Common waterhemp (Amarathus rudis Sauer) is a frequent weed in glyphosate-resistant (GR) crops in the midwestern USA due, in part, to the delayed emergence of its seedlings. Variable waterhemp emergence was simulated by transplanting seedlings into both corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] and bare plot areas at differing crop growth stages during two growing seasons in western Minnesota. Growth and fecundity were measured. As expected, late planted weeds produced little dry matter and few seeds, and competition from corn or soybean reduced waterhemp dry weight and fecundity by a 90% compared with isolated plants. Interestingly, common waterhemp was affected differently by crop and transplanting date. Common waterhemp grown with corn was always shaded by the crop canopy but produced seeds even when transplanted as late as the V10 growth stage. In soybean, weeds transplanted before the V4 growth stage were taller than soybean and produced more seeds than those transplanted into corn at a comparable growth stage; however, those transplanted after V5 produced no seeds. Consequently, control of late-emerging common waterhemp plants in soybean may not be needed, whereas control of late-emerging plants in corn may be justified because of relatively high levels of seed production.