatively new Dupont preemergent herbicide active on fleabane, malva, yellow nutsedge, and marestail. Due to its contact activity on selected grasses and several broadleaves (when newly emerged, not a foot tall!), it appears to have a good fit for fall applications where management of the mentioned noxious weeds is required. It is a dry flowable formulation applied at 4 ounces product per broadcast acre. A second application or use of another preemergence product would be needed in the spring for extended weed control. Trees must be established for one full year before treatment. Spray solution should not exceed a pH of 8.0.
Visor is registered for non-bearing preemergent use and has a grass control spectrum similar to Oryzalin 4AS. In addition, Visor controls several broadleaf weeds common in pistachio orchards. The higher label rates also suppress purple and yellow nutsedge and emerged field bindweed.
Gallery Trellis, a broadleaf preemergent herbicide registered for use on almond, pistachio, and walnut, and provides similar weed control as Goal. Tests indicate Trellis provides better control of chickweed, fleabane, horseweed, Russian thistle and spurge than Goal. It is less effective than Goal on clovers and nightshade. Gallery is the same mode of action as Trellis, but registered for non-bearing use on tree nuts.
Alion SC, by Bayer CropScience, is a newly registered preemergent. It is a cellulose inhibitor with broad spectrum, including fleabane and marestail (horseweed). Application is recommended between November and January before seed germination. Rain incorporation of one quarter inch or greater should occur within 21 days after application. The recommended application rate is five ounces per broadcast acre. During this introductory season, Bayer has limited applications to trees which have completed at least three complete growing seasons. Residual data shows control for five months. Since the complete weed spectrum has not yet been identified, a tank mix may be needed to cover misses. This herbicide is currently not registered for use for the first 36 months of tree growth.
Growers electing to dispense with a preemergent herbicide this winter and apply multiple postemergent treatments throughout the season have a good selection of herbicides available, including Roundup, Touchdown, Sandea, Gramoxone, Shark, Fusilade, Rely, Goal, and 2,4-D. Postemergent application frequency, product selection and cost will vary greatly depending upon weed species and pressure. Sandea provides better control of both yellow and purple nutsedge than glyphosate (Roundup, Touchdown). Glyphosate is moderately effective on purple nutsedge with repeated applications prior to the six-leaf growth stage. Read the Sandea label carefully and consult your crop advisor before applying to sandy soils.
Those electing not to apply a preemergent herbicide should tune up their application equipment to insure optimal coverage. Low rates of Goal in combination with glyphosate have been documented as more effective in burning down existing weeds than if used alone. Use of a high quality adjuvant is also essential to achieving maximum performance. The addition of ammonium sulfate at about 10 pounds per 100 gallons of spray solution also improves glyphosate efficacy.
What’s better? Repeated postemergent applications or a good preemergent program? I would strongly advise you to use the preemergent program. Weeds have a way of getting away from you. Wet weather makes this even more likely. Cost comparisons between pre and postemergent programs often show that the expense of repeated contact application equals or exceeds the onetime cost of the preemergent treatment. This is especially true if you have noxious weeds like fleabane which are best controlled with the newer preemergent materials. Also, experience has shown glyphosate-based materials to be risky on first year trees, even with tree wraps. Most PCA’s avoid this recommendation, so do not plan to make it your first postemergent material of choice for the first season.