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Allelopathic effects of various plants can be exploited for use against weeds; however, the selectivity in different crops is also important. In the current study, the effects of lemon balm (Melissa officinalis) allelochemicals on seed germination and seedling emergence of three biomass crops and three legume species were evaluated. Seed germination of rapeseed was reduced by 19, 30, 56, and 80% in the concentrations of 1, 2, 5 and 10%, respectively, as compared to the control group, whereas sweet sorghum seeds showed a more intermediate response and sunflower germination was affected only by the highest concentration. Seed germination of common bean was by 25, 34 and 60% lower at 1, 2 and 5% extract concentrations, respectively, in comparison to the control whereas up to 85% reduction of seed germination was recorded in 10% concentration. Peanut seed germination percentage ranged between 72 and 47% of control in 5 and 10% concentrations, respectively, while soybean germination was least affected from M. officinalis leaf extracts since it was reduced by only 25 and 41% in 5 and 10% concentrations, respectively, as compared to the control. Seedling emergence of rapeseed was reduced by 14, 25, 46, and 79% in the concentrations of 1, 2, 5 and 10%, respectively, as compared to the control whereas lemon balm extracts showed increased selectivity on the sunflower. Soybean emergence was reduced by only 27 and 46% in 5 and 10% concentrations, respectively, in comparison to the control whereas common bean’s seedling emergence was reduced up to 35% even in 2% concentration. Allelopathic response index values confirmed that sunflower and rapeseed were the least and most sensitive biomass crops to lemon balm allelochemicals, respectively, whereas sweet sorghum showed an intermediate response. Increased was the selectivity of the aqueous leaf extracts on soybean, whereas seed germination and seedling emergence of peanut were more affected and common bean was the most sensitive crop. Further research is needed to investigate the selectivity of M. officinalis and other aromatic plants’ allelochemicals on various crops and under different soil and climatic conditions to optimize their efficacy as tools of more eco-friendly weed management strategies.

The extended and in many cases unjustified use of herbicides has resulted in herbicide resistance development and serious environmental concerns. Therefore, the need for implementation and wider adoption of several agronomic and cultural practices is imperative. Ecologically-based crop management practices like crop rotation, intercropping, delay of sowing date and increased crop density can be also the basis for effective and sustainable weed management. In the present review, several cases are presented and the key points of each method are discussed. Special attention is given to the fact that the efficacy of each practice is depended on the specific soil and climatic conditions along with the field history of each site and crop. Alternative methods of weed management should be further studied and optimized to include them in both organic and conventional production systems and ensure the sustainability of agroecosystems.

The effects of aqueous extracts of the aromatic plant Melissa officinalis were studied on the seed germination and early seedlings growth of ten summer annual species in Petri dish bioassays and pot experiments. The in vitro experiments on the aqueous extracts from M. officinalis on seed germination shows that the extracts of 5 and 10% were the most active inhibitors for all the studied weed species. Seed germination reduction by the aqueous extracts was up to 54% of the untreated seed lot for each species. On the contrary, the concentration of 1% resulted in germination reduction ranging from 1 to 11%, while in some of the weed species (P. minor, S. nigrum, P. angulata and C. albida) the effect was rather stimulatory. This finding is in full agreement with "novel weapons hypothesis" and supports that native plants compared with invasive (like C. albida and P. angulata) are affected more due to the absence of tolerance or resistance to the allelochemicals. The allelopathy RI of aqueous extracts of lemon balm was negative in most cases, while in the case of 10% concentration, emergence was reduced by 58, 54, 48, 46 and 43% for X. strumarium, C. album, S. faberi, C. canadensis and C. bonariensis, respectively. The allelopathic activity of M. officinalis could be exploited in future studies, to identify and isolate the allelochemicals, as models for future herbicides for integrated weed management.