Acorns are seeds used in the Farming skill and acquired from bird nests, mammoths, seed packs or by opening supply crates obtained from subduing the Wintertodt. It requires 15 Farming to grow an acorn into an oak tree.
Using it on a plant pot while a gardening trowel is in the inventory, and watering it with a watering can, does not grant any experience. After several minutes (in the bank or the player's inventory), it will then turn into an oak sapling. It can then be planted in a tree patch and left to grow. A farmer can be paid 1 basket of tomatoes to watch over it and ensure its survival.
An oak tree takes 200 minutes to grow (5 stages of 40 minutes each) and grants 467.3 experience when its health is checked. In order to plant a new one, the tree must be chopped and then dug up, rewarding the player with oak roots. The amount of roots given depends on the player's Farming level (boosts can be used):
- At level 15, a player obtains one oak root.
- At level 23, a player obtains two oak roots.
- At level 31, a player obtains three oak roots.
- At level 39, a player obtains four oak roots.
Acorns may be given to the giant squirrel or the squirrels on Fossil Island. It provides no benefit, but grants some special dialogue.
For an exhaustive list of all known sources for this item, see here