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 plant a yew seed in a plant pot filled with soil, water it with a watering can, then wait for the yew sapling to germinate. Players must then plant the seedling in one of six tree patches, and wait for it to grow to maturity. Paying a gardener the required fee of 10 cactus spines is debatable, since if the gardener is not paid, generally only around 5% of seeds will die (when using ultracompost). Finally, 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.
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