Agricultural production is facing many pressures, including the reduction of chemical plant protection agents. Weed control requires new, effective measures with more favorable ecotoxicological properties. Phenolic acids, a group of plant phenols, show an inhibitory effect on weed species. Previous research has identified three phenolic acids (p-coumaric, vanillic, ferulic) with the strongest inhibitory effect on the weed species Ambrosia artemisiifolia L. (ambrosia). The aim of this in vitro research is to determine the effect of phenolic acids in simultaneous application with reduced doses of the herbicide thiencarbazone-methyl on the germination and initial growth of ragweed. The results of the preliminary experiment indicate that p-coumaric acid (200 x 10-7) in combination with 1/8 dose of the herbicide thiencarbazone-methyl inhibits the hypocotyl length of ragweed significantly better than other phenolic acids. With the addition of p-coumaric acid to the herbicide thiencarbazonemethyl, ragweed radicles were significantly reduced (92.4%) compared to the pure herbicide without the addition of p-coumaric acid (86.1%). At the lowest dosages of herbicides (1/32 x and 1/64 x), a significantly better reduction of ragweed hypocotyls was found when applying the combination of herbicide and p-coumaric acid compared to the pure herbicide. The results of the research indicate the possibility of reducing the dose of thiencarbazone- methyl when combined with p-coumaric acid, but the research needs to be carried out in in vivo conditions as well.