If you’re not familiar with kohlrabi, it’s this alien-looking plant that you might not know how to use in your kitchen, let alone grow it. But don’t let the looks stop you. Kohlrabi is so juicy, fresh, and versatile that it would be a shame not to try it at least once.
Or maybe you’ve tried to grow kohlrabi, but with little success, as a lot of things can go wrong with it. I found the best way to increase your chances is to start the seeds indoors.
Kohlrabi transplants well, and this is actually a preferred method to extend your season in spring. Start your seeds indoors, as early as February, because kohlrabi needs cool weather to thrive. Direct sowing it in the ground won’t give it enough time to develop until the weather starts to warm up.
Kohlrabi loves cool nights in the 40°F (5°C) to 50°F (10°C) range, and doesn’t do very well when the day temperatures reach 80°F (27°C). For this reason, if your area has a short spring season, it’s best to plant your kohlrabi, as well as other brassicas such as cabbage, kale, and turnip as soon as you can.
If the weather is too hot, or too dry, your kohlrabi plants may not form bulbs, or the bulbs may crack and become woody. You can still eat your kohlrabi plants even if this happens, as the leaves and part of the fruit are still edible. But it’s best to prevent this from happening by sowing kohlrabi at the right times of the year and using transplants instead of direct sowing.
Here is all you need to know about kohlrabi transplants:
1. Kohlrabi transplants should be at least 4 weeks old.
For most temperate climates with long winters and short springs, the soil is workable in March, and even sooner than that. I garden in zone 6B, and it makes sense for me to start my kohlrabi seeds in mid to end of February and transplant them outside at the end of March.
While you can grow kohlrabi seedlings indoors for longer, if you are using smaller modules, 4 weeks is the optimum time to transplant them outside. Otherwise, you risk getting the seedlings rootbound and stunt their growth. You can check this by gently lifting a plant and seeing if roots have taken over the exterior of the starting mix.
If you still have snow in the ground and your kohlrabi seedlings are already 4 weeks old, pot them on in larger containers or plant them under a cold frame. But generally, they can last 1-2 weeks longer in the same modules.
Kohlrabi seedlings that are started indoors need to be hardened off before transplanting them outside. You can do this by leaving them outside for a couple of hours at a time, gradually increasing their exposure until they’ve spent a full day outside.
Once in the ground, you can ease the transplant shock by covering them with one or two layers of fleece directly on top, no hoops needed.
2. You need to thin kohlrabi transplants early on.
It’s a good idea to sow a few more seeds of kohlrabi than needed in your module trays. This way, you can ensure a higher rate of germination.
Unlike beetroot and radishes that don’t mind being grown in clumps, kohlrabi grows best as a single seedling, so you will need to thin them. If the seedlings compete for space and nutrients, they won’t develop as sturdy and thick.
But don’t just discard the other seedlings – you can transplant them to another module and multiply your kohlrabi seedlings using this method.
If you’re looking to get your kohlrabi starts from your local garden center, look for the ones that have multiple seedlings inside one cell and separate them to obtain more plants than what you paid for.
Often times, the seedlings at the garden center are older, with bulbs already starting to form. In spite of getting older plants, you can still separate them. Even if you’re breaking a few roots, they will recover.
Try keeping a few spare seedlings after transplanting kohlrabi in the ground – some might not do so well, and they might need replacing. It’s always good to have spare seedlings.
3. You can fix legginess when planting kohlrabi seedlings in the ground.
Starting seeds isn’t always straightforward. Often times we can lose seedlings to damping off disease, or they grow tall and leggy because they’re not getting enough light.
Just like any other seedlings, kohlrabi needs a lot of light to grow, but the base stem will still be a little leggy when the seedlings are young. If you have kohlrabi seedlings that seem healthy but can’t stand upright and look spindly and floppy, don’t worry, it’s normal.
You can easily fix this issue by burying them up to their first leaves when transplanting them in your raised beds. This will strengthen their base stem. Be careful not to only cover the future bulb with soil. Kohlrabi doesn’t grow roots in the ground, but rather, bulbs that swell above soil level. So make sure not to plant them too deeply.
Young seedlings, especially the ones suffering from transplant shock, can look floppy for a couple of days. Watering also causes them to fall over. But once they establish and start growing, you’ll no longer have that problem.
Make sure to transplant them 6 to 8 inches apart, on a cool, overcast day, preferably in the evening, as the strong sun is their biggest enemy at this stage. This way, they will have time to establish the roots in the ground overnight.
The same goes for transplanting kohlrabi seedlings from small modules to bigger pots – let them rest without growing lights for at least half of the day.
4. You can transplant kohlrabi as a fall crop as well.
We think of kohlrabi as a spring vegetable, when in fact, its cold hardiness makes it an excellent fall crop. Kohlrabi is frost-resistant and can withstand a light autumn freeze with no problem at all. If anything, it becomes sweeter and better tasting.
To grow kohlrabi from transplants as a fall crop, count backward from your last frost date 45 to 60 days, and start your seeds. For a good fall harvest, start your kohlrabi seeds in late July or the beginning of August.
This way, you can harvest great kohlrabi that is enhanced by the cold instead of becoming woody or going to seed from the hot weather. Fall harvests also have the advantage of storing for longer.
5. The reason why direct sowing kohlrabi is difficult.
If you have short springs and hot summers, directly sown kohlrabi won’t do well in places where the ground is still covered with snow in March. Sowing seeds directly in the ground in mid-March is already too late, as April and May will bring along warm days.
The only way direct seeding can work in spring is if you live in a cool, humid area with plenty of rain and mild summers. If your soil isn’t frozen in February then you have a good chance of growing kohlrabi that doesn’t become woody.
Sowing kohlrabi seeds in the summertime to grow them as a fall crop is equally as difficult, as germination often doesn’t occur at all in high temperatures. If it’s hotter than 90°F (32°C) outside, you won’t get any kohlrabi seedlings.
In this case, either wait for the weather to cool down, or start your seedlings indoors, where you can control the temperature.
Generally, starting kohlrabi and other brassicas indoors, in module trays, instead of sowing them outside, is a great method to increase the yield of a small garden, because you have absolute control.
You won’t have to worry about uneven germination or thinning and you will be able to place your kohlrabi plants wherever you want in the garden, as they don’t occupy a lot of space.