It's very interesting.... i like a lot your idea about "Plant Health" too (but i would change the "destroying plants" concept to "pruning plants" 
). It could be called "Minor Focus: Green Thumb" xD
I love it!
Edit: Yet, the more i think about it, more it seems like a Major Focus... D:
I would define Plant Health likely as a major as well. But I'd define it as healing/harming, curing/diseasing, and growing/withering any plants.
You could take a subset of those, or also restrict the plants it applies to, to bring it down to a minor.
Like ... Tree Health, since her interest in Forests and her Dryad lineage for example.
Or instead of Unnatural Plants, Unnatural Trees perhaps to be spells/effects which create or transform Trees in unnatural ways or give them distinctly unnatural properties.
I would define Plant Health likely as a major as well. But I'd define it as healing/harming, curing/diseasing, and growing/withering any plants.
Yeah...
You could take a subset of those, or also restrict the plants it applies to, to bring it down to a minor.
Like ... Tree Health, since her interest in Forests and her Dryad lineage for example.
That would work, ofc. But the most useful plants to affect with it are the small ones xD But the thematically appropiate ones would be the big ones...
Or instead of Unnatural Plants, Unnatural Trees perhaps to be spells/effects which create or transform Trees in unnatural ways or give them distinctly unnatural properties.
I like how that sounds, but my issue is that "Unnatural" ends being a very vague term. That invariably leads to too many discussions about when it applies and when it doesn't xD
I prefer something more specific... what about Minor Focus: Animate Plants? It cover ReHe, but just partially, since it only covers living plants and just animating them, not making them move like throwing a tree above somebody or like that.
I think with animation as a focus you could go broader than "plants" and say "Animating Plant Matter" so that it would cover things made of wood or plant fibers as well as whole plants etc. and it would still be a Minor, but that is just my opinion. 8D
That would be the icing xD But i would be happy just with my version I will wait for more opinions on the matter.
Well... meanwhile, with what i have the character sheet (just finishing the Gauntlet) would be something like...
Alba Ex Merinita
Age: 25 (17)
Warping: 0
Size: -1
Personality Traits
Sympathy Traits:
Virtues
The Gift (0), Faerie Magic (0), Hermetic Magus (0), Faerie Background (0), Skilled Parens (1), Affinity with Herbam (1), Puissant Herbam (1), Unaging (1), Cautious Sorcerer (1), Good Teacher (1), Minor Magical Focus: Animating Plant Matter (1), Strong Faerie Blood: Dryad (3)
Flaws
Faerie Upbringing (1), Sheltered Upbringing (1), Social Handicap: Expressionless (1), Small Frame (1), Weak Spontaneous Magic (3), Faerie Heritage (3)
17 spell points left
As long as it only covers animation and not direct control (example: walking trees yes, picking up a tree/log with Rego directly, no) then I think it is a sufficient subset of Rego/Muto Herbam?
16 feels... excessively high for a character just out of apprenticeship. 10 before modifiers has always been a good rule of thumb in my experience. Other than that, I like what I see?
Humm... as written that would be essentially the Level 10 guideline of Rego Herbam, but I think that's not your intent (and alo seems too much restrictive to me).
My initial reading put the following spells from the corebook as valid for the focus of Animating Plants (and none of the ReHe spells I'm not mentioning):
If people thinks that Herbam 16 it's too much to start with, I could reduce it to Herbam 12+3 (using just 52 exp).
Then, lowering Vim to 0 for that single exp point, i could buy:
So... as people prefer. I can adapt 
With Animating Plants i was thinking more in line with the Levels 4, 5 and 10 from Rego Herbam. So, those spells that you said, plus: Twist the Living Tree
With Animating Plant Matter it would also include Level 3. So, to the previous list, it would also include spells like:
Humm... I left Twist the Living Tree out because it doesn't animate the tree, it chages it's position for the duration. And I also understand Plant Matter to include the two spells you pointed out. So, it essentially leaves only wards outside of the scope of the focus (I'm including "Repel the Wooden Shafts" as a technical "ward").
That's why I said it covers a lot of Rego Herbam.
Technically... Wards, craftings with rego, flowers/fruits outside the season and any spell that throw wood or plants around (or turns them into projectiles).
Thanks @Arthur! 
That superpower to move messages is very cool xD
Btw, u haven't given your opinion yet... D: What do you think?
I haven't had time to catch up yet. 
