Assuming she is utilizing a combination of earth and unicorn magic is the only way to not shit all over existing lore.
Well, depending on the canonicity of Gillian Berrow’s books, she very well might be. Do you remember one of the first chapter books she wrote,
Twilight Sparkle and the Crystal Heart Spell?
If I may offer a quote:
She was officially a part of a special breed of pony called an Alicorn. This meant that Twilight was now able to harness the magical powers of the Unicorns, the flight abilities of the Pegasi, and the strength of a good, true heart of an Earth Pony.
I mean, it may be a bit of a stretch, but if you consider alicorns as embodying a part of each traditional pony tribe, who’s to say they don’t also have access to some of those group’s unique magical skills? Plus, lest we forget, Twilight is clever and talented where magic is concerned. If anyone was going to learn something outside the typical unicorn variant, I’d put money on it being her.
Alternatively, it could be that the account we’re given describing the earth ponies as the sole possessors of plant growing magic is a misleading or simplified version of the historical reality. You don’t need to delve into all 47 scrolls of the Unicornia Accords or review historical documents pertaining to the specific details of Princess Platinum’s politically heated renegotiations with the farming communities to put on a seasonal play, after all. Heck, for all we know the strict division depicted only existed in that brief time period. We as viewers only join the story when the tribes have become bitterly divided, inviting the windigos to come and cause further trouble. Given that they had survived a fair amount of time prior to that (at least long enough to have kingdoms), maybe there was a time when unicorns farmed with their earth pony friends.