Preemergence herbicide concepts for orchard crops

Cross post from the UC Weed Science blog 11-24-14. It’s that time of the year when we’re planning preemergence (aka “residual”) herbicide programs for orchard and vineyard crops in California.  Typically, these are the herbicides that are applied in the fall, winter, or early spring BEFORE weeds emerge (preemergence) and they usually affect weeds as they germinate or are just beginning to emerge from the soil. [often, people mistakenly think these herbicides kill seeds or sterilize the soil which is not actually the case]. As you’re planning the specific program for the weed problems in your orchards and vineyards (or any site, really), I thought it would be a good time to review some of the concepts of residual weed control with preemergence herbicides.  Hopefully the concepts and ideas presented in the following line drawings will help us think about what PRE herbicides can and can’t do, and how to best use them in the orchard and vineyard production system.

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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. 

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Dormant Weed Control in Tree Nut Crops 2012-2013

 Written by Mick Canevari and Brent Holtz, UCCE San Joaquin. 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 http://www.ipm.ucdavis.edu/.      

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January is a good time for Pre-Emergent Herbicides

This article from last year sums up the considerations and herbicides available for pre-emergent use. Remember that pre-emergent applications must be followed by rain – usually around a quarter inch. Some other weed resources available are as follows:Almond Weed Photo Guide – With Common and Scientific Names,Susceptibility of Winter Weeds to Herbicides,and Weed Susceptibility Chart to Registered Herbicides. Also, check out the weed science blog authored by UC Weed Specialist, Brad Hanson.

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