In my second build I will show you how to play maokai as an incredible tank. Most players build ap with maokai, which isn't wrong, but isn't always what your team needs. Tanking makes much more sense to me, especially when you consider his support capabilities with his ult.

Maokai's AP scaling is rediculously low, not to mention his high cooldowns. Q has 0.4 scaling, W has 0.8, E has 0.4, R has 0.5. This is very low when considering other AP champs. Also, his cooldowns are much too high. 6 seconds on his Q and over 10 seconds for his other abilities. Going AP would mean that after your burst, all you can do is auto atk for 6 seconds. Only thing I like about AP Maokai is landing the funny as hell sapling toss kill.

Pros
good harrasser
excellent at supporting team

Cons
No damage output alone
Items are pricey

Maokai is a very mana hungry champ, which is why i chose mana regen per level quins. You should always get mana regen per level instead of flat mana regen because after hitting level 5, they would be equal.

All my other runes are magic resist because i get way more armor than magic resist from items, and need these to compensate. The armor items just give out more benefits, such as Sunfire Aegis or Randuin's Omen.

The defensive tree needs to be maxed out on maokai because its important for him to be able to take a lot of punishment. I take 1 point in improving Exhaust because it makes a huge difference in a fight. Making a target more killable is an important part of being a tank. Perseverence in the utility tree works very nicely with Sap Magic

Sapphire Crystal works great on maokai because of how mana hungry he is.
Developing it into a catalyst the protector can help keep you in lane longer. I keep catalyst for as long as possible to help me lane until i need to shop.
Mercury's Treads are, in my opinion, the best boots in the game, especially for tanks. Grabbing boots early increases your ability to harass.
Sunfire Aegis provides great damage. You're gonna be near enemies a lot, initiating with Twisted Advance and keeping a team fight within Vengeful Maelstrom, which I'll go more in depth on how to do that later.
Banshee's Veil is another great item on Maokai. after initiating, your foes will be tempted to use their spells on you. Stopping a huge ability, such as ashe's ult, can really give your team momentum.
At this time, you should be finding yourself in need of mana a lot. Frozen Heart provides this much needed mana, as well as huge armor and a debuff to nearby enemies.
Randuin's Omen is great because you want to be able to debuff you enemies. Landing a slow, especially on multiple targets, gives great chasing and escaping potential
Forces of Nature gives great health regen and a nice speed boost. (Also, seeing as how you're a tree, it only makes sense to get this)

arcane smash Arcane smash
This should be maxed out second, right after sapling toss. this has the lowest cooldown, and, since this isn't an AP build, the AP scaling doesn't matter.
Twisted advance
I start with 1 point in this early to help ganks, but leave it at 1 until the end. The bind isn't that important when you can follow it up with your slow. It just needs to be at 1 so you can use it to start a fight. IMPORTANT: wait until your bind runs out before using arcane smash to get the maximum crowd control effect.
Sapling toss
Your primary harassing tool. Max this out whenever it's available. It's important to try leading with this. When an enemy is running away, you have to throw in front of them in order for both the landing and explosion to hit.
Vengeful maelstrom
Max it out standardly, at lvls 6, 11, and 16. When ulting, you have to remember that all champions are going to be moving. Ult where you want the fight to take place. Remember, you are the tank, and all team fights should start with you initiating. Sometimes, when a fight has already started and your team needs help, ult where they are running to.

Clarity is a must on Maokai. His ult costs a lot to maintain, and if you arent watching it can quickly drain all your mana. He needs this in order to stay relevant in a fight. After his mana is drained, Maokai becomes quite helpless. Also, Maokai has a lot of crowd control, which can determine how a fight goes.

Exhaust is, in my opinion, the best summoner spell in the game. This can make or break a team fight. It not only sets up a slow, but decreases damage from the target. Used correctly, it can keep the fight within your ult longer and help kill the other teams main damage output.

Well, I hope that this guide helps you with your battles. Maokai has great tanking potentials, and, in my opinion, is one of the top tier tanks. Not only is he a great initiater, he's a great supporter as well.