I made this Gromsblood farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. It's recommended to have around Herbalism 250, so you won't get herb fragments.
Felwood is the best place to farm Gromsblood, but this zone have large variety of other herbs in large quantities. You will also get Golden Sansam, Dreamfoil, and a few Purple lotus, which makes this a really good zone to farm for lower levels who wants to level Alchemy because you need all 3 of those herbs.
You can also find some Gromsblood at Blasted Lands and Desolace, but there are so few I don't think it's worth to go there.