Influence of Duration of Inter-species Interference and Determination of the Critical Period for Weed Control in Peppermint (Mentha piperita L.)

Peppermint (Mentha piperita) is considered as one of the oldest medicinal plants. It is herbaceous and perennial belonging to the Lamiaceae family. The production of secondary metabolites in plants is influenced by the environment. Weed competition is of the most effective factors affecting crop yield in medicinal plant production systems, and can influence the quantity and quality of the essence constituents. The extent of crop yield loss depends on presence and competition duration of weeds. A period during the crop growing season in which weeds control is essential to prevent yield loss is called the critical period of weeds control. Respect to the importance of weed presence duration in yield loss, and the variability for critical period of weeds control depending on crop species and cultivar, and characteristics of weeds communities in different areas, and also because of increased interest in medicinal plants production and extraction, and limitation for herbicide application in medicinal plants, it is essential to understand different aspects of weed- peppermint competition. Therefore, the objective of this research was to acquire information about the effects of weed competition duration on peppermint growth and yield, which possibly can lead to integrate various approaches into weed management programs, and improve weed control strategies.

This study was conducted as a randomized complete block design with 12 treatments and three replications at research farm of the University of Guilan. Treatments were arranged in two series including weed-free and weed-infested treatments which respectively hand-weeded and un-weeded from the beginning of the growing season up to 16, 32, 48, 64, 80, and 96 days after crop planting. Peppermint cuttings with 6-8 cm height were hand-planted on 50-cm apart rows with 20 cm spacing between plants (density of 10 plants.m-2) on early May. Peppermint plants were hand-harvested 96 days after planting when 50% flowering occurred at the full-season weed-free plot.

Dominant weed species included barnyard-grass (Echinochloa crus-galli (L.) Beauv.), yellow foxtail (Setaria glauca (L.) Beauv.), annual nutsedge (Cyperus difformis L.), crab-grass (Digitaria sanguinalis (L.) Scop), knotgrass (Paspalum distichum L.), common cocklebur (Xanthium strumarium L.), spurge (Euphorbia indica Lam.) and redroot pigweed (Amaranthus retroflexus L.). Maximum height of peppermint (71.2 cm) was observed in treatments with at least 32 weed-free and at most 32 weed-infested days from the beginning of the growing season. In the other treatments, final height of peppermint plants reduced by 9.1%. Nod number per stem (22 nodes) was not influenced by weed control or interference. The maximum branch number per plant (37.1) was observed in treatments with at least 32 weed-free and at most 48 weed-infested days. The minimum branch number per plant was observed in the full-season weed-infested treatment (14.6) and also in the treatment that hand weeded just 16 days from the beginning of the growing season (17.6). The maximum dry weight of peppermint at harvest (193.62 g.m-2) was observed in treatments with at least 48 weed-free and at most 32 weed-infested days. The minimum dry weight of peppermint was 30.06 g.m-2 and belonged to the treatments including at least 64 weed-infested days, and also to the treatment was weed-infested from 16 days after planting up to the end of the growth seaso. Biological yield of peppermint in full-season weed-free treatment was 2044 kg.ha-1. For control treatments, weedy condition during 16, 32, 48, 64, and 80 days led to a biological yield loss of 0.3, 3.2, 7.8, 27.9, and 87.2%, respectively, whereas for infested treatments, weedy condition during 16, 32, 48, 64, and 80 days caused 4.8, 15.6, 41.5, 75.1, and 87.7% biological yield loss, respectively. Also 91.3% reduction was recorded for biological yield of full-season un-weeded treatment. The highest percentage and yield of peppermint essence were observed in treatments of 64, 80, and 96-day weed-free (3.38% and 68.30 kg.ha-1). The least essence percentage was 0.48% and was recorded for 96-day (full-season) weed-infested treatment. The least essence yield was belonged to 96, 80 and 64-day weed-infested, and 16-day weed-free treatments (4.4 kg.ha-1).

In general, the growth and yield of peppermint diminished with decreasing weed-free duration and increasing weed-infested duration. Full-season weed competition compared to the full-season weed-free control, reduced height, branch number, biological yield, essence percentage, and essence yield of peppermint by 13.5, 61.1, 91.3, 86.3, and 98.8%, respectively. These results support the importance of weed management in peppermint, as weeding was necessary from 22 to 49 days after peppermint planting by accepting up to 5% yield loss, and from 29 to 42 days by accepting up to 10% yield loss.