The experiment with forecrops for spring barley was conducted at the Agricultural Experimental Station of Uhrusk. It was set in the method of split-plots which was designed with four replication, on 10 m2 plants. The experiment concerned five forecrops for spring barley: pea, potato, spring barley, spring barley (2x), spring barley (3x), and two methods of weeding - mechanical and chemical. Cultivation of spring barley after itself or potato increase the number of weeds in relation to remaining forecrops. Applied in spring barley herbicides decrease number and air dry mass of weeds effectively than harrowing of canopy. In every year of investigations and for everybody forecrops in the canopy of spring barley occurrence: Galium aparine, Stellaria media and Fallopia convolvulus (V degree of constancy). Additionally after potato, pea and in position after itself, in V and IV degree of constancy occurrence: Melandrium album, Veronica persica, Amaranthus retroflexus, Chenopodium album, Avena fatua and Lamium purpureum. In positions, in which barley was sown after itself (2 or 3 times) numerous weeds were in III - I degree of constancy.