What someone sees as affirming their gender is entirely up to them. Plenty of men do feel more masculine doing things like not using a straw, eating certain meals, listening to certain music, ect. Women might wear clothing that emphasises their figure. Some trans men feel affirmed in their gender by growing a beard, trans women might train their voice to feel more feminine. Gender is a performance and we all constantly make decisions to reinforce it. Thinking growing plants is gender affirmation isn't a problem, it's just a reality you're not living. If someone grows those kinds of plants and it reinforces their gender identity, that's valid and more power to them.