Triffids exist, maybe add a drop that accelerates growth for trees (with a chance in accidentally making a hostile triffid)?
And plant mutants could use their nutrition to boost tree growth (unlike the triffid version, this might spawn a friendly, kinda like how slime mutants can. Maybe even a plant NPC, not just a simple mob?)
Basically how the Blob increases healing speed, so triffids (and plant mutants, because you pretty much use triffid parts in plant mutagens) might be able to speed up plant growth.