Hi Stacey,
Thank you for your catmint question. In general, catmint does not need to be fertilized. The following publication describes how to care for catmint:
Catmint — A “Must-Have” Perennial | Piedmont Master Gardeners

If your plant is older and large, you may want to consider reinvigorating the plant by dividing it. The recommended time to divide catmint is in the early spring or immediately after the first bloom. Source: How to Divide and Transplant Perennials | Yard and Garden (iastate.edu)

If you have additional questions, I recommend you contact the Plant Clinic at the OSU Deschutes County Extension in Redmond. You can call the phone number listed below or send your photos and question to <personal data hidden>

The person on duty will review your information, conduct necessary research, and contact you with findings or management options.

Summer Hours: Monday through Thursday, 9:00 to 5:00 pm, Friday 9:00 am to 1:00 pm.

Phone:<personal data hidden> x79630

We always welcome your home garden questions at the OSU Plant Clinic.