Cabbage seeds should typically be started indoors in the spring, about five to seven weeks before the last frost date predicted for your area. Plants started in April or May can be moved outside in about four to six weeks. Don't worry if the weather's still a little cool, because cabbage is a cool-weather crop that can tolerate a light frost. Starting seeds indoors gives them a good head start before being moved outside and tends to strengthen the young seedlings to the point of resisting damping off. These seedlings also mature earlier than plants started directly in the garden. Your cabbage will mature in about 40 to 95 days, depending upon variety.
Fill the cells of a plastic seed-starting six-pack to within 1/4 inch of the top with a commercial seed-starting potting mix. Set the pack in a shallow tray of warm water. Remove it from the water when the soil surface feels moist. Allow it to drain for a couple of hours. The soil should be evenly moist, but not soggy or wet.
Press a single cabbage seed about 1/4 inch deep into the soil of each cell. Most seeds should be planted about four times deeper than their widths. Cover with moist soil, firming gently with your fingertip.
Close the seed pack up in a clear plastic bag to conserve moisture. Set it in a brightly lit spot out of direct sun with a temperature of about 65 to 70 degrees F. The top of your refrigerator or above a hot water heater are good locations. Your cabbage seeds will germinate faster and more dependably when supplied with bottom heat. They'll sprout in about 10 to 14 days.
Check the seed pack every couple of days to make sure that the soil surface remains evenly moist. Seeds shouldn't be allowed to dry out.
Remove the plastic bag when all the cabbage seeds have germinated. Move the pack to a brightly lit but cooler spot, around 50 to 60 degrees F. Keep the soil evenly moist.
Transplant the cabbage seedlings outside as soon as they have three real leaves. This may be as early as a couple of weeks before the last expected frost. If daytime temperatures consistently reach a high of 50 degrees F, your cabbage plants will be fine. They'll thrive best with temperatures between 50 and 60 degrees F.