I made this Siren's Pollen farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick Siren's Pollen with BfA Herbalism skill 1, but the rank 2 and rank 3 requires higher Herbalism skill. Higher ranks allows you to gather this herbs more effectively, so you will get more herbs overall.
Siren's Pollen only grows on trees, so don't look for it on the ground. Sometimes a Pollen-Coated Drone or a Pollen-Coated Swarm can spawn while gathering, always kill them because they drop 7-11 Siren's Pollen.
Siren's Pollen
Siren's Pollen ranks
You will learn rank 1 from your Herbalism trainer at Daz'Alor / Boralus.
Rank 2 is a reward from 
Learn From the Best. You can get this quest after you loot one 
[Dead Pollen-Covered Bee] while gathering Siren's Pollen. It's not 100% drop, and you will need around Herbalism 50 before it can drop for you.
Rank 3 is a reward from Pollen Punching. You can get this quest after you loot one 
[Disgustingly Damp Flower] while gathering Siren's Pollen. It's not 100% drop, and you will need around Herbalism 145 before it can drop for you.
How to speed up farming
Battle for Azeroth introduces three new items that you can use to speed up farming herbs. You should use these if you can find them cheap at the Auction House, or craft it yourself if you can.
[Monel-Hardened Stirrups] - You can gather while mounted.
[Monel-Hardened Hoofplates] - Increase mount speed by 20%.
[Coarse Leather Barding] - Prevents you from being dazed. (mobs won't dismount you)
The one I would recommend the most is [Monel-Hardened Stirrups], which lets you gather herbs while mounted. Every mob scales to your level in Battle for Azeroth, so it will be hard to avoid the mobs attacking you while you try to farm herbs. If you have the 
[Sky Golem] mount, you can use that instead of the Monel-Hardened Stirrups
If your class has a tanking spec, you should switch to that while farming, so you won't get dazed and you don't have to use Coarse Leather Barding.
Don't forget to enchant your gloves with 
[Enchant Gloves - Zandalari Herbalism] or [Enchant Gloves - Kul Tiran Herbalism]! This enchant brings gathering time down from 1.5 seconds to 0.5 seconds. This is really important if you gather while mounted because mobs can still interrupt your casting.
Drustvar
You will also get a lot of Winter's Kiss and some Riverbud on this route. Following the yellow route is enough to allow the herbs to respawn from where you started, but you can take a bigger loop around the zone. (marked with the red line on the map)
Zuldazar
You will get a mix of Star Moss, Riverbud, Siren's Pollen. This route is mostly recommended for Horde players.
Tiragarde Sound
This is a good route if you just want a little bit from everything. You will get mostly Sea Stalk and a few dozen from all other herbs. (no Akunda's Bite)
Nazmir
You will also get a lot of Anchor Weed following this route.
