Efficacy of herbicides and flaming against weeds and their effects in oregano (Origanum vulgare ssp. hirtum) tolerance

Two field experiments were conducted in Greece over three growing seasons and two contrasting environments to evaluate the efficacy of preemergence (soil applied herbicides before weed emergence and oregano transplanting) and postemergence (foliage applied herbicide and flaming on both emerged weeds and oregano) against weeds and their effects on oregano tolerance. Weed presence reduced oregano's dry weight and essential oil yield by 37–42 % and 42–55 %, respectively, as compared to weed-free control treatment, whereas none of the weed management treatments provided acceptable weed control. The preemergence applied metribuzin + pendimethalin reduced dry weed weight by 51–56 % in both locations, while the respective reduction due to flaming treatments was 41–69 % in both locations and growing seasons. However, the postemergence applied glyphosate and the mixture of metribuzin + cycloxydim provided the best control by reducing weed dry weight by 61–77 % in all studied cases. Regarding oregano tolerance to weed management options, all treatments were phytotoxic and most of them reduced the number of survived plants, their dry weight, essential oil yield and carvacrol content. The flaming treatments caused the greatest reduction in these oregano growth parameters, whereas glyphosate and the mixture of metribuzin + cycloxydim produced the highest, which were in most cases similar to those of weed-free control plants. These results indicated clearly that glyphosate and mixture of metribuzin + cycloxydim could be used for weed management in oregano, but before their use, the fate (metabolism and residue levels) of these herbicides in oregano plant parts should be further studied.