Also, a harrow or rotary hoe may be needed to incorporate a preemergenceapplied herbicide if rainfall adequate to move the herbicide into the soil does not occur. Weeds that typically emerge from soil depths of greater than 1 inch such as common cocklebur, volunteer grain, volunteer sunflower, and wild oat are difficult to control with shallow tillage. Shallowemerging weeds need to be controlled soon after germination and before the root system is well established. At this stage, weeds have not emerged but are just below the soil surface. Check fields for this stage by lightly wiping your hand or a spade across the soil surface.
Weeds also are easy to control a few days after emergence.
Generally, grassy weeds should not exceed the oneleaf stage and broadleaf weeds the cotyledonleaf stage of growth to expect adequate control with a harrow or rotary hoe. A third tillage pass seven to 10 days later is optional. A compromise may have to be made between optimizing weed control and minimizing crop injury when using a harrow or rotary hoe after crop emergence. Also, a short window of opportunity normally exists for timely use of tillage due to unfavorable weather conditions. Crop species and stage is an important consideration when timing mechanical weed control operations.