Just because the plant is in a pot doesn't mean it can't fight. This plant seems intent on confronting Thor, if its expression is anything to go by.

It's smaller than Groot. Toothier, too.

This plant is also much less vocal.

The Piranha Plant stays still for a moment, as if sizing up Thor. Then the plant coughs up a spiked ball in Thor's general direction. For something with no eyes, it has some decent aim.