The problem of practical application of weeds in agriculture is considered. Weed flora, just like other vegetation, performs a number of ecological functions and, according to the definition of biogeochemical science, is a phytomass carrying large reserves of biophilic (C, O, N, H, Ca, P, S) and a number of other chemical elements. The function of weed vegetation as a component of ecological stability of agrocenoses remains incompletely studied. In this regard, the article presents data on the results of using weed vegetation in adaptive-biological farming as a green manure fertilizer. Examples of positive effects of weeds on soil fertility and yields of cultivated crops are given.