Silverwood trees cannot be grown with bonemeal. You need to use a hoe of growth to grow them on demand.
Pure nodes, when unjarred in a new area will not, afaik, do anything. To cleanse land you use ethereal blooms that will convert tainted and eerie biome back to the biome it was before being tainted / made eerie.
A silverwood tree, grown from a sapling, has a high chance to convert an area around itself to magical forest. This effect takes a while to manifest, and can happen regardless of whether the tree spawns a pure node or not.