Weeds constitute a huge group of undesirable plants, widespread throughout the world. They represent a big problem for most farmers, who implement different methods to fight against them. Thanks to their wide occurrence, weeds however, can be an excellent indicator of the quality of soil and the whole environment where they are present. In this paper, we present the impact of four alkylimidazolium chlorides with a natural terpene component introduced into the soil: (1R,2S,5R)-(–)-menthol and alkyl substituents containing 1, 4, 9 or 12 carbon atoms, on the growth and development of selected weed species. Compounds with the highest phytotoxic activity towards gallant soldier, white goosefoot and common sorrel were chlorides with methyl and butyl substituents, while compounds with nonyl and dodecyl substituents demonstrated a weak effect on these weeds. Phytotoxicity of the salts tested was largely dependent on the applied concentration of the compound and the genetic make-up of plant species used in the experiment. This was reflected in the inhibition of plants’ length and their roots, as well as changes in the content of dry matter and photosynthetic pigments.