Kohlrabi thrives when seeded four to six weeks before the last expected spring frost. Knowing exactly when to plant kohlrabi can mean the difference between tender, sweet bulbs and tough, woody ones. This cool-season crop grows fast, so timing your planting correctly is essential for a succesful harvest.
In this guide, you’ll learn the best planting windows for spring and fall, how soil temperature affects germination, and simple steps to get the timing right every time. Whether you are a beginner or an experienced gardener, these tips will help you grow perfect kohlrabi.
When To Plant Kohlrabi
Kohlrabi is a cool-weather vegetable that grows best when temperatures stay between 40°F and 75°F. It can tolerate light frosts, which actually improve its sweetness. The key is to avoid hot weather, which causes the bulbs to become tough and bitter.
For most gardeners, there are two main planting seasons: spring and late summer for a fall harvest. The exact dates depend on your local climate and your last frost date.
Spring Planting Window
In spring, you want to plant kohlrabi seeds directly in the garden as soon as the soil can be worked. This is usually about four to six weeks before your last expected spring frost. The soil temperature should be at least 40°F, but germination is faster at 60°F to 70°F.
- Count back from your last frost date by 4-6 weeks
- Check soil temperature with a simple thermometer
- If soil is too wet, wait a few days to avoid rot
- Seeds germinate in 5-10 days under good conditions
Fall Planting Window
For a fall harvest, plant kohlrabi seeds about 8 to 10 weeks before your first expected fall frost. The cooler temperatures of autumn often produce the sweetest, most tender bulbs. Fall planting avoids the heat stress that spring crops sometimes face.
- Find your first fall frost date
- Count backward 8-10 weeks
- Plant seeds directly in the garden
- Water regularly if the weather is dry
Succession Planting Tips
You can plant kohlrabi every two to three weeks during the spring and fall seasons for a continuous harvest. This works well because kohlrabi matures quickly, usually in 45 to 60 days depending on the variety.
For example, plant your first batch in early spring, then another batch two weeks later, and a third batch two weeks after that. Stop planting once daytime temperatures regularly exceed 80°F.
Soil Temperature Requirements
Soil temperature is more important than air temperature for germination. Kohlrabi seeds will germinate in soil as cold as 40°F, but they do best between 60°F and 70°F. If the soil is too cold, seeds may rot or take a long time to sprout.
How To Check Soil Temperature
Use a soil thermometer inserted about two inches deep. Take the reading in the morning, as soil temperatures can fluctuate during the day. Wait until the soil has reached at least 40°F before planting.
- Buy a cheap soil thermometer online or at a garden store
- Check at different spots in your garden
- Warm soil in a raised bed faster than ground soil
- Cover soil with black plastic to warm it up faster
Cold Soil Solutions
If you live in a cold climate and want to plant early, you can warm the soil by covering it with black plastic or row covers for a week or two before planting. This can raise the soil temperature by 5°F to 10°F.
Another option is to start seeds indoors in pots and transplant them outside once the soil warms up. This gives you a head start on the growing season.
Regional Planting Guides
Your local climate determines the best planting dates. Here are general guidelines for different regions of the United States.
Northern Climates (Zones 3-5)
In cold northern areas, spring planting happens in late April to early May. Fall planting should be done in late July to early August. Kohlrabi grows well here because summers are mild.
- Spring: 4-6 weeks before last frost (usually mid-April to early May)
- Fall: 8-10 weeks before first frost (usually late July to early August)
- Use row covers to protect from late frosts
Moderate Climates (Zones 6-7)
In these zones, spring planting can start in March or early April. Fall planting is best in August or early September. You may also be able to grow kohlrabi through winter with protection.
- Spring: Mid-March to early April
- Fall: Mid-August to early September
- Mulch around plants to keep soil cool
Southern Climates (Zones 8-10)
In warm southern areas, kohlrabi is best grown as a winter or early spring crop. Plant in late fall for a winter harvest, or in late winter for an early spring harvest. Avoid summer planting entirely.
- Fall/Winter: October to December
- Spring: January to February
- Choose heat-tolerant varieties like ‘Kolibri’ or ‘Winner’
Step-By-Step Planting Guide
Follow these steps for the best results when planting kohlrabi seeds.
