Here's an article from the San Joaquin County Field Notes newsletter (Feb. 2014.)
Dormant Weed Control in Tree Nut Crops 2014
Mick Canevari, Farm Advisor Emeritus
Brent Holtz, Pomology Farm Advisor and UCCE County Director
Brad Hanson, Extension Weed Specialist, UC Davis
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 in December, 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 and 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 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!
Manufacturer labels providing essential information about the proper use and application rate for all pesticides can be accessed at http://www.agrian.com or http://www.cdms.net. 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 at http://ucanr.org/blogs/UCDWeedScience/. UC Davis also has a UC Weed Research and Information Center: http://wric.ucdavis.edu/, a Weed Identification tool: http://weedid.wisc.edu/ca/, and current CA Tree and Vine Registrations: http://ucanr.org/t&v-registrations
For a full length newsletter article on dormant weed control in tree nut crops from last winter, go to http://cesanjoaquin.ucanr.edu/newsletters/Field_Notes_Newsletter44986.pdf (see pages 8-9). Or, call or come in and ask for the November 2012 Field Notes newsletter.