My first thought was too much water, but you said you water only once or twice a week. Maybe it doesn't even need that. How is the drainage of the soil?

IME Nepetas do well with a hard cut-back just after bloom -- like down to the ground scalping cut-back. Fresh foliage will very quickly emerge, and the plant will then hold that fresh foliage more neatly the rest of the summer without getting all sprawly.

This plant was nearly three times larger last summer. About 10 days ago, I lightly trimmed it after it finished blooming. Now, I’ve noticed some yellow leaves at the base. Should I cut it back all the way to the ground? The area gets watered roughly once a week and is located on a decent slope.

I'm too new to perennials to give advice, but what is your climate like? Do you get summer rain?

My guess would be excess watering. What is your climate? How has the weather been. Also yellow leaves at the base are not necessarily a problem. Leaves don't live forever and the older ones will naturally die off eventually.

We had heavy rain a week ago for a day or two. I am in PNW zone8

Thanks for the responses.

Only other question is should it remain so small compared to year ago

Do you have very dry summers? Nepeta shouldn't need any supplementary water in a climate with regular rainfall. Twice a week is a lot.

Yes. Typically dry summers starting now. This year june has not been so dry.

It is good to know twice a week is a lot.

I hardly ever water my established nepeta and they do fine.

For me they planted in 2024. Next year they may do better