Where your arguments with your DM on what you can do with the various plant kingdoms are based entirely on either party’s botany knowledge, there are a couple of fun plot angles that could be had. Many poisonous plants have nonpoisonous parts that are widely used today for food and medicine.  A player can also use things such as the berries of an assassin vine for diplomacy. Thanks to how dangerous these plants are to a common NPC, the danger required to obtain these materials alone is worthy of haggling with the local alchemist.