Very easy actually.
First thing that needs to be pointed out is that ash is actually a fertilizer, and there is a lot of it there.
That being said, ash is not suitable for plants that are yielding sweet or sour/acidic fruits so me must choose adequate species.
Spruces, Junipers , Oaks, Aspens, Maples would be perfect, and i am sure Night Elves would like those species for their new old home.
We can use fertile sediment rich soil from Nazmir, Swamp of Sorrows, Dustwallow Marsh, Loch Modan mixed with grass and shrub seeds to fill paper sacks and then to carpet bomb remains of Teldrassil with those bio bombs.
After few years when grass and shrubs ‘kick-start’ micro processes within soil we can proceed(or shall i say proseed) with plantation of actual trees.
Obviously sufficient fleet of zeppelins must be made by Gnomes and Goblins.
That’s about it, not really a rocket science.
