Kohlrabi, a member of the brassica family, is a cool-season vegetable prized for its swollen, bulbous stem that grows above the soil line. Cultivating a successful spring crop requires timing the growth cycle precisely to avoid the plant’s natural tendency to bolt, or prematurely flower, when exposed to summer heat. Starting the seeds indoors provides the necessary head start, allowing the plant to develop its edible stem before temperatures consistently exceed its preferred cool range. This indoor process ensures the kohlrabi matures quickly and consistently, which results in a sweeter, more tender harvest rather than a woody, bitter one.
Calculating the Optimal Indoor Start Date
Determining the time to sow kohlrabi seeds requires calculating backward from your region’s average last spring frost date. This date serves as the anchor point for the entire planting schedule. The recommended window for starting kohlrabi seeds indoors is four to six weeks before this final expected frost.
Kohlrabi needs fast, uninterrupted growth to produce a quality bulb. Sowing too early can cause the seedlings to become rootbound or overly leggy, which stresses the plant and may encourage bolting once they are moved outside. Sowing within the four to six-week window ensures the young plants are robust enough for transplanting but have not yet outgrown their starter containers. Gardeners looking to extend their harvest can initiate new batches of seeds every two to three weeks within this spring period.
Setting Up the Indoor Seed Starting Environment
Select sterile seed-starting mix rather than heavy garden soil, which can compact and introduce pathogens. Sow the seeds at a shallow depth, generally between 1/4 and 1/2 inch deep, and keep the medium consistently moist but not saturated.
Kohlrabi seeds germinate effectively across a soil temperature range of 55–75°F. Kohlrabi does not require a heating mat for successful germination, thriving well in cooler room temperatures between 60°F and 70°F. Using higher temperatures can accelerate growth too rapidly, often resulting in weak, elongated stems that struggle to support the plant.
Once the seedlings emerge, they require immediate access to strong supplemental lighting, such as fluorescent or LED grow lights, to prevent stretching. The young plants need 14 to 16 hours of light per day. Position the light source very close to the tops of the seedlings, just a few inches above the growing leaves, to prevent the plants from becoming leggy.
Transitioning Seedlings Outdoors (Hardening Off)
Before moving the young kohlrabi plants permanently into the garden, they must undergo “hardening off.” This gradual acclimation toughens the tender indoor-grown tissue to withstand direct sun, wind, and fluctuating outdoor temperatures. The procedure should take approximately seven to 14 days.
Start by placing the seedlings in a shaded, sheltered outdoor location for just one or two hours on the first day, gradually increasing the duration of exposure each subsequent day. This slow introduction prevents leaf scorch and transplant shock, which can severely stunt the plant’s growth.
After the hardening-off period, the seedlings are ready for the garden bed. Kohlrabi transplants can be moved outdoors one to two weeks before the average last frost date, as they tolerate cool conditions. The plants should have developed at least four to five true leaves before they are set into the garden soil.