The chemical method is the most popular method of weed control due to its efficiency and low costs. A broad assortment of herbicides allows to choose right preparations to control weeds in field production of ornamental shrubs. The experiment on the use of the foramsulfuron- and methylsodium iodosulfuron-containing herbicide was carried out in 2015. Plants of coniferous: Picea pungens f. glauca, Pinus mugo subsp. mugo and Thuja occidentalis ‘Smaragd’ were planted in the field and their height was measured. When plant growth ceased they were measured again (in mid-September) and plant material for biochemical analyses was then sampled. The results show that spraying plants with the herbicide in the dosage of 0.15 kg.ha-1, in April, before growth started, was not toxic for conifers Picea pungens f. glauca and Pinus mugo subsp. mugo while Thuja occidentalis ‘Smaragd’ proved sensitive to the treatment. The preparation applied together with the recommended adjuvant efficiently controlled most of the weeds. The weeds resistant to the herbicide were: Convolvulus arvensis, Equisetum arvense and Cerastium holosteoides. In two insensitive taxa the herbicide foliar application of the herbicide resulted in an increase in total soluble sugars, free amino acids and polyphenolic acids as well as an increased peroxidase activity in plant shoots while the content of chlorophyll and catalase activity were decreased. In the sensitive Thuja occidentalis ‘Smaragd’ the content of free amino acids and polyphenolic acids as well as activities of peroxidase and catalase increased while the levels of chlorophyll, total soluble sugars and hydrogen peroxide fell.