Harvest your cabbages before they split

Cabbage should be "tight as a drum head" when harvested. Don't wait until they split open.

You can test each cabbage to see if it is ready to be harvested by pressing down on the center of the head. There should be no give. If there is, leave it to grow some more.

If you need to leave cabbage in the ground after it is ready to harvest, twist the head 180 degrees. This hopefully will break enough of the vascular system to slow the expansion of the head and reduce the chances of the head splitting open. Your other option is to cut the head off from its main root and store it in the refrigerator.

Waiting even a day after the head is "as tight as a drum", might result in the heads of your cabbage splitting open.