Gnarlroot

Tanks: Swap after each complete cast of Dreadfire Barrage.
Tanks: The boss will always spawn Tainted Lashers in Cardinal Directions (in front, behind, and to the left and right).
Tanks Heroic: Position the boss to spawn Tainted Lashers in existing Controlled Burn areas to spawn them faster.
Tanks: Pick up Tainted Lashers when they spawn and tank them on top of the boss to let DPS cleave them down.
Tanks: The Tainted Lashers apply a stacking Bleed Shadow Spines. This bleed also increases damage taken. Coordinate AoE stuns to lower damage taken.
Avoid Shadowflame Cleave.
Avoid Flaming Pestilence, this spawns Tainted Lashers.
Heroic Tainted Lashers are inactive and will only be attackable once they have been hit by Controlled Burn.
Run away with Controlled Burn.
Heroic Place Controlled Burn on top of Tainted Lashers to make them spawn.
Healers: When Tainted Lashers spawn each of them will trigger a small amount of raid wide damage with Tainted Bloom.

Don't stand on top of Gnarlroot; there is Toxic Loam there.
Gnarlroot takes 99% less damage until all branches are burnt.
Heroic Soak Searing Bramble to get Ember-Charred.
Whilst you are Ember-Charred run into the branches to burn them.
When all of the branches have been burnt and have despawned, Gnarlroot will take 100% increased damage for 20 seconds. Save damage cooldowns for this point.
Gnarlroot now goes back into their main phase and deals 10% more damage.