I would define Plant Health likely as a major as well. But I'd define it as healing/harming, curing/diseasing, and growing/withering any plants.
Yeah...
You could take a subset of those, or also restrict the plants it applies to, to bring it down to a minor.
Like ... Tree Health, since her interest in Forests and her Dryad lineage for example.
That would work, ofc. But the most useful plants to affect with it are the small ones xD But the thematically appropiate ones would be the big ones...
Or instead of Unnatural Plants, Unnatural Trees perhaps to be spells/effects which create or transform Trees in unnatural ways or give them distinctly unnatural properties.
I like how that sounds, but my issue is that "Unnatural" ends being a very vague term. That invariably leads to too many discussions about when it applies and when it doesn't xD
I prefer something more specific... what about Minor Focus: Animate Plants? It cover ReHe, but just partially, since it only covers living plants and just animating them, not making them move like throwing a tree above somebody or like that.