If the nodes are scripted correctly, and I believe they are (or at least they seemed to be OK during beta), each zone as a minimum number of plants that have to be spawned at any given moment, which makes it possible for an herb to respawn instantly (not necessarily in the same spot, nor as the same herb) when you gather it.
Kinda like the mechanism in place for Black Lotus, except it's only zone-wide here.
This somewhat like a "dynamic respawn" mechanic that Blizzard implemented later for mobs respawns. Only time will tell if this is enough, but I might help to provide herbs for a large population

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