I made this Zin'anthid farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick Zin'anthid with BfA Herbalism skill 1, but the Rank1, Rank 2 and Rank 3 requires higher Herbalism skill. Higher ranks allow you to gather this herb more effectively, so you will get more herbs overall.
Zin'anthid
Zin'anthid ranks
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The rank 1 requires Herbalism 140. You can learn it from your trainer
Rolm / 
Instructor Ulooaka at Nazjatar. (You have to finish the first few intro quest at Nazjatar before you can see these NPCs.) -
You can get the rank 2 from Carnivorous Lasher. To summon this rare, you have to loot a
Germinating Seed from mobs in Nazjatar. (random drop) It's a rare mob so you might not be able to solo him if you have bad gear. -
Rank 3 is from a quest item that drops from gathering Zin'anthid.
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.
Nazjatar - route 1
I got the most Zin'anthid following this route. Bunch of mobs will be everywhere, so it's pretty bad to farm this herb until you get flying. Use the enchants I mention above or you will get dismounted every second.
Nazjatar - route 2
This is basically just a route that loop around the area that's not covered by route 1. This is not as good as the first one, but you can expect a lot of people farming route 1, so you could give this a try.
