you will need pure nodes from growing silverwood trees.
post-merge, the type will be the one with higher current vis.
you may need to combine a few pure nodes, then drain the dark node, so when they combine, the dark node will be destroyed because it has less current vis than the pure node. make sure the vis levels are such that the combined node isnt smaller than the originals.
to combine them, you just need to place them close enough together. they will then merge by moving toward each other.
warning: it will produce a LOT of flux, and will drain nearby nodes. big time.