The fastest way to grow berries is planting them with growth mulch and making sure the soil never completely dries out. In gen 4, the berries' growth times range from 8 to 72 hours (ignoring event exclusive berries), so growth mulch speeds it up to 6 to 54 hours. Fast growing berries, like Razz and Bluk, dry out in 3 hours (2 with growth mulch), and slow growing berries, like Occa and Passho, dry out in 17 hours (11 with growth mulch). Every type of berry plant can grow at least 5 berries when watered enough.
Another way to farm berries is taking them from NPCs that give you free berries every day. These NPCs are in Floaroma Town, Route 208, Amity Square, and Pastoria City. You can change the date on your DS if you don't want to wait for midnight. There's a few berries you can get in the Veilstone department store for P20 or the Battle Frontier for 1 BP.
If you can connect your Pt game to other games (including other copies of Pt), you can get berries from things like Pal Park, Union Room, and Pokewalker.
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