Cover crop planting date did not affect maize yields or the ability of interrow tillage plus cover crops to suppress the development of weed densities. Except for 1993 at l'Assomption interrow tillage plus cover crop treatments had consistently lower weed biomass than the weedy control when the weed pressure was not excessive. Most of the weed control was due to the interrow cultivation performed prior to seeding of the cover crops. The cover crops provide additional weed control but the interrrow tillage or some herbicide application may still be necessary. The lowest weed density occurred in the herbicide treated plots. The ability of interrow tillage plus cover crops to suppress the development of weeds was affected by the level of weed infestation, the growing conditions and location