Jangerberries are a berry used mainly as a secondary ingredient in Herblore. If eaten, they temporarily boost Attack by two and Strength by one, restore one Prayer point, and reduce Defence by one.
According to the book Gielinor's flora - flowers, jangerberries are berries native to Feldip Hills and taste sour and bitter when eaten.
Jangerberries are used in Herblore to mix Zamorak brews, as well as 5 can be used as payment for snape grass patches. They can also be used on a compost bin to make supercompost.
Jangerberries are also used in the Watchtower quest and may be randomly selected as an item required in the quest Fairytale I - Growing Pains. If you do get this item for the latter quest, it would be advisable to bring several jangerberries (or bring Druid pouches) due to the Ghasts in Mort Myre Swamp which might turn your berries into rotten food.
There are four spawns on a small Ogre island in a lake west of Yanille. This island requires that you take a rope each time you want to get onto it; use the rope on the branch of the long branched tree located west of the island. A mith grapple will not work in place of a rope. Alternatively, you can use Telekinetic Grab to transfer the berries to your inventory.
Jangerberries can also be looted when stealing from the fruit stalls outside of the food shop in Hosidius or in the Kourend Castle courtyard.
Jangerberries can be grown using Jangerberry seeds with the Farming skill at level 48 Farming. A player can pay the farmer six watermelons to look after the bush. Players will receive 50 experience for planting and 285 experience for checking the bush's health. Players will then get 19 experience for harvesting each Jangerberry; one seed can produce a minimum of 4 berries, with a higher Farming level reducing the chance of using up one of the four "lives" of the patch, much like harvesting herb patches. As with all bushes, the berries will grow back over time.
For an exhaustive list of all known sources for this item, see here