Berries are for making poke blocks of for crushing if you want to make poke bloks you go to the contest house in lilly cove if you want to crush them you go upstairs and talk to the middle lady for that you will need a wirless connection.
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You say hi to it.
http://www.psypokes.com/rs/berrystats.php
In emerald you get them in the contest hall by making them with berries.
You have to wait for it to grow.
Mulch
Feed them good pokeblocks from durin berries.
The leichi berry raises the attack stat if held by a Pokemon (that what it says in heartgold but I am not sure if it's the same in emerald)
Just use super rare berries to get gold pokeblocks.
No you can water them infinite times and nothing bad will happen.
Lum berries are randomly given out by the berry masters wife.
You can find berries in these loamy patches throughout the game. You can also plant them afterward and water them, after a while you can receive more from the one you planted.