Determining the Critical Period of Weed Control in Thyme (Thymus vulgaris)

In order to determine the critical period of weed control in Thyme (Thymus vulgaris)، an experiment was carried out in a randomized complete blocks design with 18 treatments and three replications in 2008-2009 at the research field of the medicinal plants departments، Shahed University، Tehran، Iran. Two series of the treatments including weed-free and weed-infested were applied in the regular time distances of 15، 30، 45، 60، 75، 90، 105 and 120 days right after the beginning of thyme growth at spring. Two check treatments including full and no control of weed during the whole season were also included. The nonlinear models kind of logistic and Weibull were applied to determine the start and end of critical period of weed control respectively. Major weed species were field bindweed (Convolvulus arvensis)، flixweed (Descurainia sophia)، mon groundsel (Senecio vulgaris) and prickly lettuce (Lactuca scariola) with greater biomass and size. The results showed that periods of weed- free and weed- infested significantly affected the dry weight and number of weeds in all treatments. Considering 5% and 10% of the acceptable yield loss in the models، critical periods of weed control were between 41-90 days and 54-76 days for fresh weight، 12-94 days and 29-78 days for dry weights، and 20-105 days and 30-86 days for essence yield after the beginning of growth at spring.