WEEDS are considered as one of the serious pests in crop production. The weeds affect crops through allelopathy and competition. Weed competition is monitored for economic threshold in terms of density and duration. The duration for which weeds actively grasp the resources of crop should be critically monitored to avoid economic losses.Fennel being one of the important medicinal and multi purpose crop of winter season also require the critical competition period to be fixed for devising an eco friendly weed management plan. This book encompasses the weed competition effects in fennel and the time when it becomes crucial to manage weeds.