Is it any legal way from the official books, to add a Plant type to Human race, or make a Humanoid Plant, with minimal influence and big LA growing? It may be useful to know how to slightly modify original races. Thanks!
The Adu'ja (Dragon #317, p.24) are a humanoid plant race with a level adjustment of +2. They photosynthesize, have all the benefits of plant type such as immunity to poison and critical hits, and have the ability to regrow missing limbs.
The following prestige classes and effects may also technically fulfil the requirement of a plant type PC without a level adjustment:
The Verdant Lord prestige class (Masters of the Wild p.73) grants plant type at level level 10. The minimum requirements would normally allow this by total level 17. The class gives 10/10 spellcasting progression.
Thrall of Zuggtmoy
The Thrall of Zuggtmoy prestige class (Dragon Magazine #337, p.45) grants plant type at level 10. The minimum requirements would normally allow this by total level 15, and the class gives 9/10ths spellcasting progression.
The Forest Master prestige class (Faiths and Pantheons, p.194) gains the plant type at 7th level. The minimum requirements normally allow this by total level 17.
Garden of Respendent Hues
A location called the garden of resplendent hues (Drow of the Underdark, p.189), can transform a druid or ranger into a plant type, no level adjustment. However, it only has a duration of 30 days.
The 5th level druid spell plant body (Savage Species, p.69; Spell Compendium, p.159) allows a temporary transformation into plant type (10 minutes per caster level).
I don’t believe there is any option for this. I was wrong, see Quadratic Wizard’s answer. There is a woodling creature template in Monster Manual III with LA +3, which is notably not LA +2 or less.
Monster Manual III also has the dread blossom swarm, which is the notoriously overpowered option for the symbiote template from Savage Species, but that doesn’t technically get you the Plant type, and in any event is a clear rules abuse (because dread blossom swarm listed various features of its Plant type and Swarm subtype as separate special qualities, technically symbiote gives those to the host). Anyway, the host of a dread-blossom-swarm-symbiote has LA +1, because three free feats, +5 to a skill, and a bunch of powerful defensive attributes is totally reasonable for LA +1. (A lot of that isn’t even “abusing” the rules so much as abusing the fact that Savage Species was originally written as a separate product line and ham-fistedly shoved into D&D 3.5e when that plan was scrapped, so it just doesn’t have the same standards that the rest of 3.5e does.)
Otherwise I think you’re stuck with wild shape or other shape-changing magic to get any kind of plant-ness. There’s a wood elemental template in Manual of the Planes, but that makes you an Elemental (wood) creature and it’s got LA +5 besides.
Your best bet is probably to tone down the woodling template and use that. It’s not remotely worth LA +3, or even LA +2 really, but it’s certainly better than a lot of other LA +2 templates out there, so if you’re getting a free LA +2 to play with or something, it might be overpowered (but not as overpowered as, say, Book of Exalted Deeds’s saint, which actually is LA +2). Plant comes with a lot of nice immunities, plus woodling gets DR 5/slashing and a bunch of spell-like abilities.