If you aren't using ACE, I don't know how to help, but if you are, you have to use the exploration garden to unlock new plants. It explains it pretty well, but you need basic fertilizer to use it. When you click on it you have to assign a herbalist, and choose whether to research native or exotic plants. The native plants are the ones that spawn in your selected region, and don't take long to research. The exotic ones are every other plant, and they take a while. You don't need the seeds for the flower pots anymore, you only need the plants unlocked. Hope this helps!