Threshold of horseweed (Conyza canadensis) in an established 'Thompson Seedless' vineyard in the San Joaquin Valley of California

Abstract

In recent years, horseweed (Conyza canadensis L. Cronq.) has become a very common weed in vineyards of the San Joaquin Valley, California. Up to 16 horseweeds m-2 have been observed on berms (raised beds) in some vineyards, but the effect of different horseweed population densities on grapevine productivity is not known. A three-year study was conducted at the Kearney Agricultural Center, Parlier, California from 2006 to 2008 to evaluate the effect of different horseweed densities (0, 4, 7, 12, 16, and 18 plant m-2 within the vine rows) on grape yield components, fruit quality, harvest interference, and pruning weights of 30-year-old flood-irrigated 'Thompson Seedless' grapevines. Horseweed seedlings were transplanted to the vineyard berms in early spring each year and allowed to grow throughout each season. Horseweeds, regardless of planting density, had no effect on grapevine growth, yield, yield components, fruit quality, or the time needed to harvest the fruit in any year of the study or as a cumulative effect of three years. However, the above-ground dry mass of individual weeds was reduced by intraspecific competition beyond densities of 7 plants m-2. Therefore, the threshold of horseweed in flood irrigated mature raisin vineyards may exceed 18 horseweed plants m-2. Copyright © Taylor & Francis Group, LLC.