Treants can provide a very substantial amount of DPS in a boss fight, but it is important to time the cast so that the treants are out of danger of being annihilated by AoE. It is also important that you cast them at the back of the boss, so they don't die due to cleaves.
Using treants effectively in PvP can be difficult, and should be approached differently for each opposing class. Generally speaking, it is a good idea to use them in a panic situation when your opponent is low on mana, running away from something or someone, or focused on healing/damaging someone else.
Against priests of all specs, it is a good idea to cast treants soon after they cast [Psychic Scream] so that they cannot instantly nullify your treants.
Balance Treant should be more responsive while chain casting. The global cooldown on their version of Wrath has been reduced to 0.5 seconds, down from 1.5 seconds.
Restoration Treant should no longer cause their target to taunt enemies.
Hotfix (2013-05-21):Force of Nature now summons Treants that have 40% of the Druid's health, up from 10%.
Balance Druid Treants: Cast time for Wrath has been increased to 2.5 seconds, up from 2.0 seconds, and damage dealt has been increased by 25%.
Patch 5.3.0 (2013-05-21): No longer on global cooldown and summons a single Treant. The Treant no longer has a control bar, immediately uses its special abilities on the Druid's current target, and accumulates 1 charge every 20 seconds up to a maximum of 3 charges.
Restoration version of the Treant now casts [Swiftmend] on the Druid's target when summoned. This version of Swiftmend does not require or consume a heal-over-time effect on the target.
Patch 5.2.0 (2013-03-05): The Treants summoned by Force of Nature now deal more damage and healing, and the Force of Nature tooltip will report the capabilities of these summoned pets.
Patch 5.0.4 (2012-08-28): Now available to all specializations as a talent. Cooldown reduced from 3 to 1 min. Can be used in all shapeshift forms and the abilities of the treants varies by specialization.
Hotfix (2011-01-26):Force of Nature and Wild Mushroom can now be cast correctly even if the target is on a different height of terrain from the druid.
Patch 3.3.0 (2009-12-08): Health on the treants has been increased.
Patch 3.0.2 (2008-10-14): Cooldown reduced to 2 minutes, down from 3.
Patch 2.3.0 (2007-11-13): This ability will no longer automatically break stealth on victims in its casting area. Nearby stealthed characters may still be attacked if they are too close to the summoned Treants.
Patch 2.2.0 (2007-09-25): Using this spell in a neutral town will no longer cause its victim to be attacked by town guards. It also will no longer cause nearby party members to lose stealth.
Patch 2.1.2 (2007-06-19): This spell will no longer causes players it is cast on to enter combat with nearby creatures. All of the treants summoned will now attack a neutral creature they are summoned on.