Summer planting of Picea abies container-grown seedlings: Effects of planting date on survival, height growth and root egress

In order to make mechanized planting economically viable, the present spring planting period of Norway spruce (Picea abies (L.)) seedlings in Fennoscandia needs to be enlarged. To evaluate the possibilities of enlarging the planting period, seven field experiments were established in the course of 3 years, during which actively growing or short-day-treated seedlings were planted regularly from the beginning of July to the middle or end of September, depending on the experiment, and also the following spring. The survival of seedlings differed between experiments, but the planting date had an effect only in the experiment where seedlings were planted on a mounded site with no short-day treatment before planting. Seedlings planted after mid-July were more susceptible to damage from night frosts. Seedlings planted in July and early-August had greater root egress and growth in height after the planting in comparison to seedlings planted after mid-August and during the following spring. For plantings in July, actively growing seedlings can be used without negative effects on their field performance. In order to avoid possible frost damage, short-day treatment is recommended before planting for seedlings used in plantings performed in August and later in the autumn.