When Should You Control Weeds?

Successful weed management requires attention to detail such as growth stages of the weeds and crop and excellent time management. On many farms we tend to concentrate first on planting and then get around to the weed control later. Weeds are easier to kill when they are small, don't wait too late. For a weeder harrow or rotary hoe we can get good control at the white thread stage (germinated but not emerged) but control is relatively poor after annual seedlings are more than 3 cm tall and their roots are established. Row cultivation works well but weeds in the row can get too big and escape if there is no management of the weeds between planting and cultivation.

Research has shown that each crop has a defined period when yield losses will be greatest due to weeds. This usually starts soon after emergence of the crop and continues throughout part or most of June for spring planted crops.

Removing weeds from the crop after this critical stage will have a minor effect on the crop's yield but may be important to prevent weed seeds for next year or harvestability of the crop, depending on the crop. Mid-summer weeding should concentrate on weeds with those potential effects.

There are many reasons for this crop competition effect. Competition for light moisture and nutrients are part of it but research has also shown that the reflection of red and far -red light between weeds and crops also affects the crop's growth habit and yield development.

Bottom Line: Control weeds as early as possible.