The leaves of the cabbage are essential for the size and quality of the heads. They make chemicals that keep the heads fresh longer, so leave them on unless you need to take them off. This is very important for cauliflower, which can have 30% less yield if you take off even one leaf.
The only time you should take off the leaves is when they are partly rotten. This can happen when there is too much rain and the leaves start to rot. You can save the cabbage by cutting off the rotten parts.
When it is too sunny and dry, the cabbage covers its head with two or three leaves to protect it from the sun. If this lasts for a long time, the leaves will dry out, but you should wait until you pick the cabbage and prepare it for storage before you take them off. The dry leaves stop fungus from infecting the cabbage, so leave them on while the cabbage is still in the garden.
Sometimes, gardeners take off leaves that are damaged but still juicy, but this is wrong. The juice that comes out of the cuts draws in bugs and germs that can enter the plant through the cuts. By taking off the leaves, you make the cabbage more exposed to pests and diseases. Also, each leaf adds weight and nutrients to the head because of photosynthesis.
