Taint dose spread, but very, very slowly. If you have some way to change biomes then I suggest turning an area into tainted lands and throwing a few bottles of taint around. Fibrous taint spreads very rapidly within it's biome. If you can't do that, then chunk loaders could work. And if you don't have chunk loaders then I guess you could always move the tainted nodes around.
You can also configure the rate of taint spreading in the Thaumcraft.cfg file. Drop the number down to "1" and taint will always spread on check.
Its kinda "cheaty" but it's an option.