Kohlrabi is a unique cool-season vegetable belonging to the Brassica family, sharing ancestry with cabbage and broccoli. The edible part is an enlarged, bulb-like stem that grows above the soil line, offering a crisp texture and a mild, sweet flavor. Successful cultivation hinges on precise timing, which is determined by the USDA Hardiness Zone. For gardeners in Zone 6 (average minimum winter temperatures between -10°F and 0°F), understanding local frost dates is the most important factor for planning the planting schedule.
Determining Spring Planting Dates
The goal for the spring crop is to ensure maturity before the intense heat of summer arrives. In Zone 6, the average last spring frost date typically falls between April 1 and April 21. This date guides all early-season planning.
To get a head start, sow seeds indoors four to six weeks before the average last frost date. Seedlings thrive in cooler room temperatures, developing strong growth without extra heat. Direct sowing outdoors can begin once the soil is workable and has warmed to at least 50°F, often two to four weeks before the final expected frost.
Young plants are moderately tolerant of light frost, but rapid growth is necessary for quality. The ideal temperature range is between 40°F and 75°F. Planting early allows the stem to form and be harvested before summer temperatures consistently exceed 75°F, which causes the edible portion to become tough and woody.
Maximizing the Fall Crop
The fall crop is often favored because cooler, diminishing temperatures enhance the vegetable’s sweetness. Planning the fall harvest requires counting backward from the average first fall frost date for Zone 6, which generally occurs between October 17 and October 31.
Standard kohlrabi varieties require 45 to 60 days to reach harvestable size. To determine the latest planting date, subtract the days to maturity from the first expected frost date. Add an additional two weeks to account for the slower growth rate in decreasing light and cooler autumn temperatures. This places the fall planting window in mid-to-late summer.
For a continuous supply, sow seeds in succession every two to three weeks until the calculated cut-off date. This staggered planting prevents all bulbs from maturing simultaneously, providing a steady harvest that can withstand light frost, further improving flavor.
Essential Growing Conditions
Kohlrabi production requires optimal environmental conditions to support its fast growth cycle. The soil must be consistently well-drained to prevent rot, yet rich in organic matter to retain sufficient moisture. An ideal soil pH range is slightly acidic to neutral, specifically between 6.0 and 6.8.
While kohlrabi tolerates very light afternoon shade, full sun (at least six hours of direct sunlight daily) promotes the fastest growth and best bulb development. The plants are heavy feeders, benefiting from incorporating compost or a balanced fertilizer into the soil before planting.
Consistent and regular watering is necessary for producing tender, flavorful bulbs. Drought or water stress during stem formation interrupts growth, causing the bulb to become stringy, dry, and unpalatable. Sow seeds about one inch deep, and thin seedlings or transplants to allow six to eight inches of space between each plant.
Avoiding Premature Bolting
Premature bolting is a primary risk in kohlrabi cultivation, where the plant prematurely sends up a flower stalk. Bolting shifts the plant’s energy from developing the edible stem to producing seeds, stopping bulb growth and rendering it tough and inedible.
The most common causes are prolonged exposure to high heat (generally above 80°F) or significant environmental stress. Stress can be triggered by inconsistent watering, nutrient deficiency, or overcrowding. The plant interprets these conditions as a threat, prompting it to reproduce quickly.
Prevention is linked to correct spring timing, ensuring harvest is completed before peak summer heat. Maintaining consistent soil moisture, particularly during early stem enlargement, is an effective preventive measure. Avoiding temperature extremes and providing uniform care minimizes stress, allowing the plant to focus on rapid, high-quality bulb formation.