Sorry if this is a repost, but I searched and found nothing.
Okay well, first you need to get Soul Preserver - Item - World of Warcraft trinket, which is available from regular CoT:Strat. When the effect procs(Soul Preserver - Spell - World of Warcraft), it will reduce the mana cost of your next spell by 800. But for some reason, when you use Wild Growth when the buff is up it wont go away after using Wild Growth. Surely, this is not intended.
So, pretty much just get that trinket, and heal regularly until it procs. Once it does you can have a hayday spamming Wild Growth with no mana cost!