We investigated the effect of soil mulching with rye straw (Secale cereale) applied at a rate of 5 t·ha^-1 on a weed infestation in cabbage (Brassica oleracea L. var. capitata L. f. rubra) cultivated in the first year, and onion (Allium cepa L. var. cepa Helm.) grown in the second year after mulching. Straw mulch was applied during the final 10 days of July and plough-incorporated on three dates (in the autumn, the final 10 days of October; in the spring of the next year, the second 10 days of May; mulch non-incorporated until the time of cabbage harvest). The study was conduced at the experimental farm in Zawady located in centraleastern Poland, in 2002-06. The mulching effect of 5 t·ha^-1 rye straw was compared to conventional farmyard manure fertilization at a dose of 40 t·ha^-1, as well as a control without manure fertilization or mulching. The weed infestation of plots where straw mulch was left until the time of cabbage harvest was significantly smaller in comparison with the autumn- and spring-incorporation of straw. In the period of cabbage growth, the fresh mass and number of weeds in the plots with non-incorporated mulch were significantly lower than in the control without mulching and farmyard manure-fertilized plots. A significant mulch effect on weed infestation was found in the first year following mulch application. No significant secondary effect of straw mulch and date of ploughing down on weed infestation in onion was found. The cabbage and onion yield achieved after straw mulch was similar to the yield after farmyard manure applied at a dose of 40 t·ha^-1.