Biennial wormwood has become an important weed problem in the northern Great Plains, but little is known about its biology. Biennial wormwood seeds were collected from Fargo, ND, and Fergus Falls, MN, for field experiments in 1999 and 2000 to determine the influence of transplanting date on growth, biomass, and seed production. Seeds were seeded in a greenhouse every 2 wk, and seedling rosettes were transplanted to the field 2 wk after emergence from April 30 until September 15 to simulate season-long emergence. Weekly destructive subsampling started 2 wk after transplanting and ended on September 29 in both years. All seedlings that grew for at least 5 wk after transplanting produced flowers by mid- to late August of the same year. Late-transplanted seedlings with less than 5 wk of growth did not flower or survive the winter. Biennial wormwood biomass allocation patterns resemble those of an annual species, with about 15% of the total dry weight allocated to roots, 20% to stems, 25% to leaves, and 40% to flowers. Transplant date had a substantial influence on biomass partitioning. Seedlings transplanted early in the growing season produced more biomass and seed than late-season transplants. Biennial wormwood seedlings transplanted on April 30 produced over 435,000 seeds per plant, whereas seedlings transplanted on August 15 produced 500 to 3,000 seeds. Biennial wormwood was photoperiod sensitive and flowered when the day length was about 14 h or less, between August 18 and 25, in both years.