Whoa. Um, this is way too good. At 1 mana you can have Wild Growth or Llanowar Elves; to get you need 3 mana, either upfront on Greenweaver Druid or Overgrowth, or spread over 2 turns on Joraga Treespeaker. ETBT and 0 power have no effect at all in reducing the absurdity of this costing 1 mana.

Is reasonable?

It's still boundary-pushing. It'll be format-defining in Block and Standard, and probably spawn a draft archetype. It should also probably be uncommon. But if you're okay with all that, yeah, it could be reasonable.

Powerful; but as long as there's some nice cheap creature kill (I think Death Spark would be perfect) might just about stay under control.

But having, say, two of these in an opening hand would be "grin and win" methinks.

I don't know. It's a close call. It would certainly be used. It's probably better than Sekura Tribe Elder, and that card is pretty damn good... but the comparison isn't perfect.

That said, I'm going to take off my 'just another submitter on Multiverse' hat, and put on my 'game reviewer' hat for a moment. If I come off as rude, please remember, I'm trying to be helpful. I'm secretly a nice guy. ;)

From a marketing standpoint, the fact that this comes into play tapped is a really bad idea. I understand it's there as a limiting factor so that this card can push the envelope for green mana acceleration. But the only time that's going to matter is when this card is drawn late in the game, can't chump block for a turn, and makes you lose. This card, right now, is a "Always play four in block" card with a feel bad moment. That's a bad combination, since it makes players feel trapped, forced to play with a good card that will sometimes make them lose. Too many cards like that, and players stop playing.