Working up to having plants in his dorm was emotionally trying. This put him in charge of an actual living thing, something that needed to be constantly cared for and supported, things he had no experience in.
His childhood homes usually lacked a garden, and when they did miraculously contain one it was cared for by someone else, someone who did not appreciate him trying to nose his way into the process.
So it made sense to introduce himself to having plants around before having living plants around? Or something along those lines.

Sweating slightly due to lurking outside of Petit Petales for so long, Kyouhei entered the extravagant flower shop, eyes bulging out of their sockets at the amount of flowers and the quality of each type.