Although a long-cultivated vegetable, kohlrabi is only recently becoming more popular. It’s a very easy crop to grow, is a little more resilient to pests than its cabbage and kale cousins, but one of the most common problems people encounter with kohlrabi is when the kohlrabi bulbs are not forming
I had this problem for the first two seasons I tried growing kohlrabi. Whenever I run into this problem, I look it up and usually find a solution, but with kohlrabi, there are multiple factors that can lead to small or non-existent bulbs. I had to do my own experiments in my home garden to determine why my kohlrabi wasn’t bulbing (spoiler: I was growing them in small containers and they were drying out too much).
Once you give your kohlrabi the right growing conditions, it practically takes care of itself and you just have to wait until you’re ready to harvest.
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Why Your Kohlrabi Bulbs Are Not Forming
1. Your kohlrabi hasn’t been given enough space
Although kohlrabi is not a large, sprawling plant, the leaves of some varieties can get quite large and spread out. If planted too close together, this may lead to competition with larger plants shading smaller ones, who will end up with small bulbs or no bulbs at all.
And while kohlrabi doesn’t have a very large root system, it’s quite sensitive to competition and kohlrabi planted too close together will stunt each other’s growth as they compete for nutrients, or you may notice one kohlrabi outcompeting all the rest.
Planted in the ground, kohlrabi should be given a minimum spacing of 8 inches (20 cm) apart. More space is better, and some growers recommend a foot (30 cm) of spacing between each plant. Similarly, if growing in a container, kohlrabi should be grown in at least a 2-gallon pot that is at least 10 inches (25 cm) deep. I have grown kohlrabi in smaller containers but with inconsistent results and much smaller bulbs.
2. Your kohlrabi isn’t getting enough light or nutrients
Kohlrabi grows best when it grows vigorously. While it tolerates partial shade, you are more likely to get bigger bulbs if it’s grown in full sun. As mentioned above, kohlrabi that are planted too close together will shade each other out, preventing the slow growers from reaching their full size. Kohlrabi also needs adequate nutrients, especially nitrogen (the first number in the NPK values on fertilizer). If the soil is poor or potting mix is depleted, this will lead to smaller kohlrabi.
Ensure your kohlrabi grows in full sun, which means at least 6 hours of unshaded sunlight per day, to maximize growth. Along with enough sunlight, plant your kohlrabi in rich soil or potting mix. You can grow kohlrabi in pure compost, but you can also amend the soil with a well-balanced fertilizer of equal NPK numbers, but kohlrabi will grow even more vigorously if you use one with more nitrogen (N) than phosphorus (P) and potassium (K).
Good examples of continuous-release fertilizer good for kohlrabi are Dr. Earth Premium Gold All Purpose Fertilizer (4-4-4), and Miracle-Gro Shake ‘N Feed All Purpose Plant Food (12-4-8). For faster-acting liquid plant food, try Neptune’s Harvest Organic Hydrolized Fish & Seaweed Fertilizer (2-3-1), Miracle-Gro Water Soluble All Purpose Plant Food (24-8-16), or Urban Farm Fertilizers All-Purpose Vegetable Fertilizer (4.5-2.5-6.0). Solid granule or spike fertilizer can be applied every month or three months, depending on the brand; liquid fertilizer should be applied more frequently, usually every one to two weeks.
3. Your kohlrabi has been undergoing sustained environmental stress
Any time your kohlrabi undergoes stress, such as a long heat wave or lack of water, that slows down growth as the kohlrabi plant focuses on survival. If this happens consistently, you may end up with no bulb forming at all.
Other factors, like weed competition and compacted soil can also impact kohlrabi bulb formation, but these are related to the above factors, such as weeds overcrowding your kohlrabi and stealing nutrients away from them.
Environmental stress is harder to control. Nowadays, I usually plant my kohlrabi in mid to late summer to avoid too much heat stress and them mature in cooler weather, which kohlrabi prefers. Always make sure your kohlrabi bed or pot is kept moist but not soggy (it can cause root rot). If you are growing kohlrabi in pots, you can put them into a shady area if you’re expecting a heat wave. For kohlrabi planted in-ground, gardeners can use shade cloth to cover their plants during periods of intense heat – shade cloth is graded on how much sunlight it will block, which will keep your kohlrabi (and other plants) cool.
My Kohlrabi Is Still Not Bulbing – Can Kohlrabi Leaves Be Eaten?
It takes time before the stem begins to swell into a bulb, but if you’ve already passed the average days to maturity for your variety of kohlrabi, and there are still no bulbs, suffice it to say, it likely won’t grow much more. That said, I would still let it be for a few weeks longer and observe its growth.
If it still looks like kohlrabi bulbs are not forming, you can still harvest and eat the leaves. The stem won’t be as tender and sweet without the bulb, but you can remove any woody parts and eat the inner stem like broccoli or kale stems. The leaves have the same taste as kale, however they are much thicker and tougher. I highly recommend cooking kohlrabi greens (steamed, boiled, or stir-fried) until they’re tender. I usually saute them in oil with a little garlic, and drizzle lemon juice over them when tender.