During 47 years (1960-2007) there were carried out profound and multilateral studies of the weeds of the Republic of Moldova. It was established that there were over 500 species of weeds, 10-20 of which dominated in crops. Their degree of hazard to the agricultural cultures is different. Some species (C. cyanus L.) when spread at a low weeds level, increased the wheat and rye crops. C. cyanus L. is part of genus Centaurea, which includes 550 species, 26 of which are spread on the territory of the Republic of Moldova. Different species of this genus (C. cyanus L., C jacea) are very important for folk medicine while are less important for the scientific one. Generally, the studies carried out in the field of chemistry demonstrated that the therapeutic actions of species were various and very fragmented and further thorough scientific research is necessary. From this point of view, the weeds must not be totally destroyed, but should be maintained at a low level of negative influence over the agricultural cultures (biological threshold of danger), protected and even cultivated for the medical purposes.