Kohlrabi, a member of the Brassica family, is a cool-season vegetable often called a German turnip because of its swollen, globe-like stem that forms just above the soil line. Successful cultivation depends on precise timing to ensure the edible stem, or “bulb,” develops before heat or cold stress occurs. The goal is to provide a consistent period of cool, moderate growth, as any interruption can compromise the quality of the harvest. Planning a spring or fall planting schedule is the most important step for growing sweet, tender kohlrabi.
Timing Kohlrabi for Spring Harvest
Maximizing the spring growing season requires coordination between indoor seed starting and outdoor planting to ensure the crop matures before summer heat arrives. For the earliest harvest, start seeds indoors four to six weeks before the last expected frost date. Starting seeds in a cool room (60°F to 70°F) promotes stocky growth, making seedlings ready for transplanting around the time of the last frost.
Transplants are moved outdoors two to four weeks before the last spring frost, provided they have been properly hardened off. Hardening off is the process of gradually exposing young plants to sun, wind, and cooler temperatures to prevent shock. Direct sowing can be done earlier, typically one month before the last spring frost, once the soil becomes workable.
The primary risk for a spring crop is that the bulb will not fully develop before temperatures become consistently hot. Prolonged heat causes the stem to become woody, tough, and bitter. The full growing cycle of 45 to 60 days must occur during cool spring weather. Select fast-maturing varieties for the spring crop to ensure harvest before the summer solstice, and plant a succession every two to three weeks for an extended harvest period.
Timing Kohlrabi for Fall and Winter Harvest
The fall crop is often preferred because the cooler, more stable temperatures of autumn enhance the kohlrabi’s sweetness and texture. Fall planting requires a backward calculation based on the region’s average first frost date. Kohlrabi varieties typically require 50 to 60 days to reach maturity.
To determine the correct planting window, subtract the variety’s days to maturity from the average first fall frost date. Subtract an additional buffer of two to four weeks to account for the slower growth rate as daylight hours decrease and temperatures cool. This calculation results in a target window for direct seeding, usually between mid-July and mid-August.
For example, if the first frost is expected on October 20th, and the variety takes 55 days, planting should occur around August 25th. Adding the two to four week buffer places the final planting date around the end of July. Planting in succession every two weeks until the cutoff date ensures a continuous harvest that often extends beyond the first light frost, as mature kohlrabi plants are frost-tolerant.
Temperature and Timing Requirements
Kohlrabi thrives in a narrow range of environmental conditions, which dictates strict timing for both spring and fall planting. The ideal air temperature range for optimal growth is between 60°F and 75°F. When temperatures consistently exceed 75°F, especially with inconsistent watering, the plant produces a fibrous, woody stem that is unpalatable.
Seed germination occurs best when the soil temperature is between 55°F and 75°F. Seeds can germinate in soil temperatures as low as 45°F, but the process is faster in the ideal range. Young plants tolerate mild cold, but prolonged temperatures below 50°F can trigger premature bolting, causing the plant to flower before the edible stem fully develops.
The requirement for cool, consistent growth means the entire 45-to-60-day development cycle must avoid the heat of mid-summer and the hard freezes of late autumn. Selecting a planting date that allows the bulb to mature entirely within the cooler periods ensures a sweet, crisp texture. This focus on temperature management prevents the stresses of extreme heat or extended cold exposure.