Abstract The effects of relative air humidity (RH) and supplementary lighting were studied in 23 foliage species at 24°C air temperature during winter. Increasing the RH from 60 to 85% significantly increased the dry weight in four species and decreased it in one. Plant height was increased in five species when the RH level was raised, while dry weight and/or height were affected by RH in nine out of the 23 species. Six of the species developed a lighter green colour at high RH compared with low RH. Supplementary light (60 μmol m −2 s −1 during 20 h day −1) increased plant dry weight and height in all but two species.