They probably are not frost hardy but usually they sprout pretty quickly as the ground warms up. A blanket on cold nights probably would get them through.

If you are talking about a low of 35 degrees, you might get by. Anything below 32 degrees would kill them. Self-sown Salvia coccinea seeds germinate in zone 7 around late May, well after late frost.

If you have bottom heat, they might get by in a cold frame.

Thank you for the feedback. I was afraid that they needed to be treated more like, say, tomatoes than cole crops.

Tim