From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-156756-425940
Question: I would like to start seeds for some ornamental cabbage for
fall planting. When should I start the seeds? Do I need to pinch back
the seedlings to get that nice, compact head?
Answer: Ornamental kale and cabbage grow best in cool weather and
generally take about 60 days to mature. To avoid stressing the
seedlings during the hottest days of summer, wait until 4 to 6 weeks
before your average first frost date to sow seed. You don't need to
pinch back the seedlings. The plants should form a nice head on their
own.