If Trundle can't Q tower, neither should Garen

speaking of people that shouldn't be able to Q a tower

He is a carry, not a tank.

You realize Trundle is inherently better at taking towers than Garen, right? Even if you ignore the attack speed buff from Frozen Kingdom.

Just Q a minion first to gain the bonus AD, and you have several seconds to eat the tower faster than Garen ever could.

I can't remember if Garen's Q does extra damage to the tower or not, but it's best used as an auto reset. Trundle can still use his Q as an auto reset actually, he just saves his bite until he attacks anythings but a tower and autoattacks instantly instead.. FYI Trundle used to be able to bite towers but Riot patched that out a few patches ago.