It's a sword so Steel type makes sense, but grass? Why grass? It doesn't represent a plant or a vegetable.
Kartana may be based on origami and samurai. In particular, it may be based on noshi, a ceremonial origami attached to gifts for congratulatory occasions as a token of good luck, or shikigami, paper figurines cut into the shape of a person or animal and imbued with spiritual power, often used by priests to ward off negative energy. Its sharp, paper-like qualities may be a reference to paper cuts.
Source
Origami is made from paper, paper is made from trees, trees are plants.
