Wisconsin’s short growing season requires starting kohlrabi seeds indoors four weeks before the final spring frost. Finding the best time to plant kohlrabi in wisconsin is key to getting a good harvest before the heat or cold stops growth.
Kohlrabi is a cool-season crop that grows fast. In Wisconsin, you can plant it in both spring and fall. The trick is timing your planting so the vegetable matures in mild weather, not during the hot summer or deep winter.
This guide covers exact dates, soil temperatures, and step-by-step instructions for Wisconsin gardeners. You will learn when to start seeds indoors, when to transplant, and when to sow directly in the ground.
Best Time To Plant Kohlrabi In Wisconsin
The absolute best time to plant kohlrabi in wisconsin depends on whether you want a spring or fall crop. For spring planting, start seeds indoors 4 weeks before your area’s last frost date. For most of Wisconsin, that means starting seeds in early to mid-April.
Transplant seedlings outdoors 2 to 3 weeks before the last frost. Kohlrabi can handle light frosts, so you can get them in the ground early. For fall planting, sow seeds directly in the garden about 8 to 10 weeks before the first fall frost.
Wisconsin’s climate varies by region. Southern Wisconsin has a longer growing season than the north. Always check your local frost dates for the most accurate timing.
Understanding Wisconsin’s Growing Zones
Wisconsin spans USDA hardiness zones 3b through 5b. Northern areas near Lake Superior have shorter, cooler summers. Southern areas near Madison and Milwaukee have warmer, longer seasons.
- Zone 3b-4a (northern Wisconsin): Last spring frost around May 15-25. First fall frost around September 15-25.
- Zone 4b-5a (central Wisconsin): Last spring frost around May 5-15. First fall frost around September 25-October 5.
- Zone 5b (southern Wisconsin): Last spring frost around April 25-May 5. First fall frost around October 5-15.
Adjust your planting dates based on your specific zone. A week or two can make a big difference in kohlrabi quality.
Spring Planting Timeline
Spring planting gives you a harvest in early summer. Kohlrabi grows best when temperatures are between 60°F and 70°F. It bolts or becomes woody if it gets too hot.
- Start seeds indoors: 4 weeks before last frost. For southern Wisconsin, that’s late March to early April. For northern Wisconsin, that’s mid to late April.
- Harden off seedlings: 1 week before transplanting. Set them outside for a few hours each day, increasing time gradually.
- Transplant outdoors: 2 to 3 weeks before last frost. Kohlrabi can survive temperatures as low as 20°F, but it grows best above 40°F.
- Direct sow (optional): 2 weeks before last frost. Plant seeds 1/4 inch deep, 1 inch apart, then thin to 4-6 inches apart.
For example, if your last frost is May 10 in southern Wisconsin, start seeds indoors around April 12. Transplant outdoors around April 25. Harvest will be ready in 45 to 60 days, around mid-June.
Fall Planting Timeline
Fall planting is often easier because the soil is warm and you don’t need to start seeds indoors. Kohlrabi actually tastes sweeter after a light frost. The cool autumn weather improves flavor.
- Count back from first frost: Kohlrabi needs 45 to 60 days to mature. Count back from your first fall frost date.
- Direct sow seeds: 8 to 10 weeks before first frost. For southern Wisconsin, that’s early to mid-August. For northern Wisconsin, that’s late July to early August.
- Thin seedlings: When they have 2-3 true leaves, thin to 4-6 inches apart.
- Mulch for protection: If a hard freeze is forecast, cover plants with row covers or straw.
Fall-planted kohlrabi can be harvested well into October or even November if you protect it. The bulbs store well in the ground for a few weeks after maturity.
Soil Preparation For Kohlrabi
Good soil preparation helps kohlrabi grow fast and stay tender. The vegetable prefers loose, well-draining soil with plenty of organic matter.
Soil Temperature Requirements
Kohlrabi seeds germinate best when soil temperature is between 50°F and 75°F. In cold Wisconsin springs, the soil may still be chilly when you transplant.
- Minimum soil temp for germination: 45°F
- Optimal soil temp for germination: 65°F to 75°F
- Transplanting soil temp: Above 50°F
Use a soil thermometer to check. If the soil is too cold, seeds may rot or germinate slowly. You can warm the soil by covering it with black plastic for a week before planting.
Amending Wisconsin Clay Soil
Many Wisconsin gardens have heavy clay soil. Kohlrabi does not like compacted or waterlogged soil. It needs loose, friable ground for the bulb to swell properly.
Mix in 2 to 3 inches of compost or aged manure before planting. This improves drainage and adds nutrients. If your soil is very heavy, consider raised beds or containers.
Kohlrabi prefers a soil pH between 6.0 and 7.5. Test your soil and add lime if needed to raise the pH. Sulfur can lower it if it’s too alkaline.
Planting Kohlrabi Seeds And Seedlings
Whether you start seeds indoors or direct sow, follow these steps for the best results. Kohlrabi is easy to grow if you give it consistent moisture and space.
Starting Seeds Indoors
Starting indoors gives you a head start in Wisconsin’s short spring. Use seed-starting mix and small pots or trays.
- Sow seeds 1/4 inch deep
- Keep soil moist but not soggy
- Provide 12-16 hours of light per day
- Maintain temperature around 65°F to 70°F
- Seeds germinate in 5 to 10 days
When seedlings have 2-3 true leaves, they are ready to harden off. Do not let them get root-bound. Transplant them before they become leggy.
