I can't speak for Big Jim but it's very very specific. Most expert classes are more specific than base classes but this feels like a much more narrow concept. I think only the Beastmaster comes close and that is a much broader concept than you have here.
I'd disagree -- the Alchemist, from which this class quite clearly grew, is plenty narrow in its focus: elixirs and a small smattering of spells. The Bloodbound does nothing else except take hits for someone else to the point it's insanely potent as a source of abilities for a personal lieutenant. The Dead Eye shoots things a long way away. The Monk and the Swashbuckler are actually relatively alone amongst published classes in possessing any sort of broad utility
Not mechanically, concept wise. You would probably play a Bloodbound as a straight up bodyguard but there's nothing stopping you from saying that your character mystically transfers wounds from his charge to his own body. Same mechanics, different concept. A deadeye can be a steely eyed rifleman or an arcane archer. Even the Alchemist can be a Jeckyl and Hyde type or a healer although the mechanics mean you'll probably end up as a bit of both.
Your guy though is pretty much a plant potion maker though. The mechanics would work perfectly for other concepts that are excluded by requiring the plant type. That's the problem. I can't think of another class that you can change the requirements for and create an entirely new and appropriate character types without altering the mechanics at all.
This is elixirs, poisons, food & healing, plus a useful and ultimately transformative connection to nature that isn't terrain dependant. It's exactly the sort of thing you want out of a backer class -- someone to buff you, and then fix you.
Right, there's no mechanical problem here. It's totally a problem that the concept is much too narrow for the mechanics.
Ok, except for the Venom Master trick that is in the requirements and doesn't have much of a payoff.
Change the type to construct and you have a Living Caudron/Kitchen. Drop the type and the trick and add The Gift for a hedge wizard, Black Cat for a very classic witch, Bandage for the kindly old healer stereotype. BSM: Bluff skill for the Traveling snake oil salesman who has the occasional real trick up his sleeve.
If you dropped the trick and type and added on say, Yeoman's Work, you could accommodate all of those character types without the mechanics or losing flavor.
It almost feels like a class for a single character.
Well, yes, you could argue that Poison Ivy is a very strong design influence. But I disagree with that single character tag, just because I'm choosing to focus on something other than vanilla folk most people tnd to opt for.
Mechanically, it's good. But if you dropped the plant type, I could see this being an Unborn who's essentially an animated cauldron/kitchen, the "kindly old lady" witch, a hedge wizard, or just an alternative take on the alchemist.
Well, yes, you could do that. But the idea behind it is the fantastical comic book possibilities of the biology of the plant type.
The abilities don't feel particularly plant-like. That may be unfair of me considering how easy it is to change fluff and my own willingness to ignore fluff on anything if it fits mechanically.
The thing is, this is a really cool alternate take on an alchemist or hedge wizard type that is unfortunately limited by its type requirement.
For a modern campaign that never took place, I came up with the idea of Rootwalkers using chemical fertilizers as recreational drugs, leading to embarrassment when they start to fruit.
Heh, that's quite funny.
Thanks. My group would go straight for the toilet humor though so it's best it's never been used.
Without commenting on the class, I have to say I'd be interested to see a Legendary Rootwalker master class

.
Challenge accepted (see 1st post).
Thoughts on the two in comparison?
Ok, this feels plant specific. I almost think you could have multiple master classes based around different aspects of ents and such creatures.
The ability names are very obscure. I got that the level two was norse but the level four and five ones were unknown to me. I know, this is rich coming from me.
Why is Wisdom more important than Constitution to them?
What does the level 5 ability do? Do you mean you gain the bullet pointed breeds or do you gain individual benefits from those breeds in a mix-and-match fashion?
Other than that, looks good to me. I'd love to see an alternate wrath of nature take on a rootwalker specific master class and may do one myself though I'm not that great with classes.