Thaumic grafters from Thaumic Bees double your chances of getting a sapling from silverwood and greatwood tree leaves. Using a Repair 2 thaumic grafter last night I was able to get a silverwood sapling and two greatwood saplings within 1/2 a trees worth of leaves each. The drop rate is still incredibly low, and the grafter eats vis like its going out of style when it repairs, but your chances are better with a thaumic grafter.Azanor has taken deliberate steps to keep silverwoods from being renewable by any means. Tools such as grafters and chainsaws won't work, and the drop rate is very low to keep you from farming them.