A Green Father is a massive, living Tree. Similar to a Treant but much more powerful, a Green Father is the master of his lesser cousins and defender of the forest.
For most purposes, a Green Father is considered a "Wood Elemental". The gold-green sap of a Green Father is incredibly potent and will accelerate the growth of any plant that it touches. A single drop of the blood of a Green Father will turn an acorn into a sapling in a matter of moments and encourage growth of 'the green' for a radius of at least five feet - per drop.
The Druids call this the Blood of the Green Fathers. The Alchemists call it simply Blood of the Forest.