Effect of weed-free intervals on the quantitative and qualitatitive characteristics of pistachio (Pistacia vera)

To study the effect of weeds on the different characteristics of pistachio (Pistacia vera L.), an experiment was conducted in Qom, during 2015-2016 growing season. The experiment was carried out in a randomized complete block design with three replications. Treatments were based on the competition critical period and the phenological stages of pistachio in two series. The first series consisted of weed removed until the end of the swelling of the bud, the end of flowering, the end of nutshell growth, the beginning of kernel filling and the end of kernel filling and then weed maintained until harvest. The second series consisted of weed maintaining until mentioned stages and then weed removed until harvest. In addition, two control treatments (weed free and weed infested for all of the growing season) were applied. In this experiment, Chenopodium album and Cynodon dactylon, with 21.83%, had the highest frequency.  The most important traits influenced by treatments, based on principal components analysis, were splitting and un-splitting percentages, size at ounce and yield. Weed-free until the beginning of kernel filling treatment, with an average yield of 3.91 kg/tree had the highest and treatment weed interference until the end of the swelling of the bud treatment with an average yield of 2.69 kg/tree had the lowest yields in the experiments. Totally, with regard to the results of present research, two times weed control, one time the end of sprouting and the other at the end of nutshell growth until the beginning of kernel filling, would be recommended.