Your Companion won't get to level 17
To quote the Companion Familiar feature of Arcane Hierophant:
You add your arcane hierophant class level to your druid or ranger level for purposes of determining your animal companion's bonus Hit Dice, natural armor adjustment, and Strength/Dexterity adjustment (see the sidebar The Druid's Animal Companion, page 36 of the Player's Handbook). For example, a [11th level] character who is a 4th-level druid/3rd-level wizard/4th-level arcane hierophant has the animal companion of an 8th-level druid[.]
Of course, there is the Natural Bond feat, which would fix things:
Add three to your effective druid level for the purpose of determining the bonus Hit Dice, extra tricks, special abilities, and other bonuses that your animal companion receives[.]
However, you'd have to take it early on, before your Animal Companion becomes a Companion Familiar.
Your Companion doesn't get 17 abilities from Natural Selection
The abilities gained through Natural Selection are not cleanly gained one per level:
Each new ability has a delay in levels between when you select it and when your plant companion gains it. You may select a new ability for your plant companion at any druid or ranger level in which a previously selected ability does not manifest. Your plant companion can only evolve one new ability at a time.
This means that if you select Roots at level 1:
Roots (Ex): Your plant companion gains fast healing 1. Your plant companion gains this ability after 3 levels.
The Roots ability will manifest at level 4 (1+3) and you will be able to pick Alacrity at level 5, which will then manifest at level 7. No ability takes less than one level to manifest, meaning that at maximum, you could select one ability every odd Companion level and have it manifest every even level.
You missed the Familiar abilities
A Companion Familiar gets the abilities of a Familiar, in addition to those of an Animal Companion:
In addition, your animal companion (if any) gains many of the abilities that a familiar would normally possess. You add your arcane hierophant class level to your arcane spellcasting class level, and determine the Intelligence bonus and special abilities of your animal companion accordingly (see the sidebar Familiars, page 53 of the Player's Handbook).
By level 17, as a level 3 Wizard and a level 10 Arcane Hierophant, that would give your Companion the Familiar special abilities of a level 13 Wizard.
Your Companion won't be anywhere near a treant
With fewer abilities and lower level, the combat ability will be nowhere near a treant, in particular if you "waste" any on defensive or healing abilities, e.g. Roots. However, you do get the spell Changestaff at level 16:
You change a specially prepared quarterstaff into a Huge treantlike creature, about 24 feet tall. When you plant the end of the staff in the ground and speak a special command to conclude the casting of the spell, your staff turns into a creature that looks and fights just like a treant.
To quote Eggynack's excellent Druid handbook:
A core idea underlying plant companions is that they’re really bad, at least as compared with animal companions, at beating face. [...]
[...]
However, that’s not to say that plant companions are useless. Where they do quite a bit better is in providing you some utility. Big all stars here are distracting pollen, which lets the companion let out a cloud of red dust in a 10 ft. radius for 1d4 rounds that grants partial concealment, blindsight out to 30 feet, which can help you find hiding enemies thus allowing the party to target them, tremorsense out to 60 feet, which is similar, but mostly weaker and somewhat cheaper, and bonus feat, which can allow feat acquisition at a faster rate than normal companions get them, for all the occasional utility that provides.
[...]
There might also be something like a chain tripping style build here, using growth spurt and tendrils to get some serious reach and a lot of feats to turn that reach into battlefield control, but the lack of intelligence makes that final step difficult, both because getting an unintelligent creature to trip something non-automatically is a tricky space in general, and because both improved trip and knock-down require intelligence to take them. All that reach must be good for something though.
I'll echo that last paragraph, taking advantage of the massive reach possible for a Plant Companion should be possible, but my imagination fails me as Eggynack's did her. Perhaps going for the Domino Rush ability of the Shock Trooper feat could be an option?
Non-companion issue: You never get Wild Shape
Arcane Hierophant only improves existing ability to Wild Shape, it doesn't give you the ability:
If you do not already possess the ability, you gain no new ability to wild shape.
A fifth level of Druid before becoming an Arcane Hierophant solves that, of course, though it loses you one level of Arcane Hierophant, which means one level of Wizard spellcasting and Familiar abilities.
