Each rank requires you to have Entangling Roots of the same rank.
Procs off melee only, does not proc off spells or ranged attacks.
Entangling Roots can be resisted - so even when Nature's Grasp procs, it can fail to have an effect.
Tips and tactics
This spell is very useful against melee attackers in PvE or against Warriors and Rogues in PvP.
Activate this spell when running away. Often the attacker will be rooted and unable to follow.
This spell is also useful for gaining range against a Shaman to use Innervate.
It is entirely impractical for raiding if the druid is not a tank, given that 1) the scariest mobs are immune and 2) the Druid shouldn't aggro away from a tank, 3) the Druid is not likely to survive a single hit, and 4) a rooted mob will turn to attack other people in range, regardless of aggro, potentially killing others in addition to the casting druid. There are, however, a rare few applications, for example when dealing with blood beasts in the Deathbringer Saurfang fight.
Can also be cast in Cat, Bear, Tree of Life, and Moonkin forms.
Patch changes
Patch 3.3.3 (2010-03-23): Now has 3 charges, up from 1.
Patch 3.2.0 (04-Aug-2009): This talent will no longer be triggered spuriously by some ranged abilities such as [Heroic Throw].
/Patch 3.0.2 (14-Oct-2008): Can now be used and can proc indoors.
Patch 2.1.0 (22-May-2007): This spell no longer costs any mana to cast.
Patch 3.3.3 (2010-03-23): Now has 3 charges, up from 1. 
