Weed control windows: pre-emergent versus post-emergent

A herbicide works within a window defined by the weed stage, the crop stage and the soil moisture. Outside that window the same product with the same quantity in the same field can do very little.

Pre-emergent products form a barrier in the top layer of soil before weed seed germinates. They need a well-prepared, reasonably level seed bed and adequate moisture. Applied to a dry, cloddy field they cannot form a continuous layer, and weeds simply come up through the gaps.

Post-emergent products act on weeds that are already up, and almost all of them work best on young, actively growing weeds. Once a grassy weed has tillered out, control falls away sharply — which is why fields that looked clean in week two can be lost by week five.

Two practical habits pay for themselves. Walk the field and identify your dominant weeds before choosing anything, because grasses, broadleaf weeds and sedges are not controlled by the same chemistry. And rotate the chemistry you rely on across seasons; continuous use of one mode of action is how resistant weed populations are built.