Growing kohlrabi in a hydro tower: from seed to bulb
I have real success with kohlrabi in the tower. Here I show you how I let it germinate warm, raise it in the seedling station, put it into the tower with a support ring and harvest nice big bulbs.
Why kohlrabi does so well in the hydro tower
I have real success with kohlrabi in the tower. I let it grow nice and big, and I eat the leaves too. Kohlrabi is a brassica, but stays much more compact than cauliflower or broccoli. The bulb grows above the planting spot, the roots hang freely in the nutrient solution. That’s exactly what it likes.
Because water and nutrients arrive evenly in the tower, the bulb grows calmly and tender. What often goes wrong in the bed, woody or split bulbs, happens much less often in hydroponics. Three things are important: germinate warm, don’t keep it too cold afterwards and fit a support ring in time, because a big bulb gets really heavy.
| Sorte: | Hersteller: |
Dünger, EC und pH
| Phase | EC (mS/cm) | Frankies Dünger | pH |
|---|---|---|---|
| Anzucht (erste Blätter) | 0,6 | 1 ml pro Liter | 6,0 bis 6,5 |
| Jungpflanze | 1,1 | 2 ml pro Liter | 6,0 bis 6,5 |
| Wachstum im Tower | 1,7 | etwa 3,5 ml pro Liter | 6,0 bis 6,5 |
| Knolle bis Ernte | bis 2,2 | nach EC nachdosieren | 6,0 bis 6,5 |
Zeitplan und Beobachtungen
Pflege
- Samen in den Schlitz legen, Deckel drauf: Kohlrabi keimt im Dunkeln.
- Nach dem Keimen nicht unter 15 °C stellen, sonst kann er schießen.
- Stützring setzen, sobald die Knolle dicker wird.
- Erst EC messen, dann pH.
- Gleichmäßig versorgen, dann platzt die Knolle nicht.
- Draußen bei Hitze Pumpe 15 an / 15 aus.
Kohlweißling: grüne Raupen fressen Löcher in die Blätter. Draußen Netz über den Tower.
Erdflöhe: viele kleine runde Löcher, die Käfer springen weg. Früh im Jahr oder ab August pflanzen.
Kohlblattläuse: grau, mehlig, an Blattunterseiten. Mit Wasser abbrausen.
Weiße Fliege: kleine weiße Fliegen unter den Blättern, drinnen und im Gewächshaus. Gelbtafeln aufhängen.
Mein Tipp für draußen: Einen Ring mit 40 cm Durchmesser auf den Deckel schrauben und ein feines Netz mit Reißverschluss darüber spannen.
Wasserwechsel und Messwerte
| Datum | Wasser neu | EC | pH |
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| Datum | Wasser neu | EC | pH |
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Kohlrabi profile
| What | Value |
|---|---|
| Plant group | brassica, leafy plant with bulb |
| Germination type | dark germinator, cover seeds lightly |
| Germination temperature | 20 to 25 °C |
| Germination time | about 5 days for me |
| Seeds per cube | 1 |
| pH value | 6.0 to 6.5 |
| EC value | 0.6 (seedlings) to 2.2 (bulb) |
| Light indoors | 200 to 300 PPFD, 14 to 16 hours |
| Water temperature | 18 to 22 °C |
| Harvest | about 10 to 12 weeks after sowing |
| Special point | needs a support ring, the bulb gets heavy |
You’ll find all values to print on the growing card above. The values for all other plants are in the pH and EC table (German).
My tip for your kohlrabi
Lots of space and lots of water: the HydroFlex Tower 9+
37 litre tank and room for 27 plants and more. Kohlrabi needs lots of nutrient solution in the bulb phase, with the big tank you top up less often.
Your growing plan
Here you can see all phases at a glance. Tap a phase, below I explain each step in detail.
Step 1 · in the seedling box
Let kohlrabi germinate warm and dark
- One seed per cube.
- Put the seed in the slit.
- Lid on the seedling box, then it lies in the dark.
- Place warm, 20 to 25 °C.
- As soon as it germinates: lid off, lamp on.
Not warm for too long: After germination the box comes away from the warmth, otherwise the seedlings get long and thin.
What you need now: seedling box with lid or for many plants the seedling mat with 100 cubes.
Read in detail ›Step 2 · in the seedling station
Raise strong young plants
- Put the cube into the net pot.
- Start with 1 ml nutrient per litre.
