Deep planting decreases risk of drought damage and increases growth of Norway spruce container seedlings

Despite several studies demonstrating that deep planting has no negative effects on seedling development, conifer seedlings are still typically planted to depths of 3-5 cm in boreal forests. Planting machines plant seedlings deeper. While machine planting has become more common in boreal forests, there is still a need to demonstrate that conifer seedlings can be planted using this method without reduced field performance of seedlings. To investigate the effects of deep planting on Norway spruce (Picea abies (L.) Karst.) container seedlings, we established two experiments in Central Finland in 2006 and 2009. In the experiment on a site with medium coarse soil, planting depths were 0, 2, 5 and 8 cm. In the experiment established on fine-textured soil, planting depths were 3, 6 and 10 cm. During the first 4 years after planting, deeper planted seedlings grew better and at the end of the fourth season initial differences in shoot length among planting depths had disappeared. Incidence of insect damage was relatively low, and no differences among planting depths were found. During the dry summer, there was a positive relationship between increased planting depth and improved survivability of seedlings. In conclusion, deep planting improved growth of Norway spruce seedlings and was an advantage in dry conditions.