Question
can you use 8 yr old horse manure when planting potted colorado spruce trees
20 inches tall for fertilizer?

Answer
I do not recommend any fertilizer applied at planting. It runs the risk of burning the roots and the seedling already has a good amount of fertilizer if you got it from a nursery. Horse manure is not really a great fertilizer but if you want to use it I would till it into the garden spot soil--flowers and vegetables.

Over fertilizing evergreens can damage them--the blue tint of Col. spruce could change to green. I would stay away from the horse manure for trees.