Two names I haven’t seen before! Welcome @tie and @elspood, have a great time and please feel free to tag me if you have any trouble or would like an outside opinion, gl hf to you both!
You need a hero to play a spell (a good way to remember this is to mention in the turn write up which hero “casts” the spell).
Also, most spells (Rampant Growth included) don’t stay in play when you use them, but go immediately to discard. Only ongoing spells stick around.