The black-robed young woman's control over plants and vegetation had to do with her Dao of medicine. Coupled with the technique she cultivated, it created something like an innate divine ability that she could use to communicate with and manipulate all types of plants.
Although the Sky River Court was adept with spatial manipulation magic, their ultimate reserve power was the Dire Skybanyan. It was only natural for people who practiced cultivation on the huge tree to develop a connection with plants and vegetation. In fact, all disciples of the Sky River Court eventually formed a bond with a lesser skybanyan, which they could use as a weapon in battle!
Such an arrangement was less of an external manipulation of the plant, and more of a bonding in which the plant assisted the cultivator!
