Effects of aromatic plants incorporated as green manure on weed and maize development

A 2-year field experiment was conducted in northern Greece to study the mulch effects of seven annual [anise (Pimpinella anisum L.), sweet fennel (Foeniculum vulgare P. Mill.), sweet basil (Ocimum basilicum L.), dill (Anethum graveolens L.), coriander (Coriandrum sativum L.), parsley (Petroselinum crispum (P. Mill.) Nyman ex A.W. Hill) and lacy phacelia (Phacelia tanacetifolia Benth.)] and three perennial [mint (Mentha X verticillata L.), oregano (Origanum vulgare L.) and common balm (Melissa officinalis L.)] aromatic plants, used as incorporated green manure, on the emergence and growth of barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.], common purslane (Portulaca oleracea L.), puncturevine (Tribulus terrestris L.), common lambsquarters (Chenopodium album L.) and maize (Zea mays L.). In addition, the phytotoxic potential of the abovementioned aromatic plants extracts was determined in the laboratory using a perlite-based bioassay with maize and barnyardgrass. The bioassays indicated that germination, root elongation and fresh weight of barnyardgrass were reduced by the most aromatic plant extracts. However, maize growth parameters were only affected by the extracts of anise, sweet fennel, coriander, common balm and oregano. In the field, emergence of barnyardgrass, common purslane, puncturevine or common lambsquarters was reduced by 11–50%, 12–59%, 26–79% or 58–83% in green manure-treatment plots, respectively, as compared with green manure-free plots (control). On the contrary, maize emergence was not affected by any green manure. At harvest, maize grain yield in green manure-herbicide untreated plots was 10–43% greater than that in the corresponding green manure-free plots. In particular, maize grain yield in anise, dill, oregano or lacy phacelia green manure-herbicide untreated plots was 27–43% greater than that in the green manure-free-herbicide untreated and slightly lower than that obtained in the corresponding herbicide treated plots. These results indicated that green manure of aromatic plants, such as anise, dill, oregano or lacy phacelia could be used for the suppression of barnyardgrass and some broadleaf weeds in maize and consequently to minimize herbicide usage.