It could be a drainage problem. It sounds like you have the mulch away from the base of the catmint. I've had faint yellowing on a few plants during periods of frequent rain. They recovered just find. I've never had to fertilize catmint. I have around 27 Walkers Low in my garden.

If you've had rain, or watered the plants a lot, make sure they are really in the ground and have not been pushed out and loosened by the water. Just stomp your foot around the sides of the base to make sure they are still completely planted. Air getting to the roots is another possibility.

Cameron

Gosh, I had my Walker's Low in pure clay that was nothing but rocks after the first 4". And it actually seemed to do really well there. Maybe it's just a little transplant shock.

Tracy