Dear Lord, how many people accuse the girl for being an idiot... ironic.
The garden has been there for long time, has been a well-known thing for long time, and there are hundreds of witches stronger than our protag in the world. If nobody is asking the witch to do something about it, other witches likely failed.
If it's still there after all that, it means that these gardens aren't so easy to take care of. If they were, they'd been gone long ago. But they are still there, so the only thing you can do is accept they are going to stay there for a long time.
And unless she is an utter fool, she won't randomly try to burn them. She is immune to the flower field's charms, but that doesn't mean that it's harmless to her. There's no saying what these plants could do if they felt endangered, both to her and others. Unlike what Shounen stories like to tell you, attacking something like a fool is usually a death sentence, not the magical solution to all problems. And inexperienced youths especially should not try to solve problems like these.