I made this Silkweed farming guide to help out players who want to farm the herbs instead of buying them from the Auction House. You can pick this Herb from skill 1, but you will only get [Silkweed Stem] until you reach Herbalism skill 450-500 (10 makes one herb). Keep in mind you will not have access to flying mounts until you reach level 90, so you won't farm Silkweed that efficiently before you reach level 90.
Valley of the Four Winds
Valley of the Four Winds is pretty much the best and only place to farm this herb. You can find Silkweed in a few others zones too, but there are so few of them, you will always have to wait for respawns.