The main RuneScape Wiki has an article on: Woad Leaf
The Old School RuneScape Wiki also has an article on Woad Leaf.

Woad Leaves can be bought from Wyson The Gardener who can be found in Falador Park. He will sell the player 1 Woad leaf for 15 coins, or 2 Woad leaves for 20 Coins. They are used to make Blue dye when given to Aggie in Draynor Village along with 5 coins.

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