Lighting method and devices can be used to treat a seedling in a growth chamber or greenhouse prior to the introduction of the seedling to outdoors. A light 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 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 this light, they are prepared to Sun light and do not need to spend a few weeks under the sunshade curtains. Thus, the growth cycle of tree seedlings is shortened, the proportion of viable seedlings is enhanced, and one work phase in the growth process is eliminated, thereby improving the economics of seedling cultivation.
