Bok choy, a fast-growing Asian green, thrives in the cool temperatures of spring and fall. Starting seeds indoors maximizes the short, mild growing window before summer heat arrives. This head start ensures plants mature before high temperatures trigger bolting—the premature production of flowers and seeds. The goal is to produce robust, transplant-ready seedlings that quickly establish themselves for an early harvest.
Calculating the Ideal Indoor Sowing Date
The precise moment to begin bok choy seeds indoors is determined by referencing the local average Last Spring Frost Date (LFD). This date serves as the primary calendar marker for spring gardening activities. The optimal window for sowing is generally four to six weeks before the LFD, giving seedlings adequate time to develop before moving outside.
Consulting local agricultural extension offices or reliable online resources provides the specific LFD for your region. This four-to-six-week period allows bok choy to reach three to four true leaves, indicating the necessary root structure for successful transplantation. Starting too early risks seedlings becoming root-bound before the outdoor weather is suitable. Starting too late may expose young plants to high temperatures that cause bolting.
For a fall harvest, the indoor sowing timeline is calculated backward from the average First Fall Frost Date. Bok choy requires 45 to 60 days to reach full maturity. Seeds should be started indoors or direct-sown approximately 50 days before the first expected frost date. This timing ensures plants mature during the cooler late-summer and autumn weather, minimizing the risk of heat-induced stress and bolting.
Creating the Optimal Indoor Nursery Environment
Once the ideal sowing date is established, focus on creating the specific environmental conditions bok choy seeds require. Seeds should be sown about a quarter-inch deep in shallow containers filled with a sterile seed-starting mix. This mix provides a fine, disease-free medium for young roots. Using individual cells or soil blocks helps prevent root disturbance during transplanting, which reduces shock in brassicas.
For optimal germination, maintain the soil temperature around 70°F, encouraging quick sprouting within five to ten days. As soon as seeds sprout, lower the ambient temperature to a cooler range, ideally between 60°F and 65°F. Maintaining this cooler temperature after germination mimics bok choy’s preferred outdoor conditions. This helps establish stocky, strong growth instead of soft, weak stems.
Supplemental light is required for indoor growing to prevent etiolation, where seedlings stretch excessively toward a light source, resulting in weak, “leggy” plants. Seedlings require 14 to 16 hours of light daily, best provided by fluorescent or LED grow lights positioned a few inches above the plants. Consistent moisture is maintained by watering from the bottom. This technique prevents “damping off,” a fungal disease that destroys young seedlings at the soil line.
Preparing Seedlings for the Garden
The final stage involves transitioning young bok choy from the controlled environment to variable outdoor conditions through hardening off. This procedure prevents transplant shock by gradually exposing seedlings to direct sunlight, wind, and cooler temperatures over seven to ten days. On the first day, place the plants in a sheltered, shaded outdoor location for only one or two hours, then bring them back inside.
The duration of outdoor exposure is extended each subsequent day, slowly increasing the amount of time the plants spend in direct sun and wind. This gradual acclimation strengthens the plant cell walls and thickens the cuticle layer on the leaves, making them resilient to the harsh garden environment. Hardening off is especially important for bok choy because temperature fluctuations can trigger bolting, so the plants must be physically prepared for the outside world.
Transplanting into the garden can occur when the seedlings have developed three to four true leaves and when the outdoor soil temperature has reached at least 50°F. This is typically a couple of weeks before or right around the average last spring frost date, allowing the cool-weather crop to take full advantage of the mild spring weather. Seedlings should be planted at the same depth as they were in their containers, spaced six to twelve inches apart, and immediately watered well to settle the soil around the roots.