Cold Harbor’s Hollow City is without a doubt the best location I’ve found for gathering your every day reagents (Mountain Flower, Columbine, Bugloss, Dragonthorn… etc). Even with 5-10 other people running about picking flowers with you, there is no shortage of plants.
With that in mind, I whipped up a quick map highlighting some of the better locations to start your search.
The Western district of the city is a great spot for gathering Water Hyacinth and Nirnroot, while the South Eastern portion (particularly outside the walls) is a phenomenal location for Bugloss, Mountain Flower, and Columbine. I mention those three reagents in particular because they contribute to creating a Health/Stamina/Magicka potion that may very well already be your best friend if you are familiar with it.