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Kohlrabi is a tasty, versatile vegetable. And one of my favorites. I love making kohlrabi fries or adding sliced raw kohlrabi to a salad.
A kohlrabi stem swells just above the soil, until a bulb forms. The leaves then extend outwards from this swollen stem. And this gives kohlrabi its unique, alien-like appearance.
Both the bulbs and leaves are edible.
Kohlrabi usually produces one bulb per plant. And it may take 25 – 30 days before the stem starts to expand.
But, sometimes, even after a month, a bulb fails to form. Let’s look at the main reasons why this happens and what you can do about it.
5 Reasons Why Kohlrabi Bulbs Don’t Form
1. You’ve Planted Your Kohlrabi Too Close Together.
Crowded kohlrabi is a common reason why bulbs fail to form.
Kohlrabi grows roughly 16 – 24 inches high and about 16 inches wide. The bulbs, themselves, need anywhere from 4 – 12 inches of space, depending on the variety.
If those bulbs are planted too close together they won’t have room to expand to their ideal size.
How to Prevent Crowded Kohlrabi
To prevent your kohlrabi from overcrowding, it’s vital you thin your plants.
Kohlrabi seedlings need to be thinned until plants are at least 4 – 6 inches apart. The further apart, the better. And keep in mind the variety planted will make a difference. The larger the variety, the more space it needs.
Start the thinning process when the plant is a couple of inches tall and has developed at least three true leaves.
Then, once you start harvesting your Kohlrabi, pick every second one. This gives the remaining plants further room to expand, which is especially important for larger varieties. These can reach up to 10 inches in diameter.
If you’ve planted Kohlrabi in a container, the container needs to be at least 16 inches wide. And, as with an inground garden, container kohlrabi also need thinning.
In general, aim for no more than 4 Kohlrabi per 16-inch pot.
2. Your Kohlrabi Plant Is Stressed.
When a kohlrabi plant becomes stressed due to temperature or too many weeds, it may not form a bulb.
Kohlrabi is a cool weather plant. Ideal temperatures for this vegetable range from 45℉ to 75℉.
If you plant kohlrabi too late in the spring season, the heat of summer can result in a tall, weak plant. When this happens, bulbs tend to be smaller and less well formed.
But, even if you’ve planted your kohlrabi early enough, unpredictable weather can also result in a poor bulb. For example, if there’s a cold spell followed by a quick rise in temperatures, your kohlrabi may bolt. In other words, it’ll go to seed without forming the bulb.
Similarly, if planted too late in the fall and temperatures turn too cold too soon, it may bolt. Temperatures consistently below 45℉ are problematic.
How to Prevent Stressed Kohlrabi
To prevent summer heat or winter cold from affecting your plant, make sure you’re planting your kohlrabi at the appropriate time of year. My beginner’s guide on growing kohlrabi has more helpful information on when to plant kohlrabi.
Kohlrabi doesn’t do well in temperatures above 75℉. Consider using a shade cover if you’ve planted your kohlrabi too close to summer, or if there’s an unexpected heat wave. This will help protect the plants from the sun’s heat.
For kohlrabi planted in containers, move them to shadier spots during the heat of the day.
In warmer climates, Kohlrabi can grow throughout the winter. However, in most places, this isn’t the case.
Kohlrabi will tolerate frost, but it can’t take continuous cold weather.
Generally, when planting fall kohlrabi, plant about 6 – 8 weeks before you expect that first fall frost. And aim to pick your kohlrabi within a week or two of that frost.
To learn more about kohlrabi and cold weather, take a look at my article on the subject. You’ll find it helpful.
3. Your Kohlrabi Has a Nutrient Deficiency
Whether in a container or in the ground, kohlrabi likes rich, well-draining soil. But, if planted too close together or crowded out by weeds, then your kohlrabi has to compete for the nutrients in that soil.
And if a plant can’t get enough nutrients it’ll have a hard time forming a bulb.
But it’s not only crowding that can drain nutrients.
Planting kohlrabi or other Brassica family members in the same spot for several years in a row can drain the soil of the nutrients your kohlrabi needs.
This is because year after year those vegetables use the same nutrients to grow. Eventually, this depletes the soil. Especially if you’re not amending the soil to replace those lost nutrients.
How to Prevent Nutrient Deficiencies
First, keep weeds to a minimum and thin your seedlings. Both actions will lessen nutrient competition.
You can side-dress your kohlrabi with compost or mulch when the stem begins to thicken. In other words, fill in around your kohlrabi, but don’t cover the bulbs.
If you’re amending your soil, you may not need additional fertilizer. But, if you’d like to give your plant an extra boost, then, by all means, go ahead and fertilize.
Use a fertilizer that’s higher in nitrogen. Nitrogen is the first number on a fertilizer’s label. So, for example, you could use a fertilizer such as a 10- -5-5.
Just as with adding compost, fertilize around the plant and not directly on the bulb.
Kohlrabi planted in containers will likely need fertilizing. This is because watering can drain nutrients out of containers. Again, use a higher nitrogen fertilizer, and fertilize around the plant and not on top of the bulb.
4. Your Kohlrabi Is Getting Too Little or Too Much Sun
Kohlrabi needs at least 6 hours of sun per day. So it’s vital your Kohlrabi is in a spot that receives this much sunlight.
But, though it likes sunshine, it doesn’t do well in temperatures above 75℉. In many locations, especially in the south, planting too late in the spring can result in temperatures becoming too hot for your kohlrabi. It may go to seed before forming a proper bulb.
How to Prevent Sun Stress
First, know your plant hardiness zone. This comes in handy when deciding what to plant and when.
Make sure you’re planting your kohlrabi where it gets lots of sunlight. Preferably morning light. As a cool weather plant, kohlrabi will like the cooler temperatures of the morning rather than the heat of later in the day.
If daytime temperatures will reach above 75℉, then consider using a shade cover for those hot afternoons.
And for kohlrabi planted in containers, consider moving those containers to sunnier or shadier spots, depending on your plant’s needs. A plant caddy is an easy tool for moving heavy planters around. For other tool suggestions, take a look at my article on handy garden tools.
5. Your Kohlrabi Is Getting Too Little Water
On average, kohlrabi needs 1 – 1 ½ inches of water per week. And it needs this water consistently while growing.
Without enough water, kohlrabi has difficulty forming a proper bulb, and that bulb may be susceptible to cracking.
However, although it likes moist soil, kohlrabi doesn’t like constantly soggy feet. So it’s important you gauge your watering based on your type of soil.
How to Prevent Water Stress
To prevent water stress, make sure you’re watering your kohlrabi regularly.
You can also mulch around your plants. Not only will this help keep the soil moist, but it will also help feed your kohlrabi.
Take note of your kohlrabi’s soil. If planted in clay soil, you may need to water less since clay soil holds water longer. Sandy soil, on the other hand, drains more quickly. So you may need to water more often.
Final Thoughts
- A kohlrabi stem swells just above the ground, forming a bulb.
- A bulb may not form properly for the following reasons,
- Planted too close together.
- The plant is stressed.
- Nutrient deficiencies.
- Improper amount of sunlight.
- Improper amount of watering.
- Measures can be taken to prevent problems with your kohlrabi forming a proper bulb.
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