Cabbage is a cool-weather crop, and its successful cultivation depends on precise timing relative to seasonal temperatures. The goal is to allow the plant to form a firm, mature head during moderate temperatures, ideally between 45°F and 75°F. Planting too late in spring causes the plant to “bolt,” or prematurely flower and turn bitter, due to excessive heat. Conversely, planting too late in the fall prevents the head from forming fully before a hard freeze. Starting seeds indoors ensures the seedlings are ready to transition to the garden at the optimal moment for growth and harvest.
Timing for a Spring Cabbage Crop
The process for a spring harvest begins indoors several weeks before the last expected spring frost date. To give the seedlings a necessary head start, sow seeds indoors approximately four to eight weeks before you plan to transplant them to the garden. This early start allows the plants to mature and form heads before the intense heat of summer arrives, which can otherwise cause the heads to split or the plant to bolt.
The soil temperature in seed starting trays should be maintained around 70°F to 75°F. Sow seeds lightly, burying them about 1/4 to 1/2 inch deep in a sterile seed-starting mix. Once the seedlings have emerged, the air temperature should be reduced to 60°F to 65°F, which encourages stocky, strong growth rather than tall, spindly seedlings.
After germination, the seedlings require ample light, typically 14 to 16 hours daily, often best achieved with a dedicated grow light positioned just a few inches above the plants. Keeping the growing medium consistently moist, but never waterlogged, is important during this initial phase to support the rapid development of the root system.
Timing for a Fall and Winter Cabbage Crop
A fall or winter crop is easier to time because the plants mature in gradually cooling temperatures, which enhances their flavor and sweetness. The correct planting date is calculated by counting backward from the expected date of the first heavy fall frost in your region. This calculation must include the cabbage variety’s days to maturity (typically 60 to 100 days), plus an additional four to six weeks for the seedling stage.
For a fall harvest, seeds are sown directly into the garden during mid-to-late summer, around 10 to 12 weeks before the first frost date. If mid-summer temperatures are consistently high, starting the seeds indoors for the first four weeks is recommended to protect the seedlings from heat stress. High heat can slow growth and reduce the quality of the mature heads.
If you opt for direct sowing, manage the high summer heat with consistent moisture and potentially a light shade cloth until temperatures moderate. For a winter harvest in milder climates, planting can be extended into late summer or early fall, allowing plants to establish before the coldest part of the season. Mature cabbage heads can withstand light frosts, which further sweetens the leaves.
Moving Seedlings to the Garden
Before any indoor-started seedling can be planted outside, it must undergo “hardening off,” a gradual acclimatization to the outdoor environment. This transition prevents transplant shock by exposing the plants to direct sunlight, wind, and temperature fluctuations over seven to ten days. Skipping this step can result in scorched leaves, stunted growth, or even the death of the young plant.
Begin hardening off by placing the trays in a shady, protected location outdoors for two to three hours on the first day. Over the following week, gradually increase the duration of their outdoor stay and the amount of direct sunlight they receive. Bring the seedlings back inside if strong winds, heavy rain, or temperatures below 45°F are forecasted, as young cabbage plants are more sensitive to cold than mature ones.
Transplanting occurs when the seedlings have developed four to six true leaves and are approximately four to six weeks old. When planting into the garden bed, set the transplants at the same depth they were growing in their containers, or slightly deeper, burying a small portion of the stem for stability. Space the plants 12 to 24 inches apart, depending on the variety’s mature size, to allow for proper head development and air circulation.