A technique developed by Hashirama Senju, founder of Konohagakure, the user forces trees to grow on any surface, easily creating a dense forest anywhere they choose. A small plant can grow into a forest in an instant. By generating chakra, the user manoeuvres it as they see fit for attack and defence; and furthermore, this omnipotent technique even allows them to capture the enemy at the same time. With their tremendous life force, the trees can pierce through earthen walls, and extend their branches at their prey in an instant. Even stating that Konohagakure wouldn't have been established without this technique wouldn't be an exaggeration.
- In Japanese, "sea of trees" (樹海, jukai) is an idiom referring to a large expanse of dense woodland. The First's technique changed the last kanji from sea (海) to world (界), probably to reinforce the greatness of the forest created.
- Hashirama uses this technique in Naruto: Ultimate Ninja 3, Naruto Shippūden: Ultimate Ninja 4, Naruto Shippūden: Ultimate Ninja 5 and Naruto Shippūden: Ultimate Ninja Storm Generations. In the first three, after trapping the opponent in a giant tree, Hashirama clasps his hands together and causes the tree to explode, causing damage to his target. In the other, after restraining the target with wooden tendrils, Hashirama delivers a chakra powered punch, sending the target crashing down through the trees.