Marestail (Conyza canadensis; aka horseweed) and hairy fleabane (C. bonariensis; aka flax leaf fleabane) are serious weed problems in vineyards in Monterey County.  Both plants are summer annuals that germinate in the fall at the onset of the rainy season (October – February), or germinate in the spring (March – May). Seeds of both species are not long lived (2-3 years) and are ready to germinate when conditions allow as soon as they mature on the plant; they germinate on the soil surface or from no greater than 0.2 inch deep. The seedlings of the two species look very similar to each other until they bolt at which time they can be easily distinguished (for detailed information on these species go to: http://anrcatalog.ucdavis.edu/Items/8314.aspx).  Mature plants of marestail and hairy fleabane produce upwards of 200,000 and 40,000 thousands of seeds, respectively; seeds are disseminated over long distances (i.e. ¼ mile) by the wind.  Both plants are more adapted to undisturbed environments and even minimal tillage in the fall or spring can effectively control them.  As evidence of this point, in a five year study that we conducted comparing a pure postemergence weed control program vs cultivation (with a Clemens^®), marestail was shown to emerge as a specific weed problem in the postemergence treatment program and was not a problem where cultivation was used (http://californiaagriculture.ucanr.org/landingpage.cfm?article=ca.v062n01p19&fulltext=yes).

Marestail and hairy fleabane can become problems in vineyards for a number of reasons. In strip sprayed vineyards, they can emerge along the edge of the sprayed strip or other untreated areas. Seeds flying in from surrounding areas can also contribute to emergence of these weeds. There are a number of effective preemergence materials that can provide good control of marestail and hairy fleabane (see table below) and these materials should be considered in areas where these weeds are a problem. In addition, there are a number of postemergence options for controlling these weeds. Glyphosate resistant strains of both of these weeds have evolved following years of use of this herbicide. This has been documented in the central valley of California and is assumed to be the case in Monterey County. Glufosinate (Rely) has been a key herbicide for controlling marestail and hairy fleabane in vineyards that are not controlled by standard postemergent programs (i.e. glyphosate and oxyfluorfen (Goal)). However, in 2013 there will likely be a shortage of glufosinate as supplies of this herbicide are being diverted to the Midwest and South to combat roundup resistant strains of marestail, Palmer amaranth, and other weeds in corn,  soybean, and cotton production systems (see http://ucanr.org/blogs/blogcore/postdetail.cfm?postnum=8877). The situation is not entirely settled, as glufosinate is now off patent and other smaller producers may start production, but indications are that there may be limited supplies of glufosinate available for the tree and vine market in 2013 and 2014. In that sense it is important to consider your options carefully and plan ahead for controlling marestail and hairy fleabane.  

Table 1. Herbicides registered for use on grapes with activity on marestail and hairy fleabane

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 UCCE Monterey County Central Coast Vineyard News