This is interesting to me because I have quite a few Walker's Low and see little seeding. Mine are grown in poor, dry soil and I cut them down in fall. They are not mulched.

All cultivars of Nepeta x faassenii are reported to be sterile however a couple of sources state "predominantly sterile", so I guess there is some room for flexibility :-)

Thank you Cecily and Gardengal. Mine are like Cecily’s…they are 5 years old and never did it before. i did prune them quite early this spring before blooming…hoping to delay the bloom and keep them from sprawling. (Like Chelsea chop?) They are just blooming now. Maybe I caused the problem and should have just waited to prune after they bloomed?

Can i spray them with something? Like a grass killer that wouldnt kill the plant?

Mine seed, too. I pull the little offenders, or let them grow up a bit if I have a spot for a new one.

A "grass killer" is unlikely to have any effect on Nepeta seedlings. They are not a grass :-)

Just use a cultivator or hula hoe to disturb the seedlings and expose their roots. That will kill them without the need for chemicals.

Pruning or not would have no effect on seed production either. It sounds like just a rare, random occurrence.