I bet LLMs would be pretty good at that since there's likely a large repository of public domain botany drawings from the past few hundred years.

I'd consider that deceptive. Botanist drawings are technical, so I tend to treat illustrations in that style as factual in some sense, but AI can ape the style quite nicely while getting all the details wrong. You wouldn't want AI-generated technical documentation, would you? It's the same here.

Can't you just find images, the same as you find plants? Or manually add them, or something. Maybe commission some, if you can't find anything for a certain plant.