Blizzard has responded to even more Season 3 trinket feedback from the Patch 10.2 PTR!
Can confirm, even with efflorescene, a treant and natures swfitness for an instant 100% regrowth preplaced for the little guy he instantly died the global after being summoned without any of my spells being able to tick past the intial regrowth hit.
This has been fixed in the next PTR build. The Seedling will now spawn at 50% HP and burn down at a much slower rate. A few other changes: