Biochar (BC), the carbon-rich by-product resulting from pyrolysis of biomass, is used for bioenergy and increasingly as a soil additive for carbon sequestration and soil improvement. However, information about the effects of BC on forest productivity and reforestation success, especially on boreal and temperate forest soils, is scant. We examined the effects of two BC types (Canadian and Finnish) added in proportions up to 60 vol.% into a common alluvial silty soil on the growth of transplanted Norway spruce (Picea abies [L.] Karst.) seedlings in a growth chamber. We found no marked differences in seedling growth among the binary growing media mixes used. Seedling growth attributes (seedling height, terminal shoot growth, root volume) differed consistently only between the BC types in the highest proportion used. The terminal shoot growth differed overall among the two BC types. These results suggest that BC may be applied into mineral soils without detrimental chemical effects on tree plantation success. Our results provide foundation for further field research on the longer-term impacts of adding BCs to boreal forest soils.