When can I sow kale seeds directly in ground in zone 6?
3-5 weeks before the last frost date, which for zone 6 is April 15th. Anytime between March 11th-25th should work. You can also wintersow kale seeds in milk jugs which may give you stronger, maybe even earlier plants. I'm trying it both ways this year for a fun experiment!
Here is a link that might be useful: How to Winter Sow Seeds