Kohlrabi, a unique member of the Brassica family, is a cool-season vegetable prized for its swollen, turnip-like stem that grows above ground. It has an appealing flavor, often described as a mild mix between a turnip and cabbage. For gardeners in USDA Hardiness Zone 5, precise timing is essential for producing a tender, non-woody harvest. Planting kohlrabi at the right moment ensures the plant matures during the ideal temperature window, avoiding late spring frost and summer heat.
Understanding Zone 5 Planting Windows
Zone 5 has significant temperature differences between summer and winter, making frost dates critical markers for cool-season crops. The average last spring frost typically falls between mid-April and mid-May, marking the start of the frost-free growing season. Conversely, the average first fall frost usually occurs between mid-October and late October.
Kohlrabi thrives in temperatures between 60°F and 70°F. Exposure to temperatures above 75°F causes the edible stem to become woody and fibrous, severely impacting flavor and texture. Planting must be timed so the 45-to-60-day maturity window occurs before the onset of summer heat and after the danger of a hard freeze has passed.
Spring Planting Strategy
Achieving an early spring harvest requires starting seeds indoors and carefully timing the outdoor transplanting. Seeds should be sown indoors four to six weeks before the average last frost date for your specific area. This early start allows seedlings to establish a strong root system before moving to the garden.
Sow seeds about one-quarter inch deep in sterile seed-starting mix, ensuring they receive plenty of light. Seedlings can be moved outdoors once the last frost date has passed, or when the soil temperature consistently reaches at least 50°F. Before transplanting, gradually acclimate the young plants to outdoor conditions over seven to ten days, a process called “hardening off.”
When transplanting, space the plants six to eight inches apart. Ensure the swollen stem portion sits just above the soil line as it develops. Direct seeding is an option, but only after the last frost date when the soil is workable. The goal is to have the kohlrabi ready for harvest by early to mid-June, before mid-summer temperatures arrive.
Succession Planting for Fall Harvest
The secondary planting window offers an excellent opportunity for a second, often sweeter, harvest, as cooling autumn weather enhances the plant’s flavor. To calculate the ideal fall planting date, count backward from the average first frost date, typically mid-October in Zone 5. Use the variety’s Days to Maturity (DTM), generally 50 to 60 days, and add a buffer of 10 to 14 days to account for slower growth in decreasing daylight and cooler conditions.
This calculation places the latest practical direct-sow date in Zone 5 between mid-July and early August for most common varieties. Planting during this period allows the kohlrabi to fully mature before a hard freeze, which would damage the crop. Kohlrabi can withstand a light frost, but it must be near maturity before sustained cold weather arrives.
A fall harvest involves direct-sowing seeds into the garden, as the summer soil is warm enough for quick germination. Consistent moisture is important during the summer months to prevent the young plants from bolting or becoming woody. Harvesting the mature bulbs, which should be two to three inches in diameter, in late September or early October yields a tender, crisp crop that benefits from the cool nights.