I would slow down if i could... but i must win to Nithyn and Vortigern... xD
I'm pretty close I think...
I don't see a problem with Herbam 16. While it does feel a bit excessive, it also denotes an extreme focus which I think, for this character, is valid. I do expect some explanation, however, about why the master didn't force the apprentice to spread the arts more, since a 16 at gauntlet means the magus can do very little outside of his expertise.
However, if my math is not wrong you are about 19xp above the starting for a magus with skilled parens (with 189 points in arts and 130 in abilities. Already considering the first 5xp in Faerie Lore and Second Sight as free).
Re: Herbam 16
I tend to agree with Nithyn about the general level of believable competence at Gauntlet being roughly 10 before modification by virtues. But I will grant readily that is just a rule of thumb and not RAW.
Sufficient investment to hit 16 to me is taking so much of the xp of the character that it almost inevitably means other things on the build are going to be light/low enough that you end up questioning why they are so low.
Of course ... we never feel like we have "enough" xp when we are the ones building something. 8P
Personally I tend to see Art scores in the following scale.
5: Apprentice
10: Journeyman
15: Professional
20: Master
25: Grand Master
To compare, Ability scale roughly analogs to this.
1: Apprentice
3: Journeyman
5: Professional
7: Master
9: Grand Master
So having a 16 in an Art at Gauntlet I would compare to having a 5 in an Ability also at Gauntlet. I won't say it is impossible, but it is a very high level of ability at that point in progression. Having Affinity + Puissant to hit it together makes it much more believable say, not that I'll say it is required in RAW by any means.
Re: Characteristic Consistency
The contrast of Strength -3 and Stamina +3 seems incongruous to me. These both reflect near minimum and near peak levels of human ability in what we would normally consider related physical capacities. The level of physical conditioning I'd expect from someone who has Stamina +3 would seem incompatible with the level of weakness or frailness I'd expect from someone with Strength -3.
Because with Weak Spontaneous Magic she would face the same difficulties having 4 or 6 into an Art than having 0. At least, with 16+3 in Herbam, she can spontcast basic spells with her best Art.
And with a Skilled Parens (that you can understand as a Pedagogic Parents too) plus Affinity, you can gain like 30exp each season. So 16 Herbam is like 3 seasons in total. Using books instead of the parens, means 4 seasons anyway. Not a lot of time.
I mean, if people wants im ok with lowering her Herbam to 10 + bonuses, that ends being 12+3. But then that exp would go into Abilities, no into Arts from which she cannot benefit anyways. Unless she ends in a very new/poor Covenant (like Tugurium xD), who doesn't have a L5Q15 Summa for each Art anyways?
Rlly? D: I did the math like 1.000 times, but i have a superpower to misscalculate a lot of time on the same places... so... xD I will copy here the detailed info and i will redo the math from 0, so i can find the issue and change values:
Abilities
Second Sight (Invisible things) 1
------- First years -------- 45exp + native language
Language: High German (Swabian) 5
Awareness (Smell) 1 -5exp-
Faerie Lore (Faerie forests) 2 -15exp-
Second Sight (Invisible things) 1 -> 2 -10exp-
Survival (Forests) 2 -15exp-
------- Later life (5 years) -------- 75exp
Awareness (Smell) 1 -> 2 -10exp-
Concentration (Spells) 2 15exp
Music (Sing) 2 -15exp-
Second Sight (Invisible things) 1 -> 3 -15exp-
Stealth (Natural areas) 1 -5exp-
Survival (Forests) 2 -> 3 -15exp-
------- Apprenticeship -------- 300exp
Faerie Magic (Charms) 1
Artes Liberales (Geometry) 1 -5exp-
Language: Latín (Hermetic) 4 -50exp-
Magic Theory (Herbam) 4 -50exp-
Organization Lore: Order of Hermes (House Merinita) 1 -5exp-
Parma Magica (Vim) 1 -5exp-
Penetration (Herbam) 2 -15exp-
175 exp remaining for Arts <--- Yeah, i can see that here is something very fishy indeed... xD
Arts
Creo 8 (36exp)
Intellego 2 (3exp)
Muto 4 (10exp)
Rego 9 (45exp)
Herbam 16+3 (91exp) +0.5exp
Ignem 1 (1exp)
Imaginem 2 (3exp)
Vim 1 (1exp)
190 exp here... yeah... 15exp missplaced... it seems like that Penetration (Herbam) 2 shouldn't be there 