Terumot and ma'aserot from one sprig of rosemary
Question
If I pick a sprig of rosemary or another herb, does a single sprig at a time require separating ma'aser?
Answer
Even if you pick one leaf of rosemary it requires the separation of terumot and ma'aserot; rosemary leaves are eaten and do not only impart flavor.
Note: the leaves are often infested and must be thoroughly cleaned and inspected before use.
If it is difficult to separate terumot and ma'aserot each time -- and especially if there is a concern that family members will use the herbs without tithing, there is a way to separate in advance once a week against the herbs you will pick for that week. Please see my explanation of the procedure here
The formula for this separation appears on p. 16 of our guide, Mitzvot of the Land in a Nutshell .
Note that advanced separation is not lechatchilah; it's best to actually separate each time you harvest.