Tarberries are small, purple berries of the tarberry plant, itself a water-grown crop ostensibly similar to cranberries.
Tarberries are a required component of Berry Mentats, which give a substantial +5 boost to Intelligence, and thus earned experience. Under normal circumstances, this works out to about a 16.666% bonus. This is likely the reason that tarberries are one of the rarer crops in the Commonwealth.
Tarberries can only be farmed at The Slog settlement. There are eight plants, and if removed they can never be re-planted. Most companions will comment on the ingenuity of farming tarberry in a former swimming pool, alluding to the general Commonwealth knowledge that tarberries are rare and difficult to cultivate commercially. The ghouls at The Slog say that they have the best tarberries in the Commonwealth, but truthfully, they have the only cultivated tarberries in the Commonwealth.
Some vendors, typically the general goods vendors such as Myrna and Trashcan Carla, occasionally carry one, two, or three tarberries for sale.
The Slog has a swimming pool containing nine farmed tarberry plants which can normally be harvested once a day. You are not required to have The Slog as a settlement to harvest the berries, but the settlers must be alive, and if you do have the settlement, settlers must be assigned to farm them like typical crops.
There are four along the northeastern shore of Dark Hollow pond. Two of the four are against rocks and are more difficult to spot.