Growth and interference of invasive Russian knapweed on Valcatorce INTA onion

Abstract: Russian knapweed is an invasive creeping perennial herb which affects crops by competition and allelopathy. Herbicides available for use in onion are not able to control Russian knapweed in a crop context. Conversely, recommended products for Russian knapweed are not selective for the crop. The aims of this work were to study Russian knapweed biomass production and propagation for a range of increasing densities in an experimental onion culture and to characterize the productive response of onion plants under these conditions. A partial additive experiment was carried out to study Russian knapweed interference (variable density, 0-64 ramet m^-2) on onion transplants (constant density, 40 pl m^-2) under greenhouse conditions in Viedma, Argentina (40? 03' S; 62? 48' O). Although no differences among treatments were found for weed final aboveground biomass, low density treatments (0, 2 ramet m^-2) were lower than 64 ramet m^-2 for belowground biomass. Final weed density was proportional to initial conditions. For onion, total (-54%) and commercial bulb yield (- 56 %) were reduced by weed competition with > 32 ramet m^-2. While size 3 bulbs (50-70 mm eq. diam.) were less represented at weed densities higher than 16 ramets m^-2, size 4 ones (70-90 mm eq. diam.) were not present in this condition. For A. repens, traits such as the rate of vegetative propagation, high competitive ability, mainly belowground, and high propagule pressure support its high invasive potential.