Dragon Flight

Free Routes:

Saxifrage - Ohn'ahran Plains
Bubble Poppy - The Waking Shore
Writhebark - The Azure Span

Shadowlands

Free Routes:

VigilsTorch - Ardenweald
RisingGlory - Bastion
MarrowRoot - Maldraxxus
Widowbloom - Revendreth

Battle For Azeroth

Free Routes:

Zin'Anthid - Nazjatar

Legion

Free Routes:

Aethril - Azsuna
Foxflower - Highmountain
Astral Glory - Krokuun
Fjarnskaggl - Stormheim
Starlight Rose - Suramar
Dreamleaf - Val'Sharah

Warlords of Draenor

Free Routes:

All WOD Herbs - Tannan Jungle

Mists of Pandaria

Free Routes:

Fool's Cap - Dread Wastes
Snow Lilly - Kun-Lai Summit
Green Tea Leaf - The Jade Forest
Silkweed - Valley of The Four Winds

Cataclysm

Free Routes:

Azshara's Veil - Abyssal Depths
Heartblossom - Deepholm
Cinderbloom - Mount Hyjal
Whiptail - Uldum

Wrath of the Lich King

Free Routes:

All WotLK Herbs - Wintergrasp

The Burning Crusade

Free Routes:

Dreaming Glory - Terokkar Forest

Vanilla

Free Routes:

MageRoyal - Darkshore
Gromsblood - Felwood
Fadeleaf - Feralas