How to Save Kohlrabi Seeds: Biennial Lifecycle & Storage Guide

Saving kohlrabi seeds is one of those gardening skills that sounds straightforward but has a catch: kohlrabi is a biennial. It doesn’t produce seeds in the same year you plant it. You need to grow the plant through two seasons — first year for the bulb, second year for the flowers and seeds. That extra step trips up a lot of gardeners, but once you understand the lifecycle, the process is actually quite manageable.

Here’s how to do it from start to finish.

Understanding Kohlrabi’s Biennial Lifecycle

Most vegetable gardeners treat kohlrabi as an annual — plant in spring or fall, harvest the bulb, done. But kohlrabi (Brassica oleracea var. gongylodes) is biologically a biennial, meaning its natural lifecycle spans two growing seasons.

Year 1: The plant germinates, grows leaves, and forms the swollen stem (the bulb we eat). It stores energy in this bulb. The plant’s “goal” in year one is energy storage, not reproduction.

Winter (vernalization): The plant needs a prolonged cold period — typically 4-10 weeks at temperatures between 35-50°F (2-10°C). This cold exposure triggers the hormonal switch from vegetative growth to reproductive growth. Without this cold period, kohlrabi won’t flower.

Year 2: After vernalization, the plant sends up a tall flower stalk (2-4 feet) from the center of the bulb. It blooms with small yellow four-petaled flowers typical of brassicas, gets pollinated by insects, and produces seed pods. The entire above-ground plant eventually dries up as energy goes into seed production.

This two-year timeline is why most gardeners never see kohlrabi flowers — they harvest the bulb in year one. Seed saving means intentionally leaving select plants unharvested and carrying them through winter.

Selecting Plants for Seed Saving

Not every kohlrabi plant is worth saving seeds from. Since you’re investing two seasons, be selective.

Choose plants that show:

• Smooth, round, well-formed bulbs
• Good size for the variety (2-4 inches for most standard types)
• No cracking, woodiness, or deformity
• Healthy leaves with no signs of disease
• True-to-type characteristics for the variety (correct color, shape, size)

Avoid saving seeds from plants that:

• Bolted in the first year (premature flowering suggests poor genetics for your climate)
• Produced small, irregular, or woody bulbs
• Showed signs of clubroot, black rot, or other brassica diseases
• Were the first to succumb to pests

Select at least 5-6 plants for seed saving if possible. More plants means more genetic diversity in your saved seed, which produces healthier, more adaptable future generations. If you only save from one plant, you’ll progressively narrow the gene pool and may see problems after a few generations.

For details on variety characteristics to look for, see our kohlrabi varieties guide.

Overwintering Kohlrabi

This is the trickiest part of seed saving: getting your selected plants through winter alive so they can flower in year two.

Method 1: Overwinter in the Ground (Mild Climates)

If your winters stay above about 10°F (-12°C) and you don’t get prolonged deep freezes, you can leave kohlrabi in the garden over winter.

1. In late fall, trim the outer leaves but leave the central growing point and a few inner leaves intact.
2. Mulch heavily — 6-8 inches of straw, leaves, or hay around and over the plants.
3. In zones 7-9, this is usually sufficient. The plant goes dormant, the bulb survives in the ground, and it breaks dormancy in spring and sends up a flower stalk.

Method 2: Dig and Store (Cold Climates)

In zones 3-6, freezing soil will kill overwintering kohlrabi. Instead:

1. Before the first hard freeze (below 25°F), carefully dig up your selected plants. Keep the root system intact.
2. Trim outer leaves, leaving the growing point and a few small inner leaves.
3. Store in a root cellar, unheated garage, or refrigerator — anywhere that stays consistently between 32-40°F (0-4°C) with moderate humidity (80-90%).
4. Pack the roots in slightly damp sand, sawdust, or peat moss. Don’t let them dry out completely, but don’t keep them soaking wet either.
5. Check monthly — remove any plants showing rot.
6. In early spring, after the last hard freeze, replant the stored bulbs in the garden, burying the roots and base at the same depth they were originally growing.

