Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
So basically all you need for a good Maokai is a lot of AP and quite a bit of health (the health is for his passive).
As far as runes go, I like to start with all mark and quints as straight up AP. This gives you a bout 20 or so AP off the bat. The rest of the runes are Glyph cool down, and mana regen seals.
I prefer to start with amplifying tome to give extra AP. I also pick up a mana potion (Maokai is mana hungry). As soon as I have the cash, I work my amplifying tome into a Will of the Ancients (yes, I'm aware Maokai's passive gives him life back, but this has saved me multiple times). I then follow up with a pair of Sorcerer's shoes. Right after the boots, I get a Crystal Scepter for the health and slow. Then comes a Deathcap. Then a void staff. And finally either a Rod of Ages or a Zhona's, depending on how the game is going.
You can pick up any kind of spells. I like to take a clarity because I spam my sapling toss, and depending on the game, a heal or a ghost.