In the search for alternatives to synthetic herbicides we assessed the efficacy of 2 non-selective, natural products. The active substances were fatty acids: one herbicide containing pelargonic acid (C9) and the other containing caprylic (C8) and capric acids (C10). The aim of the study was to determine the dose required to achieve this range of efficacy (ED80) for weed stands with different canopy heights and densities. 2 trials were carried out. Weed stand height varied between 0.05 m (trial 2) and 0.25-0.35 m (trial 1). The herbicides were applied with a logsprayer on plots of 43.5 m2. Dose-response curves were calculated using green weed coverage determined by image analyses as response variable. The ED80 increased with increasing weed canopy height: from 0.9 to 1.7 of the standard dose (n) for the caprylic and capric acid containing product and from 0.7 n to 1.7 n for the pelargonic acid containing product. In conclusion, acceptable efficacies can be achieved if the herbicides are applied on young weed stages. For larger weeds the dose, spray volume and application technology need to be further adjusted. These herbicides could be used for the stale seedbed technique, crop pre-emergence or inter-row applications. Nevertheless, the relatively high price of these natural herbicides limits their broader use.