They look very healthy and nice to me. Be patient ! It will take a while. Not the very first leaves start forming head. They need some open leaves to get light, oxygen, ..

much too early. call back in 6 weeks.

Thanks!

I'm a first time winter gardener and I've never grown cabbages before.

Would it be ok to harvest some of the bigger outer leaves to eat, while I'm waiting for the heads to form?

Do those leaves taste ok in anyone's experience?

I have eaten them before heading. They are fine, even raw in salad.

Would it be ok to harvest some of the bigger outer leaves to eat, while I'm waiting for the heads to form?
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Sure thing. you can. They might be even tastier.

Same thing that happened to me in the spring. They kept getting bigger and bigger, and I was just about ready to give up on them heading. All of a sudden, heads!

Sure. You can harvest some of the outer leaves, but maybe just a couple from each plant at a time. Those large outer leaves create lots of surface area for photosynthesis.