Leech Seed plants a seed on the target. At the end of each turn that the target is under the effect of Leech Seed, 1/16 of the target's HP will be drained, and the same amount of HP will be restored to the target's opponent (even if it was not the Pokémon that originally used the attack). Recurrent Leech Seed draining will not occur if the seeded Pokémon defeats the target. Unlike other types of damage, the recurrent damage done by Leech Seed can exceed the target's current HP. Leech Seed will not work against Template:Type2 Pokémon.
If the target has less than 16 HP, Leech Seed will drain 1 HP per turn. Leech Seed draining will occur even if the seeded Pokémon knocks itself out with crash damage, recoil damage, or self-inflicted confusion damage, or by making a Substitute before the end of its turn. Draining from Leech Seed is done after continuing partial trapping damage and recurrent poison or burn damage. Leech Seed can be removed by Haze or switching. If the target is seeded during a round that it switches in, it will take recurrent damage during that round, but only if its speed is less than or equal to the user's speed.
If the target of Leech Seed is also under the effect of Toxic, Leech Seed damage will increase every turn as the N value of Toxic increases because Leech Seed and Toxic both use the same damage algorithm ( N × max (1, int (0.0625 × Max HP)). If the current HP of the recipient of Leech Seed's HP-restoring effect is greater than its maximum HP, its current HP will be set equal to its maximum HP.
In Pokémon Stadium, Leech Seed will not work against a target with a substitute.
In a Double Battle, Leech Seed will only heal the Pokémon in the position that used Leech Seed. If the Trainer only has two Pokémon left, and the Pokémon that was being healed by Leech Seed is KO'd, the effect of Leech Seed simply disappears rather than transferring over to the remaining Pokémon.
A seed sprouts from Bulbasaur's bulb, which shoots out to the opponent. When hit, the opponent becomes wrapped in thin vines and leaves that restrain it. The vines and leaves then glow red and suck energy from the opponent. Sometimes, the vines have no leaves at all.
Skiploom shoots one or two seeds from the middle of the flower on top of its head towards the opponent. When hit, the opponent is wrapped in vines that restrain it. The vines then glow red and suck energy from the opponent.
Shroomish shoots multiple seeds from the hole on its head towards the opponent. When the opponent is hit, the seeds sprout vines which wrap around the opponent. The vines glow red and suck energy from the opponent.
Jumpluff holds its arms above its head and releases a stream of cotton from the cotton puffs on its head and arms at the opponent. The cotton sticks to the opponent and sprout vines that ties up the opponent.
Sceptile touches the opponent with the tip of its tail, releasing a seed from it and planting it onto the opponent. The seed then sprouts into vines that wrap around the opponent's body and absorbs its energy, or a vine with leaves on it sprouts from one of the seeds on Sceptile's back and wraps themsevles around the opponent, draining its energy.
Though Leech Seed cannot normally affect Template:Type2 Pokémon in the games, Grass-type Pokémon have been affected in the anime.
Leech Seed is referred to as "Mistletoe Seed" in Japanese because the mistletoe is a well-known parasitic plant that leeches nutrients from its host plant. Ironically, Grass-type Pokémon are immune to its effects.
Although Leech Seed is singular, the games depict three seeds being planted. In the anime usually just one seed is planted, but sometimes two are. In the manga, only one seed is planted when the attack is used.
Because all Pokémon that can learn Leech Seed are at least part Template:Type2, all Pokémon that can learn Leech Seed are immune to its effects.