The invention relates to lighting method and devices that can be used to treat a seedling (210, 310) in a growth chamber (360) or greenhouse (200) prior to the introduction of the seedling to outdoors. One aspect of the invention is a light (220, 320, 321) by which the plants can be prepared for the outdoors, by giving them certain wavelengths of light they do not currently receive from the light in the greenhouse or growth chambers. The light of the invention can be applied in smaller doses during the major part of nursing of the seedlings or as a sun-shock period in the end of the indoors nursing period. By giving the seedlings light from the invention, they are prepared to Sun light and do not need to spend a few weeks under the sunshade curtains. The invention thus shortens the growth cycle of tree seedlings, enhances the proportion of viable seedlings and eliminates one work phase in the growth process, thus improving the economics of seedling cultivation.
