Apparently, Croppery DOES influence your herb harvest.
It increases the yield for all herbs, where you normally would get 2 per spot (Yellow Crumb, Purple, Silvertongue) from 2 to 3.
All others (Gandlevery, Rookswort, Hairball) stay at 1 per spot, no matter if you have croppery or not.
Details see here: http://www.glitch.com/forum/general/13142/
Is that intended, or is it a bug?
There seems to have been a widespread agreement on the fact that "Croppery does not benefit herbgardens" in the forum so far. Apparently that is not the case, and the benefit is quite significant (given the relatively low yields on herbs), although quite confusing. It also does NOT match the skill description of croppery (double your harvest).
Would be nice, if some people from TS could clarify if this is intended. Also: I think it would be quite helpful if some of the skill descriptions would be a bit clearer in this regard, and give a better idea on how a skill influences the game mechanics.