I dropped off more equipment at the castle and took the time and safety afforded by its walls (despite its history) to make use of the many herbs and other ingredients that I had collected thus far. With the alchemical equipment I had just plundered from the small ruin held by Diedre cultists. I prepared not a few potions for myself, and poisons for my arrows.
Wandering about outside for the remainder of the day, I happened upon another nirnroot. Perhaps I will satisfy the Skingrad alchemist after all!