Crop–weed interactions is an important topic for introductory weed science courses. The effect of the timing of weed emergence and the duration of weed competition on crop yield are two topics usually covered when discussing competition. Students generally gain a better knowledge of these concepts through observation in addition to discussion of the underlying concepts. An additive removal and plant-back experiment was used, in the undergraduate weed science laboratory at Iowa State University, to demonstrate critical period for weed control concepts for cultivated radish. In one series of treatments, weeds were sown at the time of radish planting and removed at 2, 3, and 4 wk after planting. In another series, weeds were sown at 1, 2, and 4 wk after radish planting and allowed to grow in the flats until completion of the experiment. Two controls, one weed free and one unweeded, were also included. The results from four semesters suggested that the critical period for weed control began immediately after planting and lasted 3 wk. The timing and duration of the critical period was consistent across the four semesters evaluated. This activity was successful in demonstrating the critical period for weed control in radish.

Nomenclature: Radish, Raphanus sativus L.

Additional index words: Competition, education, weed ecology, weed-free period.