The allelopathic effects of water extracts from sweet potato (Ipomoea batatas) leaves on seed germination and seedling growth of five noxious farming weeds (Galinsoga parviflora, Ageratum conyzoides, Bidens pilosa, Digitaria sanguinalis, and Echinochloa crusgalli) in the Yunnan Province, China were examined in the laboratory. Effects were based on seed germination rate, seed germination vigor, root length, stem length, and fresh biomass. The results showed that the seed germination vigor of the five weeds was markedly suppressed by water extracts of sweet potato leaf, but seed germination rates were less influenced. Root length and fresh biomass of G. parviflora, A. conyzoides, B. pilosa, and D. sanguinalis were significantly reduced with increasing concentration of water extracts of sweet potato leaf, and D. sanguinalis was the most inhibited, with suppression rates of 92.04% (length) and 73.33% (biomass) at a concentration of 0.1 g/mL, 40.99% (length) and 46.67% (biomass) at a concentration of 0.0125 g/mL. The next most suppressed species were B. pilosa, A. conyzoides and G. parviflora; and the least suppressed was E. crusgalli. With increasing concentration of water extracts of sweet potato leaf, stem length of G. parviflora, B. pilosa and D. sanguinalis was significantly decreased. The highest inhibition was for D. sanguinalis, consisting of 86.85% and 70.64% at concentrations of 0.1 g/mL and 0.0125 g/mL, respectively, followed by B. pilosa and G. parviflora. However, the stem length of A. conyzoides and E. crusgalli was significantly increased with concentration increases in water extracts of sweet potato leaf, and for A. conyzoides rates of promotion were 86.97% and 16.03% at concentrations 0.1 g/mL and 0.0125 g/mL, respectively. For stem length and biomass of G. parviflora, the water extracts of sweet potato leaf showed a "low-promotion and high-inhibition effect" at a concentration of 0.0125 g/mL water extracts of sweet potato leaf. Generally, compared to combination of response index and synthetical allelopathic index, G. parviflora, A. conyzoides, B. pilosa, and D. sanguinalis were significantly inhibited with increasing concentration of water extracts of sweet potato leaf, and D. sanguinalis had the strongest inhibition, followed by B. pilosa, G. parviflora and A. conyzoides, however E. crusgalli exhibited positive effects (except at a 0.1 g/mLconcentration). Thus the sensitivity of five weeds to the allelopathic effects of sweet potato is in the order of D. sanguinalis > B. pilosa > G. parviflora > A. conyzoides > E. crusgalli from highest to lowest allelopathic sensitivity.