Catnip does not need fertilizer nor be kept in constantly moist soil.

FataMorgana

Perhaps take the plant out of the pot and examine the roots.. maybe it has become too crowded down there.... It sounds like this one might have been in the pot for over a year and the rhizomes might need some maintenance.

Actually, my catnip DOES need to be in constantly moist soil (though this wasn't the problem here). In any of my pots of catnip, after the top of the soil is dry for a day or two, the whole plant wilts. You water it, and it perks right back up. In my experience catnip (and I think this is really true of any mint) is one of the more water hungry plants I've ever grown.

This particular pot (a few cubic feet) has been lush for several years. Roots are too crowded? That's not going to make the whole plant die.

Interesting that it is the outside one that has collapsed. Do you have a cat? Is it a tom? Could a neighbour's cat have sprayed it? After cutting back the smell might have been stronger and more attractive.

Difference in locations. Here, catnip grows in my drier locations. But then I'm Western NY State as opposed to central Texas. Containers generally bump up the heat zone in the summer so I would think you are growing catnip in a hotter location than generally it likes. Sadly, I couldn't find a heat zone for catnip to verify. But you need to give it so much water and I never water it because of where we live not that this plant is a water piggie or a xeriscapic queen.

FataMorgana

That's a good point about what a cat could have done to it. We don't have outdoor cats, but there are several around that go nosing around in this plant. They nuzzle it and even roll on it. But the vegetation on this plant was two feet across. It didn't look like it got rolled on. So if it's about spraying, that's a LOT of spraying!

There was one healthy stem left out of ten. I ripped out those ten, and we'll see what happens with the healthy one. That in itself is pretty odd. You'd think a disease would just kill the whole thing. What gives when one stem comes through unscathed?

Yes, that's very true that containers in a hot climate make any plant thirsty. But even my catnip plants inside do the same thing. They want to be wet.

I folded my one long healthy catnip stem over and buried half of it. It's now thriving. I'll have a full pot again pretty soon.