Firstly, not to just shamelessly override your thread, feel free to take further plant-related/jungle-related ideas from Melthik and my Zyra rework Concept, as well. They're free-game, and particularly, you seem to enjoy traps. :P
As far as this concept goes, it's mechanically pleasing...but statistically lacking. I know you don't necessarily have ratios or base stats up, so I'm not going to be immediately nitpicky over your overall math, but what you have so far in terms of both damage and healing seems lackluster. I do always say that I'd much rather buff something than nerf it, as far as concepts are concerned, so don't take that as me berating you. I'd much rather tell someone to step up their concept than tell them to quit god-modding. xD
Let's see...where to begin...your damage is very insufficient, even by support standards, and can be improved significantly. You area control is pretty decent for a support, and a lot of it comes with the tradeoff of taking its sweet ass time before activating over a significant area, so I don't find it overall lacking or overtuned in that regard. Your healing is sort of all over the place...on your ult, it's stupid weak. On your E, it's a little strong when paired with the shield. And on your passive, it's pretty OP, but with the tradeoff of being so incredibly inconsistent that you either bait people into activating your starflies and healing stupid amounts all at once, or not getting any healing at all to the point they're totally irrelevant.
Let's try this, as far as my analytical suggestions:
Passive: A Light In The Dark: After standing in a bush for 3s, Eirvin releases a swarm of Starflies into the bush. Starflies last for 60/90/120s (upgrades lvl 1/7/13) before returning to Eirvin. If an enemy champion walks into the bush, the Starflies swarm the intruder, dealing 15/25/35/45/55(+7%AP) damage each second for 5s, and revealing the intruder for the duration. If an ally (not yourself) enters the bush, they instead are healed for 10/25/40/55/70(+7%AP) hp each second over 5s. If they remain for the entire duration, they speed back to Eirvin, healing him for 5/7.5/10%(+1%/150AP) of his max hp over 5s (60s cd...meaning you can have two out at once later on, and a small overlap in the mid-game) This gives you a "Bard Shrine" heal for your teammate that's consistent, as well as vision control and a damage trap/reveal on enemies who mess with it. Fancy little free spell.
Q: Natural Sanctum: (I want to rework this a bit as a primary healing spell, since I added damage ratios to the passive, but I don't want to add too much healing to this kit, so I'm making all of these HoT effects to balance them out.) Eirvin creates a natural blessing to an area for 4s, granting allies within it 10/15/20/25/30(+1per50 AR/MR) armor and magic resist and healing them for 20/35/50/65/80(+15%AP) HP each second they're within the area. (Area is about the size of a Morgana pool...this gives decent resistances and healing while you're in the area, but it also opens the counterplay for "hooker" champions to dislocate you and knock your brains out, so it's a double edge.)
W: Starfly Swarm: 4s Duration. Remove healing ratio, give shield a value of 55/75/95/115/135(+55%AP) and keep the damage-on-hit, with a ratio of 15/25/35/45/55(+15%AP) per hit...basic attack only - don't let this count for abilities. The Starflies deal double damage to minions and monsters (so this is good for jungle clears). Mana Cost: 60/70/80/90/100 mana. This is a pretty potent ability that both shields and damages, and with the significant buff to the shield strength, this is actually pretty consistent with other abilities with similar strength (like morg W, except more about preventing damage than shredding people, and since you can still use it on yourself, it's still perfectly relevant in many scenarios.)
E: Amber Trap: Leave this one be - I appreciate the devotion to dealing no damage but having a significant, game changing cc, so imo, this is fine. Make it a constant 50 mana, as this does no damage, and you're just upgrading the cd and root duration. I would however, allow 5% damage from all sources on this ability - it's not much, but it allows you to still contribute without making this ludicrously overpowered.
R: Enchanted Overgrowth: (I want to let you test a mechanic I was going to use for an upcoming concept, cause I think it'll thematically and aesthetically fit here) Eirvin channels for 1s, and creates a large area of enchanted brush that creeps out from around him over 1.5s. Enemies who connect with the zone are rooted for 1s and take 100/170/240(+75%AP) damage, and the zone slows enemies by an increasing amount as it expands, starting a 20% and creeping its way to 80% after 4s.
These are all suggestions, and they're all mostly inspired by your hard work on this. It's featuring a lot of over-time effects, which aren't really in the meta right now, but it's the best way to stack all of these heals on top of each other without making them too overbearing, and offering counterplay (grievous wounds and burst damage don't care about HoT effects...Adaptive Helm cares not for repetitive/DoT effects)