First, visit any trainer in the main cities. Ask a guard if you get lost, and learn Herbalism. The Herbalism Apprentice skill will allow you to upgrade your skill to level 75.
Briarthorn requires herbalism 70 and Stranglekelp requires herbalism 85, so you can't pick them right away.
Kingsblood requires herbalism 125 and Liferoot requires herbalism 150, so you can't pick them right away. You should only go to the red zone/lines after you passed 150.
Hope it helps.