I would definitely clear more of it away so the dahlias can come up, and assuming this plant 'nepeta' is fairly short, it will be fine sprawling under as the dahlias grow.

Most likely there will be fierce competition for available water and nutrients, though, which can invite all sorts of bug infestations if you don't watch it closely and avoid stressed conditions.

IME Nepeta do well with lots of heat, lean soil and need very little water.

then they stay reasonable compact but still spread.

but if you combine them with Dahlias and keep those happy and flowering, the Nepeta will probably sprawl as they are completely overfed.

I would wack them back now (they probably regrow quickly) to give the dahlias room and plant the tubers in another place next season.

do you happen to know what kind of Nepeta species you have?

Thanks, and I will take the advice of both of you. It's also a heads up not to extend the nepeta to the rose border, which I was planning to do. Even though it's a classic combination..