Direct Sowing Outdoors
Direct sowing is simpler and works well for fall planting. Plant seeds in rows 12 to 18 inches apart.
- Make a shallow furrow 1/4 inch deep
- Space seeds 1 inch apart in the row
- Cover lightly with soil and water gently
- Thin to 4-6 inches apart when seedlings are 2 inches tall
- Keep soil consistently moist until germination
Thinning is important. Crowded kohlrabi produces small or misshapen bulbs. You can eat the thinned leaves as microgreens.
Transplanting Seedlings
Transplant seedlings on a cloudy day or in the evening to reduce transplant shock. Water them well before moving.
- Space plants 4-6 inches apart in rows 12-18 inches apart
- Plant at the same depth they were growing in the pot
- Water thoroughly after transplanting
- Apply a light mulch to retain moisture and suppress weeds
Kohlrabi roots are shallow. Be gentle when transplanting to avoid damaging them. Water regularly for the first week until they establish.
Caring For Kohlrabi Through The Season
Kohlrabi needs consistent care to produce tender, sweet bulbs. Neglect can lead to tough, woody vegetables.
Watering Needs
Kohlrabi needs about 1 to 1.5 inches of water per week. Inconsistent watering causes the bulbs to crack or become tough.
- Water deeply once or twice a week
- Use drip irrigation or soaker hoses to keep foliage dry
- Mulch with straw or grass clippings to retain moisture
- Check soil moisture by feeling 1 inch below the surface
During hot spells, you may need to water more often. Kohlrabi that dries out will bolt or become bitter.
Fertilizing
Kohlrabi is a moderate feeder. It benefits from a balanced fertilizer at planting time.
- Apply a 10-10-10 fertilizer at planting, following package rates
- Side-dress with compost or a nitrogen source 3-4 weeks after planting
- Avoid too much nitrogen, which can cause leafy growth at the expense of bulbs
If your soil is rich in organic matter, you may not need additional fertilizer. Watch for yellowing leaves, which indicate nitrogen deficiency.
Pest And Disease Management
Common pests in Wisconsin include cabbage worms, aphids, and flea beetles. Kohlrabi is a brassica, so it shares pests with cabbage and broccoli.
- Use row covers to exclude pests
- Handpick cabbage worms
- Spray neem oil for aphids
- Apply diatomaceous earth for flea beetles
- Rotate crops to prevent soil-borne diseases
Diseases like clubroot and black rot can occur in wet, poorly drained soil. Ensure good drainage and avoid planting kohlrabi where other brassicas grew recently.
Harvesting Kohlrabi At The Right Time
Harvesting at the right time is crucial for flavor and texture. Kohlrabi is best when the bulb is 2 to 3 inches in diameter.
If you wait too long, the bulb becomes woody and tough. The leaves can also be eaten as greens.
Signs Of Readiness
- Bulb diameter: 2-3 inches (about the size of a tennis ball)
- Bulb feels firm and smooth
- Leaves are still green and healthy
- Days to maturity: 45-60 days from transplant or 55-70 days from seed
In spring, harvest before temperatures consistently exceed 80°F. In fall, you can harvest after a light frost for sweeter flavor.
How To Harvest
- Use a sharp knife or garden shears
- Cut the stem about 1 inch below the bulb
- Trim off the leaves, leaving 1-2 inches of stem
- Wash and store in the refrigerator
Kohlrabi stores well for several weeks in the crisper drawer. You can also blanch and freeze it for longer storage.
Common Mistakes And How To Avoid Them
Even experienced Wisconsin gardeners make mistakes with kohlrabi. Here are the most common ones and how to fix them.
Planting Too Late In Spring
If you plant kohlrabi in late May or June, it will likely bolt or become woody in the summer heat. Stick to early spring or late summer planting.
Overcrowding
Kohlrabi needs space to form bulbs. Thin seedlings to 4-6 inches apart. Crowded plants produce tiny bulbs or no bulbs at all.
Inconsistent Watering
Letting the soil dry out then soaking it causes bulbs to crack. Water evenly and regularly, especially during dry spells.
Ignoring Pests
Flea beetles can damage young seedlings quickly. Use row covers or insecticidal soap at the first sign of damage.
Frequently Asked Questions
Can I plant kohlrabi in Wisconsin in July?
July is too hot for spring planting, but you can start seeds for a fall crop in late July or early August. Direct sow 8-10 weeks before the first fall frost.
How late can I plant kohlrabi in Wisconsin for fall harvest?
You can plant kohlrabi as late as mid-August in southern Wisconsin and early August in northern Wisconsin. Count back 60 days from your first frost date.
Does kohlrabi need full sun in Wisconsin?
Yes, kohlrabi grows best in full sun, at least 6 hours per day. It can tolerate partial shade, but bulbs will be smaller.
Can I grow kohlrabi in containers in Wisconsin?
Yes, kohlrabi grows well in containers at least 8 inches deep. Use a quality potting mix and water frequently. Containers warm up faster in spring.
What happens if I plant kohlrabi too early in Wisconsin?
If planted too early, seeds may rot in cold soil or seedlings may be killed by a hard freeze. Wait until soil temperature is above 45°F for direct sowing.
Kohlrabi is a rewarding crop for Wisconsin gardeners. By timing your planting correctly, you can enjoy fresh, tender bulbs from your garden twice a year. Start seeds indoors in early spring for a June harvest, or direct sow in late summer for a sweet fall crop. With consistent care and proper spacing, you will have plenty of kohlrabi to eat fresh, cook, or share.