Egwene was Nynaeve's apprentice in the Two Rivers. She in the same book that your discussing recognizes the name of Asping Rot, and knows what it does. Clearly she has some knowledge of herbs.
A lot of cooking herbs, can be used not for just cooking but medicinal value. Particularly in folk remedies.

(Let me tell you the descriptions in the book about vile tasting teas does not do them justice enough...blah!)

Also a lot of wiccan/pagan/new age who use herbs and folk medicine combine their herb garden not only for cooking/but medicine. Waste not want not.

And Aes Sedai have been shown to have their eccentric hobbies as well. So Suana has a green thumb? Making things grow is very satisfying, relaxing, and rewarding.