To be able to have fun the first thing you have to do is know and accept how much this hero sucks at offensive spells and, taking that in account, play differently as you would do with, for example, a nuker (which is the extreme opposite). If you think you can play without worring on dealing much damage, and doing a more helpful role, I think you may be able to see what really does this huge tree have to offer.

Now, let's take a look at his abilities, shall we.
I'll present Cons before Pros so you get a better image of him. Not that you have to be aware of this, though.

Nature's Guise

AKA Invisibility of the Tree Huger. If you love ganking/roaming, you'll love this ability.

Cons:

• You have to move around and near trees if you don't want to lose your invis. You'll get used to it though, and if you play him enough you'll learn where the trees of different areas are the closest, and will learn routes to be able to cross half the map without losing it.
  
• At its first level it will be nearly useless, it's duration is only of 15 seconds. If you really want to exploit this ability I'd suggest putting 2 early points in this ability.
  
• Its cost mana is HUGE.

Pros:

• It only has a cooldown of 10/8/6/4 seconds, so you can set up a whole invisible team very fast at later levels.
  
• Its duration is also pretty nice... Unless you only spent one level-up point in this ability.
  
• Because of that, it can be used like a smoke gank, but without losing it if you go near an enemy hero.
  
• TREANT PROTECTOR EXCLUSIVE: You can throw spells or any ♥♥♥♥ you like, that you WON'T lose your invis. The only thing I know that breaks your invis while staying near trees is auto-attacking. THIS IS SO NEAT, YOU HAVE NO IDEA
  
• So, if you're initiating from an invis gank, throw all your spells and active items before going outside invis. They won't know where the attacks are coming from before it's too late
  
• Because of the previous point, if the pseudo-smoke gank is taking too long, you can reset its duration with throwing it again on your allies. (This is easier if you're with friends) Neato, right?

Leech Seed

AKA Leech ♥♥♥♥. The only damaging ability of Treant, and it's not even that good at it. This is why Treant is one of the least liked heroes.
It's supposed to slow enemy heroes, deals a bit of damage over 3 seconds and emits up to 4 pulses that heal nearby allied units (there won't be any pulses if there are no nearby allied units).

Cons:

• It is technically a slow, but it doesn't practically slow that much. Don't use this on fleeting heroes as a way to catch them. You won't. (unless your speeds are tied, which is improbable)
  
• It doesn't damage that much, though it's a bit more noticeable than the "slow". Use this on fleeting heroes if the enemy hero has very low HP remaining.
  
• Its cost mana is HUUUUUUUGE. Forget to use this more than twice in the first 10 or 20 minutes of the game.

Pros:

• It heals nearby allied units. It's useful.
  
• Throw it in if you have already thrown everything you could throw and still got mana left, and/or if your allies would appreciate that extra health in a fight.
  
• It can also be used as an expensive harass. I do not recommend that, though.

Living Armor

NOTE: This skill has been changed relatively recently. If you see someone talking about this ability as a passive one, they're a bit outdated. It's an active ability now.
This is a buff that blocks some damage from all sources and a HP regeneration plus for 15 seconds (or until it wears off because of enemy damage).
Cons:

• It wears off pretty fast if the target of this ability is being focused.

Pros:

• Its HP regeneration is beautiful. It is like a free Tango (but with mana cost - although negligible).
  
• It can be applied to buildings. YOU CAN HEAL TOWERS WITH THIS ♥♥♥♥
  TREANT OP RIOT PLS NERF
  
• It is EXTREMELY useful to tank enemy towers, as their attacks don't do ♥♥♥♥ to wear it off. Beware of nearby enemy heroes though.
  
• It has. FRIGGIN'. GLOBAL RANGE. YOU CAN THROW THIS ♥♥♥♥ ON AN ALLY AT THE OTHER SITE OF THE MAP.
  
• Its mana cost is negligible FROM LEVEL 1. SPAM THIS ♥♥♥♥

This is the most useful ability of Treant. A friggin' free tango that blocks damage for 15 seconds. And heals buildings too.
I'll explain why is this so beautiful in depth later.


I always call it Entangle though. Dunno why.
*Searches*
Oh wait I know: it's a disable that causes a status effect called Entangle. So yeah.
Cons:

• This is an incomplete disable, and won't hold the enemies back from throwing spells or attacking if you get in range.
  
• If the enemy uses a BKB after you've entangled him, he will get rid of the entangle.
  
• It has a long cooldown.

Pros:

• If the enemy uses BKB before you've entangled him, your ulti will go through magic immunity.
  
