Evaluating chemical and thermal weed suppression in lemon balm (Melissa officinalis L.) cultivation

The usefulness of 3 herbicides and flame weeding in lemon balm sown directly into the field was assessed in a two-year experiment. Glufosinate-ammonium (600 g∙ha–1) and flame weeding (90 kg propane∙ha–1) were applied 12–13 days after lemon balm sowing, i.e. shortly after weed emergence and 4 days before crop emergence. Bentazon (960 g∙ha–1) and fluazifop-P-butyl (150 g∙ha–1) were sprayed approximately 3 weeks after sowing, during the emergence of lemon balm, and when the weeds were in the cotyledon – 2–4 true leaves stage. All of the studied weed control methods significantly reduced the number and fresh weight of weeds growing 4 weeks after lemon balm sowing. The most effective method was spraying with glufosinate-ammonium, which controlled 69–76% of weeds. The efficiency of flame weeding was slightly lower. Bentazon caused slight, temporary chlorosis of some lemon balm cotyledons. Content of essential oil (1.9–2.1%), its composition and content of rosmarinic acid (2.08–2.44%) in lemon balm leaves, as well as content of total nitrogen (2.18–2.55%), phosphorus (0.30–0.32%), potassium (2.94–3.22%), calcium (1.02–1.60%), and magnesium (0.30–0.32%) in lemon balm raw material were independent of the weeding method. Content of essential oil, phosphorus, potassium and calcium were significantly higher in the dryer year. Studied weed control methods proved useful in the cultivation of lemon balm from direct sowing into the field. Methods with total action (flaming and glufosinate-ammonium) were more effective than those with selective herbicides (bentazon and fluazifop-P-butyl).