The aim of the paper was to evaluate allelopathic effects of weed species Johnsongrass (Sorghum halepense) and black nightshade (Solanum nigrum) on germination and early growth of sugar beet. Petri dish experiment with water extracts prepared from different plant parts of weeds in 1, 5 and 10% concentrations and pot experiment with weed residues in rates of 10, 20 and 30 g/kg were conducted. Allelopathic potential was determined through germination, root and shoot length, and fresh and dry weight of sugar beet seedlings. Water extracts from various plant parts showed significant inhibitory potential, up to 100%, on germination and growth of sugar beet. Lower extract concentration promoted shoot length up to 35.5%. Johnsongrass residues showed negative effect on shoot length, fresh and dry weight of seedlings, especially in treatment with 30 g/kg residue rate. Contrary, black nightshade residues promoted sugar beet growth. Besides concentration/rate, allelopathic effect was dependent on extraction method, growth media and plant part.