A watering can shaped like a Lotad. It helps promote the healthy growth of any Berries planted in good, soft soil.
A properly watered Berry of any species will yield more berries.
Berry farming in Uranium use Gen IV mechanics and values. A Berry plant has 4 stages of growth: sprouted, taller, flowering, fruiting.
You can find a Berry's total growth time, moisture drain rate, and yields (check the individual Berry's page) on Bulbapedia. Make sure you're looking at the Gen IV column. Moisture starts at 100 and drains away every 60 minutes, reducing their yield once it hits 0 and every hour thereafter.
Halfli Berries have a total growth time of 72 hours, a moisture drain rate of 6, and a yield of 1 to 5.
Mulch is not yet in the game, as of 1.2.4.