A Druid's powers can only be limited to their knowledge of Nature and inspiration through the Awen. To reach beyond these boundaries and to experiment with simpler or sometimes more potent things, you would need a strong background in herbal lore. Herbalism is a careful and mystic craft. It requires great knowledge, patience, and practice before it ever becomes useful.
The knowledge of herbalism is something you must first become accustomed with. There is no point foraging a salve or potion when you have no idea what it is or isn't capable of. I suggest studying all types of flora around and becoming attuned with their magical along with physical properties.
Unlike Alchemy, Herbalism requires patience beyond expected. In Alchemy you can brew a potion in matters of hours. Herbalism requires a bit more of time. Cerridwen's potion to forge inspiration of Awen took years to focus enough inspiration to forge the great legend it was. In the study of this craft being patient with how certain herbs react with each other is imperative to be successful. Combined with enough practice, it becomes a key tool in the study of Druidry.
As what anything needs to improve, practice is always required in herbalism. Every leaf, bit of sap, and grain is different than the other. Experience will teach you how to measure and apply the knowledge you've gained. Once you receive a feel for it, and trust me, you know when you will, you can began your true study of the craft. And perfect it to finally become useful.
Once you get a fair grasp of it, Herbalism can be one of the most useful aspects in a Druids life. Once you begin your more stronger part of the practice, see what combinations you can bring together that may come to your aid when your own instincts and magic fail you.
~Borfshwitz
See also: The Legend, Whispers of an Ancient Art, Making Potions