I suppose maybe the How To Play sections doesn't make this clear enough, but you're able to right click fully grown plants and instead of havesting them, they'll turn into 2x (or more if you're lucky) seeds.
This way as soon as you've bought one plant, if you're patient enough, you don't really ever have to buy more of them.
Thanks alot for the nice comment :D I'd definitely recommend giving itt a go again with this new information tho, it should make the game a lot less "grindy".