Strange has an idea of how to fix it. He's just worried about how well he'll fix it.

"It was a simple growth spell that got out of hand. I believe I can talk the plant down to a more reasonable size...though I'm a little worried the reduction spell might also get out of hand." A tiny plant is probably better than a real fuckin' huge plant, but as Strange isn't certain of how bonsai works, he's not sure how much trouble he'll get into.