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When held by a Pokemon, increases the HP recovered by the holder by using a draining move by 30%. The damaging moves affected by this item are Absorb, Drain Punch, Dream Eater, Giga Drain, Horn Leech, Leech Life, and Mega Drain. Leech Seed will also recover more HP, but will not deal any extra damage to the opponent. Despite not being draining moves, the HP gain from Ingrain and Aqua Ring is also boosted, so each of them will heal 8.125% HP per turn as opposed to 6.25%.
While theoretically a good item, Big Root is nearly entirely outclassed by Leftovers—even if using a move like Drain Punch or Giga Drain, the extra healing you will get will be lower than you would have got from Leftovers over the short-term. In addition, if you use a Choice Band or Choice Specs, you will increase the HP recovered by 50% regardless, as you are doing more damage to your opponent. Even if using a Substitute + Leech Seed strategy, Leftovers allows you to heal even when you don't have Leech Seed up, which is always helpful. Unless you have a moveset purely consisting of draining moves (not recommended), Big Root should never be seriously considered.
Inner area of Pinwheel Forest.