The influence of irradiance and light quality onin vitrocultures was studied at the rooting stage, and subsequently as it affected the field performance of birch(Betula pendulaRoth.). Forty‐five μmol m−2s−1during the rooting stage reduced the rooting time compared with 30μmolm−2s−1. The light quality treatments of the cultures, from which the microcuttings were harvested, had a larger effect on the rooting than the treatments during the rooting stage. The fastest rooting was observed in microcuttings pre‐treated with blue light (6 days), and the slowest rooting after treatments with red light (12 days), which also, respectively, gave the highest (5.1) and lowest (3.5) mean number of roots per microcutting. The largest growth rate in the field was observed in plantlets harvested from cultures subjected to blue and cool white light.