Critical period of weed competition in three vegetable crops in relation to management practices

Summary The critical period of weed competition was determined in three vegetable crops: early cabbage ( Brassica oleracea var. capitata L.), pickling cucumbers ( Cucumis sativus L.), and field‐seeded processing tomatoes ( Lycopersicon esculentum L.). There were significant interactions between weed‐removal treatments, year, and row width. Cabbage yields were reduced if plots were not kept weed‐free for at least 3 weeks after transplanting or if weeds which emerged with the crop were allowed to remain longer than 4–5 weeks, Cucumber yields were reduced if plots were not kept weed‐free for up to 4 weeks after seeding or if plots remained weed‐infested longer than 3–4 weeks. Higher crop population densities (narrower row widths) in cabbage and cucumbers resulted in smaller plants, earlier competition from weeds, and therefore a shorter period that the crop could remain weed‐infested without suffering reduced yields. Yields of direct‐seeded tomatoes were reduced if plots were not kept weed‐free for up to 9 weeks after seeding or if weeds which emerged with the crop were allowed to remain longer than 5 weeks. In each crop the timing of the critical period of competition was verified by weed removal only during this interval. There was a true critical period in direct‐seeded tomatoes, but not in cabbage or cucumbers where a single weeding was sufficient to prevent yield losses.