Brussels sprouts, a member of the Brassica oleracea family, are a long-season, cool-weather crop that rewards the gardener with a harvest that often improves after the first frost. Starting these plants from seed is the most reliable method for home cultivation, allowing for the selection of specific varieties and ensuring a healthy start. Successfully growing Brussels sprouts begins with a precise understanding of their initial requirements, particularly the depth at which the seeds are planted.
Optimal Timing and Environment for Seed Starting
Brussels sprouts require an extended growing period, typically 90 to 110 days from transplanting to maturity, which necessitates starting the seeds indoors in most regions. The optimal time to sow seeds indoors is generally four to six weeks before the last expected spring frost date, or in mid-summer for a fall harvest. This timing allows the seedlings to establish robust growth before being moved outside.
The seeds should be sown in seed trays or small pots filled with a sterile, soilless seed-starting mix. Placing the seed trays under bright grow lights immediately after sowing prevents the emerging seedlings from becoming weak and elongated, a condition known as “legginess.” Proper air circulation, often achieved with a small fan, helps to strengthen the young stems and reduce the risk of dampening-off disease.
The Specific Planting Depth
The recommended planting depth for Brussels sprout seeds is consistently shallow, ranging from one-quarter to one-half inch deep. This narrow depth range balances providing enough soil cover to maintain moisture and allowing the emerging seedling to push through the soil surface easily. A depth of one-quarter inch is often preferred for optimal emergence.
To achieve this precision, use a pencil tip or a small dibbler to create a shallow depression in the prepared seed-starting mix. The seed is placed into the depression, and the hole is then gently covered with the seed mix. Lightly tamping the soil surface ensures firm seed-to-soil contact, which is necessary for the seed to absorb water and begin the germination process.
Ensuring Successful Germination
After planting, the focus shifts to maintaining the specific environmental conditions that trigger germination. The ideal soil temperature range for successful germination is between 60°F and 70°F. This cooler temperature requirement means that a heat mat may not be necessary and could be detrimental if it causes the soil to become too warm.
Consistent moisture is equally important; the soil mix must be kept evenly moist but never completely saturated or waterlogged. Over-saturation can lead to fungal issues and prevent the developing embryo from accessing the necessary oxygen. Under ideal moisture and temperature conditions, the sprout should emerge within four to ten days after planting.
Moving Seedlings Outdoors
Once the young plants have developed their first set of true leaves and reached a height of about four to six inches, they are ready to be moved outside. Before transplanting, the seedlings must undergo a process called “hardening off,” which gradually acclimates them to the harsher outdoor conditions of wind, direct sun, and cooler temperatures. This process involves moving the seedlings outside for increasingly longer periods over about a week.
The final spacing in the garden is important for the mature plant’s development, as Brussels sprouts require significant room for airflow and full sun exposure. Seedlings should be transplanted into the garden bed with a final spacing of 18 to 24 inches apart between plants. When planting, the seedling should be set into the ground at the same depth it was growing in its container to minimize transplant shock.