well if its all organicthen u dont have to flush, but i always try to flush as it makes to the weed taste and burn betterIMO, and u flush by running 3 times the amount of water as the pot so a ten gal pot means 30 gallons of water and u do this 10-14 days before harvest some do a fast flush like 5-7 days, and after that its just water and mallases till u cut