- Prepare the soil: Loosen the soil to a depth of 8-10 inches. Mix in compost or well-rotted manure. Kohlrabi prefers a pH between 6.0 and 7.5.
- Make shallow rows: Create rows about 12-18 inches apart. The seeds only need to be 1/4 to 1/2 inch deep.
- Sow the seeds: Place seeds about 1 inch apart in the row. Cover lightly with soil and water gently.
- Thin the seedlings: Once seedlings have two true leaves, thin them to 4-6 inches apart. This gives the bulbs room to grow.
- Water regularly: Keep the soil moist but not waterlogged. Kohlrabi needs consistent moisture for tender bulbs.
- Mulch: Apply a layer of straw or grass clippings to keep the soil cool and retain moisture.
Transplanting Seedlings
If you start seeds indoors, transplant them outside when they have 3-4 true leaves and the soil is workable. Harden them off by exposing them to outdoor conditions for a few days before planting.
Space transplants 4-6 inches apart in rows 12-18 inches apart. Water them well after planting to reduce transplant shock.
Common Timing Mistakes
Many gardeners make simple errors when deciding when to plant kohlrabi. Avoid these common mistakes.
- Planting too early in cold, wet soil: Seeds may rot or germinate slowly. Wait until soil temperature is at least 40°F.
- Planting too late in spring: Kohlrabi matures in warm weather, leading to tough bulbs. Plant early enough so harvest happens before heat arrives.
- Ignoring fall planting: Many gardeners only plant in spring. Fall kohlrabi is often sweeter and more tender.
- Not thinning seedlings: Crowded plants produce small, misshapen bulbs. Always thin to 4-6 inches apart.
Harvest Timing
Knowing when to harvest is just as important as knowing when to plant. Kohlrabi is ready to pick when the bulb is 2-3 inches in diameter. If you wait too long, the bulb becomes woody and tough.
Harvest in the morning when the plants are crisp. Use a sharp knife to cut the stem just below the bulb. Remove the leaves and store the bulb in the refrigerator for up to two weeks.
Signs Of Over-Maturity
If you see cracks in the bulb or the skin becomes tough, you have waited too long. The flesh will be fibrous and less sweet. It is better to harvest a bit early than too late.
Frequently Asked Questions
Can I plant kohlrabi in the summer?
Summer planting is not recommended in most climates because high temperatures cause the bulbs to become tough and bitter. If you live in a cool coastal area, you might succeed, but generally stick to spring and fall.
How late can I plant kohlrabi in the fall?
You can plant kohlrabi up to 8 weeks before your first expected fall frost. In mild climates, you can even plant later and use row covers or cold frames to protect the plants from light frosts.
Should I start kohlrabi seeds indoors or outdoors?
Direct sowing outdoors is easiest because kohlrabi does not transplant well. However, starting seeds indoors 4-6 weeks before the last frost can give you a head start in cold climates. Just be careful not to disturb the roots when transplanting.
What happens if I plant kohlrabi too early?
Planting too early in cold, wet soil can cause seeds to rot or germinate very slowly. The seedlings may also be stunted by prolonged cold. Wait until the soil temperature is at least 40°F.
Can I grow kohlrabi in containers?
Yes, kohlrabi grows well in containers that are at least 8 inches deep. Plant one seed per pot or space seeds 4-6 inches apart in larger containers. Use a quality potting mix and water regularly.
Final Tips For Success
Kohlrabi is one of the easiest vegetables to grow if you get the timing right. Remember these key points:
- Plant 4-6 weeks before last spring frost
- Plant 8-10 weeks before first fall frost
- Keep soil moist and cool
- Thin seedlings to 4-6 inches apart
- Harvest when bulbs are 2-3 inches in diameter
With a little planning, you can enjoy fresh, homegrown kohlrabi from your garden in both spring and fall. The sweet, crunchy bulbs are delicious raw in salads, roasted, or steamed. Start with the right planting time, and you will be rewarded with a bountiful harvest.
If you are unsure about your local frost dates, check online with your county extension service or use a gardening app. Many resources provide specific dates based on your zip code. This simple step will help you plant with confidence.
Happy gardening, and enjoy your kohlrabi crop!