Blooming Elm

Blooming Elm
BuildingTreeCost: 3ATK: HP: 4
Your units and heroes with +1/+1 runes have overpower.
⤵ → Put three +1/+1 runes on a unit or one on a hero if that unit or hero doesn't have any +1/+1 runes.
GreenGrowth Tech II

Card-Specific Rulings
You can target a unit or a hero which already have +1/+1 rune(s). It just won't add more runes. — sharpo
Overpower Rulings
The excess combat damage from the first target can go to only one additional target. It can't cascade beyond that to hit even more targets. — Sirlin
The "excess" combat damage is the damage beyond the remaining HP of the thing you attacked. It doesn't matter if the thing actually dies or not, the damage beyond its HP is still counted as "excess." For example, if you attack a 1/1 indestructible unit such as Hardened Mox with a 3/4 overpower unit such as Centaur, there is 2 excess damage even though the Mox doesn't die. That 2 excess damage can hit something else that that Centaur could have legally attacked. — Sirlin
When determining what an overpower attacker "could have attacked" you still obey all the normal rules of attacking. If there are any patrollers other than the one actually attacked that would have been possible for the overpower attacker to attack, the excess combat damage has to go to one of them rather than say, a tech building. If there aren't any other patrollers that the overpower unit or hero could have attacked, then the excess damage can go to anything with HP that's controlled by the player (or team in 2v2) whose patroller was attacked. — Sirlin
Overpower does nothing when you attack a non-patroller. It also does nothing on defense (when your overpower unit or hero gets attacked). — Sirlin
Overpower damage still counts as combat damage (it can kill Gilded Glaxx for example) and it counts as an ability (so something that has overpower cannot get +2/+2 from Midori's middle ability). — Sirlin
Overpower does not stack. Having two instances of overpower is the same as having just one. — sharpo