The experiment aimed at comparison of the fresh and dry biomass of aboveground parts and roots of redroot pigweed ( Amaranthus retroflexus L.) depending on plant density. A micro-plot (1m 2 plots) experiment was set up in 2012 at the experimental station Totem Field at UBC, Vancouver, Canada, as a randomized block design with four replications per treatment. Three different densities of redroot pigweed were studied: 10, 30 and 50 pcs per 1 m 2 . Seeds were sown in two terms: I – 18 th July and II – 6 th August. Plants sown in the I term were of higher biomass and more diversed whereas plants sown in the II term developed smaller amounts of aboveground and root biomass. We conclude that in the later term of emergence root competition and seed production play more important role than aboveground competition.