Your graph may look much different than this depending on the growth conditions of your experiment and the plant species that you used to simulate the weeds. The critical weed-free period is the period where competition from the weeds reduced the radish yield. For the graph above it is roughly from two weeks after planting until four weeks after planting, although there was a slight yield reduction from competition occurring from planting until 2 weeks after planting. From a weed management standpoint, control measures must be applied by two weeks after planting and the radishes must be kept weed free until four weeks after planting to keep from losing yield.