Nightshade is all over if you go by the swamps, between riften and windhelm (closer to riften).. they are all over the place (not the waters edge).
3v3r is correct on the ninroots. the main thing I do is listen for them, they will make an annoying sound. Also if you go by riften in the water you will find 2-3, where you do the braer bee quest to burn the bee hives.. if you have not done that there is an island with houses on it.. look around there..