Dormant Weed Control in Tree Nut Crops 2014

by Mick Canevari, Brent Holtz, and Brad Hanson

Current dry weather has preempted most normal winter weed germination and growth while prolonged periods of dry soil has caused some early weeds to desiccate and die.  In most tree and vine herbicide trials conducted so far this winter–the untreated controls look similar to herbicide treatments, very clean and without weeds.  What to do at this point poses some interesting questions.  Will it rain? Most of us are worrying more about irrigating our almond trees this summer than controlling weeds right now.

If you have some weed growth that germinated with rain our one December rain, or with fall and winter irrigations, you may want to apply a post-emergent herbicide now in order to prevent hard to kill weeds from becoming established.  The warm weather could allow these weeds to establish and become more difficult to control, impacting the success of later herbicide applications by increasing trash on the berm and reducing coverage. 

If you have solid set or micro irrigation systems and available water, you could apply pre-emergent herbicides and incorporate them with a light winter irrigation.  A lower concentration or rate could be used in February, compared to a November rate, and the chance of crop injury for some materials applied close to bud break will be reduced.  The Dinitroanaline herbicides, such as Prowl or Surflan, would be a good choice to be set with winter irrigations since this chemistry controls many of our summer weeds and grasses and is very soil stable under multiple rain events and continuous irrigations.  The Dinitroanaline herbicides however, do not adequately control our more troublesome and persistent weeds such as Fleabane & Horseweed, and to some extent Willow Weed and Malva that can germinate well into spring and early summer.  If you are battling these weeds you can consider other pre and post emergent herbicides in order to control them while they are small.

Growers with drip-irrigation (surface or buried) or furrow-run flooded orchards are going to have an awfully hard time getting decent activation of pre-emergent herbicides on dry berms.  Applying herbicides under different spray patterns (solid and spot sprays) will have to be considered by each farm operator.  Hopefully, we will get some rain in February that will still allow pre-emergent herbicides to be used effectively and provide good control into early summer.  But without rain we will have to adjust reduce herbicide rates for smaller or no weeds and a reduced time of residual control.  This may be the year to stock up on post emergent herbicides!

Dormant Weed Control

Weeds have a tremendous capacity to spread within an orchard.  The first line of defense is identifying the weeds you need to control, and selecting the best herbicides or cultural practices to control those weeds.  If you use the same herbicide(s) each year, a shift to tolerant weed species will ultimately take over and a loss of herbicide effectiveness will occur.  Alternating products with different modes of action at least every couple years will improve results and insure herbicides long term viability.  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

Pre Emergent Herbicides

Prowl H2O (pendimethalin) herbicide has excellent grass control and broadleaves especially those germinating in the spring and summer time.  Surflan (Oryzalin) and Prowl are similar in their weed spectrum and residual properties.  Prowl H2O and Surflan remain stable on the soil without rainfall for 21 days.  Apply them at the higher label rates (4-6 quarts per sprayed acre) for extended weed control.  Another strategy is to treat early season November/December for winter weeds with a low rate of glyphosate (Roundup, Touchdown) with a soil residual herbicide such as Chateau, Matrix, Alion, or Pindar GT and then wait to apply the Surflan or Prowl later in February or March to achieve summer long weed control.

Chateau (flumioxazin) is a long-lasting pre-emergent herbicide available for tree, nut, and vine crops.  Applied between 8-12 oz. per treated acre, Chateau enhances burndown of small broadleaf weeds and provides residual control of difficult weeds such as fleabane and horseweed (mare’s tail) and a host of other winter weeds as they germinate.  This has made Chateau an excellent herbicide for use in the fall/early winter timing during the dormant period.  This time frame also avoids phytotoxicity to emerging bud tissue in the early spring, especially on young trees.  The addition of Rely (glufosinate), Roundup (glyphosate), Treevix ( saflufenacil ), or Gramoxone (paraquat) is needed to control emerged weeds especially fleabane and marestail.

Matrix FNV (rimsulfuron) is a pre-emergent herbicide active on many winter broadleaf and grass weeds including  fleabane, malva, willow weed, and marestail.  Its broad spectrum activity on grasses and broadleaf weeds, makes it a good fit for an early fall application timing November/December.  It should be tank mixed with a contact herbicide; Roundup, Rely, Gramoxone, or Treevix. Matrix is applied at 4 ounces product per broadcast acre.  A second application or use of another pre-emergent herbicide is generally needed in the spring for extended summer weed control.  Matrix is very safe on young trees.

