Biennial wormwood has been increasing in Manitoba, Canada, but information
is limited on the potential of biennial wormwood to reduce crop yields.
Field experiments were conducted over 5 site-yr throughout southern Manitoba
in 2010 and 2011 to determine the effect of biennial wormwood density and
relative time of biennial wormwood seedling recruitment on sunflower growth,
development, yield, and seed quality. Biennial wormwood was broadcast on the
soil surface at six densities into sunflowers planted in 75-cm rows, either
at the same time the sunflower crop was planted (early weed seedling
recruitment) or when the sunflowers were at about the four-leaf stage (late
weed seedling recruitment). When biennial wormwood emerged at about the same
time as the sunflowers, yield was reduced by up to 46%. Early-recruiting
biennial wormwood had minimal effect on sunflower growth and development,
but sunflower achene size and individual achene weight were reduced, even
when no effect on sunflower yield was observed. Biennial wormwood plants
that recruited after the four-leaf stage of the sunflower crop had no effect
on sunflower yield or seed quality.