• It prevents enemies from moving or blinking.
  
• It is very useful if it's combined with other ultis from your allies.
  
• Its cooldown is not that long.

You see this ability? Let's remember what are its pros, even though you've just read 'em and you probably still remember them:

• Free. Friggin'. Tango.
  
• Damage block.
  
• 15 secs duration.
  
• Can heal buildings.
  
• Negligible mana cost.
  
• Global friggin' range.
  You gotta max this little monstruous baby. It has to become your priority number 1. Spam that ♥♥♥♥ too. It will be the only thing you can do for the first 10 minutes of match anyway.

When to use it, by order of priority:

• If you're missing at least half your HP, use this on yourself, UNLESS the carry you're supporting is in danger.
  
• If your lane partner is fighting against lane enemy heroes and/or is being focused, throw this on him to block part of the damage (until it wears off) and heal a bit (if you have some mana left, a Leech Seed after that would be nice too).
  
• If there's a fight too far away from you (and remember, nearly everything is too far away from you since you move VERY slow - "trees are not so good with motion, you know") and you're not in trouble, don't hesitate to move the camera to the fight and throw it to the ally being focused/the one with less HP. Global range, remember? Use a bit of common sense: don't look at the fight if you're standing in the enemy's side of the lane, go to a safer area before looking over there.
  
• If there's no fight on your lane nor anywhere else, look for the tower with lowest HP (anywhere in the map - global range, remember? On the first line of towers if possible) and throw it on the tower. Common sense needed here too.

Okay, so you know how your common sense asks you, begs you to attack the enemy tower when you push the creep line too far, so you can wear out the enemy tower and take it down easier later in the game. Well, if you use Living Armor on your towers, your enemy's effort at this will be USELESS. Do you imagine how frustrating it has to be, that every time you push to the enemy tower to wear it out, each time you get back to it the tower's HP is full again?
This is one of the things I enjoy the most of this ability: healing towers and blocking damage on allies being anywhere on the map thanks to the global range.
Best of all, it's duration is exactly as long as it's cooldown, so when you can cast it again you know it has finished for the last target you used it on.

I usually start with one point on Leech Seed, and cast it right away when I see an enemy hero for the first time on the lane, for early harass and scare them away. You don't have to do it, though: it consumes a bit more than half your starting mana pool.

If you do, however, you have 2 options: wait for a bit to regen enough mana to throw it again (and empty completely your mana pool), or save it for the Living Armor spamming you're going to do. I go for the second one; you'll still be able to spam it if you go for the first option, but not as often.

Then I distribute the next 4 points as this: 3 on Living Armor and 1 on invis if I can't upgrade Living Armor (I won't use it until I have 2 points on Nature's Guise, though).

An alternative, for if you love ganking/roaming, would be to spend 2 points on Nature's Guise, instead of 1 on invis and 1 on Leech Seed. In that case, I'd recommend you to spend your first point in Living Armor, and leveling up Nature's Guise when you can't do it with Living Armor.

Whenever you can, hit the enemy heroes with auto-attack instead of enemy creeps. You hit way too hard, so it's a great harass tool.

Items:

Starting items:

• Wait until someone buys courier. If nobody does, do it yourself. It isn't nice to start a match without courier, even if you're the only one worrying about it.
  
• Valve's standard build recommends you to get a shield, but since you probably won't be getting Vanguard, I prefer to take a Gauntlet of Strength, and then building it into a Bracer, which gives you nice early stats and allows you to hit even harder.
  
• After buying Gauntlet, spend the rest of your money on 1 or 2 clarities and 1 pack of tangoes OR 1 salve. Don't buy tangoes AND salve, though, it's a waste of money: remember you have a spell that works as a tango.

Early items:

• Rush an Aquila if you can. If not, get at least a Basilius before getting your Bracer. It will help you with your mana problems, as well as helping your lane partner with the aura. It also gives armor, which is very important for you as Treant, since, even though you have one of the highest Strength in all Dota's roster, the extra HP that gives you won't be useful if you don't have any armor.
  
• After the Basilius, make the Bracer as fast as you can; not for the strength, because you already have that covered, but because it gives a nice general boost to all your stats. It stops being useful pretty early, so that's why it would have to be a fast Bracer.
  Note: Please use the extra strength to harass enemy heroes, not to hit creeps.
  
• An alternative to Bracer would be Urn of Shadows, which is a great healing tool, and helps you a bit with your mana problems. It won't be as useful if your team starts losing early on, though, since it runs on enemy hero deaths, so I prefer to make a Bracer and then building a Mekansm instead later on, for the heal.
  