The stored plants look rough by spring — wrinkled, slightly soft, maybe missing most leaves. That’s normal. As long as the central growing point is alive and not rotten, they’ll recover quickly once replanted and temperatures warm.

Method 3: Pot and Overwinter in a Cold Frame

A middle option that works in many climates:

1. In fall, transplant selected kohlrabi into large pots (at least 3 gallons each).
2. Place in an unheated cold frame, hoop house, or unheated greenhouse.
3. The cold frame provides enough protection to prevent root-zone freezing while still giving the plant the cold exposure it needs for vernalization.
4. In spring, either transplant to the garden or leave in the pots — they’ll flower either way.

Cross-Pollination and Isolation

This is critical and often overlooked. Kohlrabi crosses freely with other Brassica oleracea varieties. That includes:

• Cabbage
• Broccoli
• Cauliflower
• Brussels sprouts
• Kale
• Collard greens

If any of these are flowering at the same time within pollination range, they can cross-pollinate with your kohlrabi. The resulting seeds will be hybrids — not true to type. You might plant “kohlrabi” seeds next year and get some strange cabbage-kohlrabi mutant.

Isolation options:

1. Distance isolation — The standard recommendation is 1/2 to 1 mile between flowering brassica types. This is impractical for most home gardeners.

2. Time isolation — Only let one Brassica oleracea variety flower at a time. This works if you’re only saving seed from one brassica type per season.

3. Caging/bagging — Cover flowering plants with insect-proof mesh or row cover. Since kohlrabi is insect-pollinated, you’ll need to either introduce pollinators (a small container of flies works) or hand-pollinate with a small brush. Open the cage every other day to allow pollinators in, alternating with your other flowering brassicas.

4. Practical approach for home gardeners — If you only grow kohlrabi and no other brassicas are flowering nearby (check your neighbors’ gardens too), cross-pollination risk is low. In suburban or rural areas with no nearby brassica seed crops, you’ll usually get true-to-type seed without elaborate isolation.

Flowering and Pollination

Once vernalized kohlrabi is planted (or emerges from dormancy in the ground), the flower stalk appears within a few weeks as temperatures warm.

What to expect:

• The bulb sends up a central stalk that grows 2-4 feet tall
• Multiple branching stems develop, each covered in small yellow flowers
• Flowering lasts 3-6 weeks, with flowers opening progressively from bottom to top
• Bees, flies, and other insects are the primary pollinators
• Each flower produces a narrow seed pod (silique) containing 10-20 seeds

Support the stalk. Flowering kohlrabi gets top-heavy and can fall over in wind or rain. Stake the main stalk or use a tomato cage.

Water and feed. The plant is putting enormous energy into seed production. Keep it watered and side-dress with compost or balanced fertilizer when the stalk starts growing.

Harvesting Kohlrabi Seeds

Timing the harvest is important — too early and seeds aren’t mature, too late and the pods shatter and scatter seeds everywhere.

When to Harvest

• Seed pods start green, then turn tan/brown as they dry
• Seeds inside transition from green to dark brown or black
• The pods begin to feel dry and papery
• The plant’s leaves start yellowing and dying back

The test: Open a few pods from the lowest (oldest) part of the stalk. If the seeds inside are dark brown to black, hard, and round, they’re ready. If they’re still green or soft, wait longer.

How to Harvest

Method 1: Cut and dry (recommended)

1. When about 2/3 of the pods on the plant have turned brown, cut the entire stalk.
2. Place it upside down in a paper bag or hang it over a tarp in a dry, well-ventilated area.
3. Let it dry for 1-2 weeks. The remaining green pods will continue to mature as the stalk dries.
4. Once fully dry, crush or rub the pods to release the seeds. The pods shatter easily when completely dry.

Method 2: Harvest in stages If you want to maximize seed viability, harvest pods individually as they turn brown, checking every few days over a 2-3 week period. This is more labor-intensive but ensures each pod is harvested at peak maturity.

Cleaning Seeds

After crushing the pods, you’ll have a mix of seeds, pod fragments, and chaff.