Alion (indaziflam) is a new herbicide registered in tree nuts.  It is a preemergent, long-lasting soil residual herbicide exceptional in controlling grasses and many broadleaf weeds.  It is effective on both winter and summer weeds including fleabane, marestail, sowthistle, and willow weed.  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 contact herbicide for emerged weeds; Rely, Roundup, and Gramoxne are all compatible.  Alion is a brand new chemistry and has shown excellent results and has an inhibiting cell wall formation MOA (mode of action).  This MOA will have an important role in future weed control strategies of weed resistant management.  Dr. Brad Hanson, Extension Weed Specialist at UC Davis, performed a number of trials where Matrix and Alion were tank mixed together and better long term weed control was observed (UC Weed Science Blog at

Pindar GT (oxyfluorfen and penoxsulam) is two herbicides, having pre- and post-emergence activity for use in tree nuts and fruits.  Applied in November/December, it provides residual control lasting into spring/early summer. It is especially effective on filaree, malva, willow weed, sowthistle, and many other winter broadleaf weeds. If weeds have emerged, it is recommended to combine it with a post-herbicide Roundup, Rely, or Gramoxone.  If heavy grass pressure is anticipated in the orchard, the addition of Prowl or Surflan will benefit long term grass weed control.  Within 14 days of application, a ½ inch of water is needed to set and activate the herbicide.

Trellis (isoxaben) has been recently registered for use in bearing almonds and other nut and fruit crops.  It is a pre-emergent herbicide controlling many winter and summer broadleaf weeds.  Applied in the fall/winter time frame will provide 4-5 months of control.  It has no post-emergent activity, therefore, it must be tank mixed with Roundup, Rely, or Gramoxone for emerged weeds.  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 a pre-emergent grass herbicide; Prowl or Surflan will be needed.

Post Emergent

Rely (glufosinate) herbicide has become a mainstay for growers needing a broad spectrum burn down herbicide to control tough weeds like fillare, willowweed, or glyphose resistant fleabane and marestail.  During the 2012 season, California was in short supply of Rely due to the high demand in the midwest for planting glufosinate corn varieties.  In recent years, the development and spread of Roundup resistant weeds is forcing a change from Roundup Ready corn and soybeans varieties to planting Liberty Link varieties which require the use of glufosinate herbicide (Rely, Liberty).  With the heavy use expected in corn states, Rely is again anticipated to be in short supply for California growers in 2013.  Growers should plan on alternative weed control strategies that will replace the use of Rely.  We are confident with the post- and pre-emergent herbicide combinations we have available and used in a timely manner, we can still expect excellent weed control results.

Treevix (saflufenacil) is a new post-emergent contact herbicide offered for almond, nuts, and fruit crops.  The use is for tough emerged broadleaf weeds but no activity on grasses.  Like all post contact herbicides, treating small weeds 1”- 6” tall with complete spray coverage is important.  Treevix is excellent in burning down fleabane, marestail, and willowweed, especially in cooler temperatures beginning in fall through spring time.  It has no soil residual activity, therefore, will need to be tank mix with soil active herbicides for long term control.  If grasses have already emerged, using glyphosate or Gramoxone is needed.

Some growers may prefer multiple post-emergent treatments rather than pre-emergent treatments, if orchard access is limited during the dormant season.  Roundup, Touchdown, Gramoxone, Shark, Venue, Rely, Goal, and 2,4-D are registered for use in almond orchards.  Glyphosate is moderately effective on purple nutsedge with repeated applications prior to the six-leaf growth stage.  Yellow nutsedge can be managed by using 4qts/A of glyphosate at each application.  Sandia has shown excellent results to control nutsedge, but is not registered on almond (Sandia is registered for pistachio and walnut).  The key to nutsedge control is repeated applications before it is able to regenerate new nutlets and tree size allows for orchard shading. Care should be taken to avoid resistance in weed species by repeated use of the same herbicide year after year.  Cost comparisons between pre- and post-emergent programs often show that the expense of repeated contact application equals or exceeds the cost of the pre-emergent treatment, especially if you have noxious weeds like fleabane, which are best controlled with these newer pre-emergent materials.  Herbicide application equipment should NEVER be used for treating tree foliage!  Manufacturer labels providing essential information about the proper use and application rate for all pesticides can be accessed at or  NOTE: Before using any herbicide always check labels for any use restrictions applicable to your area or soil type.

Dr. Brad Hanson, Extension Weed Specialist at UC Davis, has created a UC Weed Science Blog.  UC Davis also has a UC Weed Research and Information Center, a Weed Identification tool, and current CA tree and vine registrations.

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