• Throw Boots of Speed in there if you suddenly get 450 gold or more on a blink, even if you haven't finished your Ring of Aquila. You NEED that speed. If you have to wait for the money, though, finish that Aquila first.

From here on it will depend on how you like to play. Remember that you won't be the one dealing damage, so don't be too carefree at teamfights (unless you're the only durable on your team; dying can be helpful IF your death can save your carry's ass).
Don't forget to max Living Armor as early as you can. I usually max invis after that, and max Leech Seed the last one; but those two are interchangeable, I guess.

Items:

Well, you know, usual support stuff.

• Upgrade your boots to Arcane Boots.
  
• Get a Mek if nobody else's getting it. It also gives armor.
  
• You can upgrade your Bracer to Drums of Endurance if you want, too.
  
• Don't forget to buy wards from time to time; specially if there's a hero with invis on the enemy team, buy a lot of Sentry Wards.
  
• If you get more money than you know what to do with, and you're near late game, build some armor: an Assault Cuirass or Shiva's Guard would be cool, and you'll need that extra armor in late game. Shiva's also helps you with your mana problems, but they disappear naturally as you get levels + your Arcane Boots + Aquila, so I prefer Cuirass' aura.
  
• Blade Mail could be an alternative armor item if you're playing against powerful nukers.

Situational items:
Depend on how the game is going.

• You can rush a Blink Dagger instead of getting Drums and/or armor, and/or if someone else's getting Mekansm, AND you're having considerable problems placing your ulti. It shouldn't be your priority item though.
  
• Vladimir's Offering is highly recommendable if your team is full of melees. Lifesteal is always nice and if, for example, there's an Ursa on your team, you're giving him/her an extra slot in their inventory since they won't be using it for the Vlad (if you don't know them let them do Vlad's instead, they probably will anyway, or maybe they'll rush it, and you will be late, and we don't want 2 Vlad's).

Items:

• Get some armor if you haven't already (Assault Cuirass/Shiva's Guard/Blade Mail).
  
• A Heart of Tarrasque would be cool for the great HP regen and the great HP max increase, but don't worry if you don't seem to be able to amass enough gold.
  
• Some people also get Refresher Orb. I guess it helps spamming your ulti a bit more, and gives you mana, Intelligenge, extra damage, and a bit of HP regen.

Other situational/expensive items:

• Heaven's Halberd. Why not? It's a nice disable, and I recall someone calling it Poor Man's Heart.
  
• Black King Bar, for if the enemy line-up is full of stun that doesn't let you place your spells/ulti, or full of nukers that make you evaporate in the flash of a second.
  
• Vanguard, in case you bought a shield as a starting item and/or you prefer this over Heart of Tarrasque since it's easier to build.
  
• Radiance. You can be a great durable, and Radiance's fire thingy will be a pain in the ass for the enemy.
  
• Pipe of Insight, Mekansm's close friend. Get this if your team needs this and there's already a Mek on your team.

It has a short range, so you'll have to learn when you will get someone in it. Sometimes I've felt very stupid when I get in and throw my ulti, expecting to entangle 1 or 2 enemies, and get nothing.

Once you do that, you have 2 ways to use it:

As an initiator:

• You go to a nearby forest, you cast invis on yourself, wait for the prey to get in position, and then you throw in your ulti. Your carry/ies come in and start wreacking ♥♥♥♥, and since they can't move, they'll have it easier to locate their AoE spells, etc.

As a chaser:

• You should be with someone else here, though. If you are alone don't try to chase, in most cases the enemy will still have time enough to run away. But if, for example, your carry jumps into some enemy, and when the emey hero is low on HP and starts to run away from him, you come in and throw your ulti. That look of desperation when they suddenly find that they can't keep running... It truly is lovable.

...It pays off, I promise.



And I guess that's all about Treant.


One last note: if your teammates are bad players, gg. You're a support, so don't expect to have much fun if your carry can't do his job. Don't give up at first try, do it at second or third. hue

Another last note: I'll keep doing small edits here and there to improve this guide as I find necessary. Come back after a while if you still have doubts or you just want to check what I may have added to the guide.

Just another one more last note: This is the Dota 2 build guide that you can follow in-game:
http://gtm.steamproxy.vip/sharedfiles/filedetails/?id=141781588
After reading this guide, the descriptions will probably feel repetitive but oh well; I couldn't just ignore the people that could see that one without reading this one.
(It seems that when you try to open it from outside Dota 2, it shows an error; just click "Subscribe" and it should appear on your guides list when playing Treant)