1. Coarse screen — Pass the material through a colander or coarse screen to remove large stem and pod pieces.
2. Fine screen — Use a fine mesh screen to separate seeds from small chaff.
3. Winnowing — Pour seeds slowly between two bowls in a gentle breeze (or in front of a fan on low). The lighter chaff blows away while the heavier seeds fall into the receiving bowl.
4. Final sort — Pick out any remaining debris by hand. Remove any seeds that look shriveled, discolored, or damaged.

Storing Kohlrabi Seeds

Properly stored kohlrabi seeds remain viable for 3-5 years, with some reports of germination after 7+ years.

Storage Conditions

The enemies of seed viability are moisture, heat, and light. Your storage strategy should minimize all three.

• Temperature: Cool is better. Room temperature (65-70°F) is adequate for short-term storage. For long-term storage (3+ years), keep seeds at 35-50°F — a refrigerator works perfectly.
• Humidity: Keep seeds dry. After cleaning, spread them on a paper plate for 2-3 days in a dry room to ensure all surface moisture is gone before sealing in containers.
• Container: Airtight glass jars, sealed envelopes inside a zip-lock bag, or small sealed containers all work. Add a small packet of silica gel desiccant to absorb any residual moisture.
• Light: Store in the dark. A closed drawer, box, or opaque container is fine.

Labeling

Always label saved seeds with:

• Variety name
• Date harvested
• Any notes (plant characteristics, growing conditions, etc.)

It sounds obvious until you’re staring at five identical jars of small brown seeds two years later with no idea which is which.

Testing Germination

Before planting saved seeds, especially those stored for more than a year, do a quick germination test:

1. Place 10 seeds on a damp paper towel
2. Fold the towel and put it in a sealed plastic bag
3. Keep at room temperature for 5-7 days
4. Count how many germinated

If 7+ out of 10 germinate (70%+), you’re good. If germination is lower, sow seeds more thickly to compensate, or consider starting fresh.

How Much Seed Does One Plant Produce?

A single healthy kohlrabi plant can produce a surprising amount of seed — typically 1/4 to 1/2 ounce (7-14 grams). Since kohlrabi seeds are small (about 250-300 seeds per gram), one plant can yield anywhere from 1,500 to 4,000+ seeds.

That’s enough to plant a very large garden for several years, share with friends, or contribute to a seed swap. For context, you only need about 20-30 seeds to plant a full season’s worth of kohlrabi for a typical household.

Why Save Kohlrabi Seeds?

Beyond self-sufficiency, there are real reasons to invest in this two-year process:

• Adaptation — Kohlrabi grown from your own saved seeds gradually adapts to your specific soil, climate, and growing conditions over multiple generations. After 3-4 cycles of saving seed from your best plants, you’ll have a locally adapted strain that outperforms generic commercial seed.
• Cost — A packet of kohlrabi seeds costs $3-5 and contains 100-200 seeds. One seed-saving plant produces 10-40x that amount.
• Variety preservation — Some heirloom kohlrabi varieties are maintained entirely by home seed savers. If nobody saves seed from them, they disappear.
• Education — Understanding the full lifecycle of a food plant is deeply satisfying and teaches you things about plant biology that reading never quite conveys.

For everything about growing kohlrabi from seed through harvest, our how to grow kohlrabi guide is the complete reference. And if you’re interested in the different varieties worth preserving, check our kohlrabi varieties guide.

Quick Reference: Seed Saving Timeline

When	What
Year 1, Spring/Fall	Plant kohlrabi, grow to bulb stage
Year 1, Late Fall	Select best plants, prepare for overwintering
Winter	Store/protect plants through cold period (vernalization)
Year 2, Early Spring	Replant stored bulbs or uncover in-ground plants
Year 2, Late Spring	Flower stalk emerges, stake and support
Year 2, Early-Mid Summer	Flowering, pollination (3-6 weeks)
Year 2, Mid-Late Summer	Seed pods develop and dry on the plant
Year 2, Late Summer	Harvest pods when 2/3 are brown and dry
Year 2, Fall	Clean, dry, label, and store seeds

It’s a long game. But the payoff — garden independence and seeds perfectly adapted to your growing conditions — is worth the patience.