Crop–weed interactions are affected by environmental alterations resulting from a crop’s presence, such as modifications in temperature, light quality and quantity, and moisture conditions that could modify weed performance. The objectives of this work were to study (1) how soybean [Glycine max (L.) Merr.] crop structure modifies the environment under the canopy and large crabgrass [Digitaria sanguinalis (L.) Scop.] plant structure, biomass, and seed production and dormancy; and (2) the relative importance of these environmental changes on the weed’s characteristics. A field experiment in a completely randomized block design with five replicates was performed to evaluate narrow and wide interrow spacing and soybean maturity groups 3 and 4. Measured variables were intercepted solar radiation (RAD); red–far red (R-FR) ratio; humidity; minimum, maximum, and alternating temperatures; and weed biomass, tillers per plant, height, and seed dormancy. Crop canopy reduced solar radiation, R-FR ratio, and daily average maximum and alternating temperatures. Soybean presence reduced the weed biomass, tillers and seeds per plant, and seed dormancy. High solar radiation intercepted by the crop during the reproductive phase was the main environmental variable related to reductions in weed biomass, tillers per plant, and fecundity. The combination of low temperature and solar radiation received by developing seeds was more related to seed dormancy than the rest of the variables. Crop management decisions focused on the fact that keeping the crop canopy alive for a longer time at the end of the season would not only reduce the weed growth but also seed dormancy.