Ah I figured it out. There doesn't seem to be a way to open the dropdown menu using the an gamepad/controller. So I switched over to using mouse and keyboard and now the dropdown works.
Viewing post in The Well jam comments
ah yeah that's one of the things on our list to change, the mouse controls for the inventory was a relic from when the game was more of a slow-paced puzzle platformer game where you had to collect each seed, then plant it, then water it, etc. We ended up shifted away from that but didn't change the way you plant the seeds. Thanks for giving it another shot, appreciate the in depth feedback!