When To Plant Kohlrabi In Zone 6

If you’re gardening in zone 6, knowing when to plant kohlrabi is the key to a successful harvest. This cool-season vegetable thrives in the specific temperature windows your climate provides, and timing it right avoids common problems like bolting or poor bulb formation.

This guide will walk you through the optimal spring and fall planting schedules, along with practical tips for soil preparation, care, and harvesting. You’ll learn how to make the most of your zone 6 growing season for a plentiful crop of this unique and tasty vegetable.

For zone 6 gardeners, the optimal planting times revolve around cool weather. Kohlrabi grows best when daytime temperatures are between 60°F and 75°F. It can handle a light frost, which actually improves its flavor, but it struggles in the heat of summer.

You have two main planting windows: early spring and late summer for a fall harvest. Missing these windows often leads to the plant “bolting,” where it sends up a flower stalk instead of forming a good bulb.

Spring Planting Schedule

Your goal for spring is to get kohlrabi seeds or transplants into the ground as early as the soil can be worked. The soil temperature should be at least 45°F. Here’s a simple timeline:

Direct Seed Outdoors: Sow seeds directly in your garden 2-4 weeks before your last average spring frost date. For most of zone 6, this falls between late March and mid-April.
Start Seeds Indoors: For a head start, sow seeds indoors 4-6 weeks before your last frost date. This is typically in early to mid-March. Transplant the seedlings outdoors 2-3 weeks before the last frost, once they have a few true leaves.
Succession Planting: To extend your harvest, plant a new batch of seeds every 2-3 weeks until late spring. Stop planting when daytime temperatures are consistantly above 80°F.

Fall Planting Schedule

Fall planting is often even more successful than spring in zone 6. The cooling temperatures are ideal for kohlrabi development. You’ll need to work backwards from your first average fall frost date.

Direct Seed Outdoors: Sow seeds directly in the garden in late July or early August. This is typically 6-8 weeks before your first fall frost, which for zone 6 is usually in mid-to-late October.
Transplant for Fall: You can also start seeds indoors in mid-summer and transplant them into the garden in late August. This can help avoid the peak summer heat on young seedlings.

The fall crop often produces larger, sweeter bulbs because the maturing plants enjoy those cool, crisp autumn nights.

Preparing Your Garden Soil

Kohlrabi isn’t overly fussy, but it does best in fertile, well-drained soil. A little prep work makes a big difference.

Location: Choose a spot that gets full sun (at least 6 hours daily).
Soil Test: It’s a good idea to test your soil pH. Kohlrabi prefers a pH between 6.0 and 7.5.
Amendments: Work 2-4 inches of well-rotted compost or aged manure into the top 6-8 inches of soil before planting. This improves drainage and fertility.
Fertilizer: You can mix in a balanced, all-purpose organic fertilizer according to package directions at planting time.

How to Plant Kohlrabi Seeds

Planting kohlrabi is straightforward. Follow these steps for best results:

Rake the soil smooth and remove any large clumps or stones.
Sow seeds about ¼ to ½ inch deep.
Space seeds roughly 1 inch apart in rows that are 12-18 inches apart.
Water the seedbed gently but thoroughly to settle the soil.
Seeds will germinate in 5-10 days, depending on soil temperature.

Thinning and Spacing

Once seedlings are a few inches tall and have their first set of true leaves, they need room to grow. Thin them so that plants are spaced 4-6 inches apart for smaller varieties, and 6-8 inches apart for larger ones. Crowded plants will compete for nutrients and fail to form nice bulbs.

Caring for Your Kohlrabi Plants

Consistent care is simple but crucial for developing those crisp, tender bulbs.

Watering: Provide 1-1.5 inches of water per week. Consistent moisture is vital, especially during bulb formation. Inconsistent watering can cause the bulbs to become woody or split.
Mulching: Apply a 2-3 inch layer of straw or shredded leaves around plants. This conserves moisture, suppresses weeds, and keeps the soil temperature cooler in spring and warmer in fall.
Weeding: Keep the area around your kohlrabi free of weeds, which compete for water and nutrients. Be careful when weeding not to damage the shallow roots.
Fertilizing: If your soil is rich, extra fertilizer may not be needed. For a mid-season boost, you can side-dress with a nitrogen-rich fertilizer like blood meal when bulbs begin to swell.

Common Pests and Problems

Kohlrabi is a brassica, so it shares pests with cabbage and broccoli. Stay vigilant for these common issues:

Cabbage Worms and Loopers: These green caterpillars can chew large holes in leaves. Check the undersides of leaves regularly. Handpick them off or use an organic Bt (Bacillus thuringiensis) spray.
Aphids: Small clusters of these insects can appear on stems and leaves. A strong spray of water from the hose often knocks them off. Insecticidal soap is also effective.
Flea Beetles: They create many tiny holes in leaves, especially on young plants. Use floating row covers immediately after planting to create a physical barrier.
Bolting: If your plant suddenly sends up a tall flower stalk, it’s usually due to planting too late in spring (exposed to heat) or a sudden cold snap followed by warmth on young plants. Stick to the recommended planting dates to avoid this.

Harvesting and Storing Kohlrabi

Harvesting at the right time ensures the best texture and flavor. Kohlrabi bulbs are ready when they are about the size of a golf ball or tennis ball, usually 1 to 3 inches in diameter for most varieties.

Use a sharp knife to cut the stem just below the bulb. You can also harvest the young, tender leaves to eat like kale or collards.
Don’t let the bulbs grow too large; once they exceed about 3-4 inches, they can become tough and woody with a strong flavor.
For storage, twist off the leaves and roots. The bulbs can be stored in a plastic bag in your refrigerator crisper drawer for several weeks. For longer storage, they can be kept in a cool, humid root cellar.

Recommended Varieties for Zone 6

Choosing the right variety can enhance your success. Here are some excellent choices well-suited to zone 6’s climate:

Early White Vienna: A fast-maturing (55 days) heirloom perfect for spring planting. It produces pale green, tender bulbs.
Grand Duke: A hybrid known for its heat tolerance and uniform bulbs, making it a reliable choice for both spring and fall.
Korridor: A purple variety that holds its quality well in the garden without getting woody. It’s also very cold-tolerant for fall crops.
Gigante: As the name suggests, this one grows very large but remains tender if harvested on time. It needs a longer season, so it’s ideal for fall planting.

FAQ: Planting Kohlrabi in Zone 6

Can I plant kohlrabi in the summer in zone 6?

It’s not recommended. Summer heat causes kohlrabi to bolt or develop poor-quality, woody bulbs. Focus on early spring and late summer plantings instead.

How do I protect my fall kohlrabi from an early frost?

Kohlrabi is quite frost-tolerant. A light frost actually sweetens the flavor. For a hard freeze forecast, you can cover the plants with a frost blanket or old bedsheets overnight to extend the harvest season.

Can I grow kohlrabi in containers?

Absolutely. Choose a container at least 12 inches deep and wide. Use a high-quality potting mix and be diligent about watering, as containers dry out faster. One plant per large pot is a good rule.

Why are my kohlrabi bulbs not swelling?

This is usually caused by overcrowding, inconsistent watering, or nutrient-poor soil. Ensure proper spacing, keep the soil evenly moist, and start with soil amended with compost.

What are good companion plants for kohlrabi?

Good companions include onions, beets, and aromatic herbs like dill and mint. Avoid planting it near other brassicas (like broccoli or cauliflower) to help minimize pest spread, and keep it away from pole beans and tomatoes.