Are you choosing the right spray for the right weeds?

Kurt Hembree, UCCE Weed Management Advisor, Fresno County

With tree nut harvest complete and preparations being made for winter, now is the time to choose the right spray for the right weeds in your orchard. We’ve already had a couple of late-fall showers, which have germinated some key winter weeds like filaree, annual sowthistle, malva, horseweed, and hairy fleabane (see photos). Hopefully you already know what weeds you’ll be up against this season. If not, try and identify them ASAP. Smart, cost-effective weed control hinges on your ability to tailor your herbicide spray program to the specific weeds in your orchard.

If you’re not familiar with the weeds in your field, ask your PCA, consultant, chemical representative, or farm advisor to help you identify them. There are also plenty of paperback and online resources that can help you visually match your weeds with their common names as they appear on herbicide labels. Weeds of California and Other Western States, Weeds of the West, and The Grower’s Weed Identification Handbook have good pictures of small and large weeds. The UC IPM website (http://ucipm.ucanr.edu) also has a nice weed photo gallery for ID purposes. Make a list of the weeds in each orchard, and keep those lists so you can reference them at a later date. It will allow you to change your spray program to match the changes in your weed flora over time. This small effort in itself, will save you time and money, by not having to be fighting the same weeds year-after-year.

Once you’re confident you have identified the weeds you want to target, choose your herbicide(s) accordingly. The preferred method of weed control is to apply a combination of soil-residual (preemergent) herbicides, followed by timely foliar (postemergent) sprays when weeds emerge later. Tank-mixing appropriate pre- and postemergent materials will control both small emerged weeds and germinating weed seeds in the soil. This generally reduces long-term weed control costs compared to all foliar burn-down programs. If you “start clean”, your orchard generally “stays clean”. Whatever herbicide spray is used, always combine at least two different herbicide mode-of-actions in the spray tank and rotate herbicide mode-of-actions every couple of years. This will help reduce the occurrence of herbicide resistance and maintain herbicide usefulness. While foliar burn-down programs can be successful if the treatments are made when weeds are less than 3-4 inches tall, many growers don’t have enough equipment or manpower to be able to get across large acreages in a timely manner to treat when weeds are small and vulnerable. This usually leads to late sprays, weed regrowth, poor or erratic control.

For preemergent herbicides to do their job, they need to be sprayed directly onto the soil where the weed seeds are. If leaf litter exists, it is necessary to mechanically blow off the leaves and other debris before spraying to assure good herbicide-soil contact. If there are patches of large, dried weed skeletons present, it may be necessary to send a hand crew in to physically remove them too. The bottom line is, you want the soil-residual herbicide on the soil, not on the dried-up weeds and other debris where the herbicides can be tied-up and carried away with the first wind. Also, spray ahead of predicted rainfall to insure the herbicides are adequately activated in the soil. Most of the preemergents can remain on the soil surface for 2-3 weeks before needing to be activated by rainfall, but it is best to treat as close to a rainfall event as possible to reduce the amount of volatilization off the soil surface and maintain herbicide integrity. Table 1 lists the herbicides registered in tree nut crops in California. Product cost alone, or spraying “on the cheap” should never be the driving force behind herbicide selection. While cost is still a consideration, selecting products should be mainly based on trying to combine materials that will give you the best control possible for the weeds you want to control. Remember, weed species, soil types, tree age, and other factors often vary from orchard to orchard, so adjust the herbicide package selected accordingly.

Table 1. Herbicides registered in tree nut orchards in California
Herbicide a.i.	Herbicide	Herbicide
mode-of-action	product example	activity	Tree nut type
group 1	Select Max, Poast, Fusilade DX	foliar	almond, pistachio, walnut
group 2	Matrix, Pindar GT (+ grp 14), Mission	soil and foliar	almond, pistachio, walnut
	Sandea	soil and foliar	walnut
group 3	Prowl H2O, Surflan,	soil	almond, pistachio, walnut
	Treflan	soil	almond, walnut
group 4	Dri-Clean, Orchard Master	foliar	almond, pistachio, walnut
group 5	Princep	soil	almond, walnut
group 7	Direx	soil	walnut
group 8	Eptam	soil	almond, walnut
group 9	Roundup Powermax	foliar	almond, pistachio, walnut
group 10	Rely 280, Lifeline	foliar	almond, pistachio, walnut
group 12	Solicam	soil	almond, walnut
group 14	Chateau, Goal, Treevix, Venue, Shark, Pindar GT (+ grp 2),	soil and foliar	almond, pistachio, walnut
	Zeus	soil and foliar	almond, pistachio, walnut
		soil and foliar	pistachio, walnut
group 21	Trellis	soil	almond, pistachio, walnut
group 22	Gramoxone SL	foliar	almond, pistachio, walnut
group 27	Broadworks	soil and foliar	almond, pistachio, walnut
group 29	Alion	soil	almond, pistachio, walnut

In preparation for a weed spray, consider this “to do” list to help make the treatment successful:

• Select herbicides that are effective on the target weeds and registered for the tree nut type and age.

• Use herbicide combinations with at least two different herbicide active ingredient (a.i.) mode-of-action group numbers in the spray tank that are effective on your specific weeds.

• Use the highest label rate of herbicide(s) for difficult weeds to control.

• Add buffering agents, surfactants, or other additives as required on the label.

• Treat when the weeds are best controlled, according to the label.

• Make sure the sprayer is calibrated and all parts are working properly.

• Select nozzles and a spray pressure that provides good spray coverage.

• Spray under favorable environmental conditions to minimize spray drift.

• Have the person spraying monitor the application closely and resolve problems as they occur.