what does it mean to have a catch on a non-toughness power, from a game system standpoint? What does it mean that the catch for his strength powers are reeds?
In this particular case, it would mean that he couldn't break reed bindings (and possibly hemp rope) because they would actively weaken him, sapping all his strength (mechanically speaking, his Might would be reduced to Terrible (-2)). I wasn't quite satisfied with this method, though.
OTOH, I hadn't considered just making it a part of his High Concept, which seems like it would be a better solution.