Modern varieties of cabbage normally rot before they flower. The older varieties the head will crack open before the seed stalk comes up through the head. You can harvest anytime the head feels hard when given a gentle squeeze.

Hi michelelc,
Farmerdill has it dialed in, but for us newbies, that thing could start bolting at any minute (because of the heat/lateness etc etc) , If you want to eat some of your cabbage, I would pick NOW, just to be on the safe side, this year sucked for me with my cabbage harvest , I lost 1500 pounds (though it made awesome compost materials), due to bolting/not completely heading up. Here are a couple of pics to show what I mean, at first it looked like it was going to head up, but alas, it bolted ;-(

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Every bed in the background is about to be tossed onto the compost pile ;-(

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What it should look like:

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I go by how tight the head feels. When its as tight as one you squeeze at the grocery store, then its done and ready to pick. I generally pull some early anyway, just to space the harvest out over several weeks. If you need to leave some in the garden and don't want to risk the head splitting, you can use a shovel and cut about half the roots so it quits growing.

It s reassuring to me that not everything that John Hughes grows is the perfect vegetable. Mine havent headed up yet, and I dont know if they will make it, we have some 90 degree weather coming this weekend.

Jon: the cabbages in your second photo look like a pointy head type. If so that is the way they head. heads are samll and looser than a round head cabbage. Good boiling cabbage, not very good for slaw and related uses.
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See michelelc,
What did I tell you about Farmerdill, he's the man ;-)
That is Exactly what happened, apparently I could have harvested them ,but I kept waiting for them to get "better", well...... live and learn, Thank You Farmerdill ;-)

I agree, thanks Farmerdill and everyone else. All the knowledge gained on this site is awesome! I plan on picking my cabbage tomorrow. It won't get hot until the weekend. I've only got 2, and only 1 is ready. I picked my cauliflower this morning and will have it soon for dinner!