Cover crops not only improve soil conditions, but can also suppress weed growth. In a field experiment the influence of two cover crops, Secale cereale (stooling rye), and Lolium multiflorum (annual ryegrass), on the growth of Zea mays (maize) and C. esculentus (yellow nutsedge) was compared to the latter crop and weed’s growth at three control treatments which involved weed residues left on the soil surface, application of herbicides and weed control by hoeing. Maize emergence and growth were delayed in the presence of residues of both cover crop species, especially in annual ryegrass residues. C. esculentus growth was significantly inhibited in the area between the maize planting rows by the cover crops for the first 14 days after maize emergence, but this growth suppressing effect diminished after 28 days. In a controlled environment study, the influence of the same cover crops, together with Avena sativa (oats) and three cultivars of annual ryegrass were evaluated. Maize and C. esculentus growth were suppressed, especially by the root residues of the cover crops with the annual ryegrass cultivar ‘Midmar’ being the most suppressive. Chemical analysis of the leachate of root residues indicated the presence of phenolic acids and benzoxazolin-2(3 H)-one (BOA). It is suggested that weed growth could be reduced by the allelochemicals leached from cover crop residues but in order to achieve prolonged, effective weed control the combination of mulch retained on the soil surface and the application of herbicides will required. In an integrated weed management approach a possible reduction in the type and number of herbicide applications required for effective weed control, could be implemented.