Cabbage plants (Brassica oleracea var. capitata) produce heads of nutrient-rich, leafy greens suitable for fresh eating, cooking or steaming. Gardeners living in cooler climates with long winters may extend their cabbage season by growing this cool-season crop in a greenhouse. Cabbage requires a germination temperature of 75 degrees Fahrenheit and a growing temperature range of 60 to 70 degrees F to mature correctly. Sow greenhouse-grown cabbage seed in the spring, 6 to 8 weeks prior to the last frost date or in the fall, 6 to 8 weeks before the first frost date.
Fill a 4-inch pot to 1 inch below its top with seedling potting soil. Tamp the soil down to remove excess air pockets. Add additional soil to the pot if needed to keep the correct level. Prepare one pot for each cabbage plant you wish to grow.
Attach a water wand to the end of a garden hose. Fill the pot two to three times with water to moisten the soil completely and evenly.
Poke a hole 1/4 to 3/4 inch deep in the center of the pot. Place two to three cabbage seeds into the hole. Fill the hole with soil, patting it down gently over the seeds.
Write the cabbage variety and planting date on a plant tag with a black permanent marker. Insert the tag into the soil against the pot's side. Cover the pot with a clear plastic bag and wrap a rubber band around the sides of the pot over the bag's edges to hold it in place. Position the pot in a location in the greenhouse that receives full sunlight and retains a constant temperature of 75 degrees F.
Open the greenhouse doors and vents as needed when the air temperature rises to 80 degrees F or warmer. Close the doors and vents once the air temperature returns to 75 degrees F.
Check the pot every one to two days for signs of moisture loss. Remove the bag and water the pot slowly once the top 1/4 inch of soil begins to dry. Replace the bag after watering. Never allow the soil to become soggy or to dry out completely. Watch for germination four to seven days after planting.
Remove the bag once the seedlings germinate. Reduce watering to when the top 1 inch of soil becomes dry. Cut through the weakest seedlings' stems at soil level with a pair of pruning snips once the plants develop three to four true leaves, thinning the planting to a single, strong seedling.
Open the greenhouse's doors and vents as needed to keep the air temperature between 60 and 70 degrees F. Close the openings in the late afternoon or early evening, before the sun sets, to allow the greenhouse to build up adequate heat to retain the proper temperature throughout the night.
Mix 1/2 teaspoon of 10-10-10 nitrogen, phosphorous, potassium and 1 gallon water in a watering can. Apply the fertilizer to the seedling every seven to 10 days. Fill the pot full of the solution. Wait for excess to drain out the bottom of the pot. Fill the pot a second time with the solution.
Transplant the cabbage start once it outgrows the 4-inch pot, generally four to six weeks after planting. Fill a pot 18 inches wide by 12 inches deep one-third to one-half full of potting soil. Tamp the potting soil down to settle it. Slide the plant from its pot carefully. Place the root ball in the center of the new pot. Add or remove soil from the pot, if needed, to position the top of the root ball 1 to 2 inches below the pot's top. Fill the pot with soil to the top of the root ball, tamping it down securely around the roots. Do not overfill the pot or bury the cabbage deeper than it was previously growing. Water the pot thoroughly to moisten the soil completely.
Place the pot in full, direct sunlight in the greenhouse. Continue to water the cabbage when the top 1 to 2 inches of soil becomes dry, applying 1 inch of water at each application. Fertilize the plant every seven to 10 days in the same manner as before.