The objective of the research was to calibrate laser systems and to investigate the effect of laser treatment on young plants. The lasers are supposed to accomplish growth depression. In order to generate the beams and to achieved right wavelength, two stationary laser systems namely diode laser (lambda = 940 nm) and carbon dioxide laser (lambda = 10600 nm) were selected. For these systems, calibrations including the connection of supplied power and achieved energy density with the beam were carried out. The background and basics of these settings are mentioned in this paper. Laser beams with spot diameters of about 6 mm - simplifying the positioning of the laser beam to the plant and the distribution of the coupled energy - were used. The investigations were done with dicotyledonous (Nicotiana tabacum) and monocotyledonous plants (Echinochloa crus-galli) each within three growth stages. This was to compare necessary energy densities betwenn different plant species and between plant ages. After proving of settings and testing manual aiming, altogether 48 boxes representing two laser systems, two plant species, three growth stages and four replicates were treated. The plants were once irradiated with either CO2- laser for one second or with diode laser for two seconds. Compared to untreated plants, lethal doses regarding 90\% minor fresh biomass were calculated for every growth stage and plant species.