I've direct sown with no chilling mistuba with success. Any failures I had with it have always been with older seed. I found the plant preferred the cooler, partial shade locations in the lee of larger plants -- and this is in my already relatively cool and wet climate.

FataMorgana

I'm in zone 6b and successfully grew mitsuba (from Kitazawa Seed company) from seed directly sown into a pot left in partial shade. I also have shiso (perilla) that has self-seeded (orginally from a seed packet from a Japanese grocery) in the garden. It comes up very late - around August - but then takes off. Now that I know that I will try to collect seeds this fall to start it earlier inside. Thanks for the reminder..