Weed and Weeding Effects on Medicinal Herbs

Competition with weeds exerts significant depressive effects on yield and quality features of Medicinal Plants (MPs). According to the crop, the part of plant to be harvested, the environmental features (including cropping technique) and the severity of infestation, yield losses due to the presence of weeds may vary within wide intervals. Furthermore, unlike the majority of other crops, MPs are cultivated with the goal to obtain relevant quantities of specific secondary metabolites, whose final quantity determines the quality level (and, consequently, the market value) of the harvested drug. Almost all papers addressed to this topic agree on the statement that unrestricted weed growth may alter MP production also from the qualitative point of view, that is, determining an overall decrease in the yield of active substances for unit area. In part, this outcome can be attributed to the general decrease of harvestable biomass, but in some cases also modifications of crop metabolic pathways have been observed, resulting in a general unpredictability of the chemical characteristics of the product obtained in weedy fields. Competition with weeds may assume a different severity according to the time and duration of competition period. In the starting phases of cultivation, the outcome of an early weed infestation is expected to be severe, since very often weeds grow much faster than crops. The maximum tolerance period, i.e. the period when weeding operations must be started, varies according to the tolerated loss values, and in annual crops the time span when fields must be kept totally weed-free may cover more than 60% of the entire crop cycle. The tools that are used for weeds removal may affect MPs production in many ways. Chemical treatments have been studied with contrasting results, but an interference of herbicides with the metabolism of secondary products was found in some cases. Furthermore, the interest in growing MPs with organic or environmentally friendly methods is increasing. Hence, besides the traditional (and highly expensive) method of hand-weeding, other non-chemical methods are studied, including mechanical treatments, mulching, flaming, and even grazing by goats or lambs. There is scope for further research, embracing a larger number of MPs and different environments, also including the effects of weeds on MPs metabolic pathways.