Most germinate in early spring. Wild mustard and wild oats are vigourous competitors. Green foxtail, lamb's quarters, and wild buckwheat are less vigourous. Weeds compete with crop for nitrogen. Buckwheat, smartweeds and wild oats are high users.
Weeds compete for moisture.
Wild mustard, wild oats use 4X as much water as cereals. Light competition begins when plants begin to shade each other. Shading reduces green foxtail, wild oat competition. Heavy canopies inhibit germination. Bindweed, red root pigweed and green foxtail seedling emergence is associated with that first warm period the spring.