Yew roots are the remains of a yew tree planted in a tree farming patch. Players need level 60 Farming to grow this tree, and cannot obtain roots from chopping down a wild yew tree. Players must chop down the tree using an axe (yielding yew logs), then dig up the roots using a spade.
The quantity of roots the player obtains depends on their Farming level. If the roots are not immediately dug up, the tree will respawn and it may be cut down again. Multiple roots can be obtained at higher farming levels. 1 root is obtained at 60, 2 at 68, 3 at 76, and 4 at 84.
Yew roots (like all tree roots) can be used to make supercompost for use in Farming or crossbow strings for Fletching, but this is a waste of the item, since it is much more valuable for use in Herblore; when added to coconut milk it is used as the secondary ingredient with an antidote+ (unf) to make an antidote+. Players need level 68 Herblore, and gain 155 experience points when they do so.
For an exhaustive list of all known sources for this item, see here