- From the third leaf go to 2 ml per litre.
- Air pump runs day and night.
- Lamp 14 to 16 hours.
Not too cold: If young kohlrabi stand below 12 °C for a longer time, they already set up flowering. Then later they bolt instead of forming a bulb.
What you need now: Seedling station.
Read in detail ›Step 3 · into the tower
Transplant with support ring
- Transplant with 4 to 5 true leaves.
- Net pot with cube into the planting spot.
- Fit a support ring straight away.
- Slowly increase to EC 1.7.
Fit the ring at the same time: The ring covers the pot against light and later holds the heavy bulb.
What you need now: Support rings from my shop.
Read in detail ›Step 4 · the bulb grows
Supply evenly
- Raise the EC step by step up to 2.2.
- Top up with ready-mixed nutrient solution.
- Cut off lower, yellow leaves.
- Outdoors a net against cabbage whites.
Even means tender: Big jumps in water or EC make the bulb split.
Read in detail ›Step 5 · harvest and enjoy
Harvest when the bulb is nice and round
- Cut the bulb off above the ring.
- Take the young leaves too.
- Dispose of cube and roots.
- Clean the net pot, put in a new young plant.
Don’t wait too long: If a bulb stands too long, it gets woody. Big varieties stay tender longer.
Read in detail ›1Germinating in the seedling box
about 5 days · warm and dark
I start kohlrabi in foam cubes. Exactly one seed goes into each cube, because later each plant grows on its own in a planting spot. I wet the cubes well beforehand and squeeze them lightly, they should be moist but not dripping.
Kohlrabi is a dark germinator. Put the seed in the slit of the cube, with the lid on the seedling box it lies in the dark. It germinates best at 20 to 25 °C, for example on a heat mat or in a warm spot. After about 5 days the seedlings are up.
What I use for this: the seedling box with lid from my shop. If you want to raise more plants, use the seedling mat with 100 cubes.
2Seedlings in the station
from the seed leaves · about 3 to 4 weeks
Once the seed leaves are open, I put the cubes into the net pots of the seedling station. The roots grow quickly through the cube into the water. An air pump brings oxygen into the nutrient solution and runs day and night.
Now temperature is important. Kohlrabi likes 15 to 20 °C for seedlings. If it stays colder than about 12 °C for a longer time, the young plant already sets up flowering. Later it then bolts instead of forming a thick bulb. On a cold windowsill in winter that happens quickly.
How I feed my kohlrabi
I start with 1 ml of Frankie’s nutrient per litre, which gives an EC of about 0.6 with my soft water. From the third true leaf I go to 2 ml per litre, then the EC is about 1.1. I change the nutrient solution in the station every 7 days.
If the leaves turn light and pale green, nutrients are missing. Then I add a little nutrient. Always measure the EC first and then the pH, because the nutrient lowers the pH.
Without a meter you’re just guessing. A combination meter shows you pH, EC and temperature at a glance.
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3Move to the tower
with 4 to 5 true leaves
When the kohlrabi has 4 to 5 true leaves and the roots have grown strongly out of the pot, it goes into the tower. I simply put the net pot with the cube into the planting spot.
Kohlrabi gets heavy. Without support it tips over or slips in the pot. That’s why I fit a support ring right when transplanting: open the ring at the predetermined breaking point, bend it open slightly, slide the stem in from the side and press the ring into the pot. The ring also covers the pot against light, so fewer algae form.
Thin and flexible, they adapt to the stem and hold kohlrabi, strawberries, tomatoes and cucumbers securely in the pot.
To the support rings ›Pump timing
Indoors my pump runs for 5 minutes, then pauses for 30 minutes. The roots stay moist and get enough air in between. If the tower stands outdoors in blazing sun, I switch to 15 minutes on and 15 minutes off.
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HydroFlex Tower for indoors and outdoors, seedling box, seedling station, support rings and my own nutrient. After you buy, I’ll help you on WhatsApp.
To the shop ›4The bulb grows
until harvest · supply evenly
As soon as the bulb gets thicker, kohlrabi needs more nutrients than lettuce. I raise the EC step by step: in growth to about 1.7, when the bulb grows up to 2.2. With Frankie’s nutrient and soft water that’s a good 3.5 ml per litre, after that you dose by EC.
Top up instead of changing
In the running tower I only top up what the plants have drunk, and straight away with ready-mixed nutrient solution. I only change it completely if the EC rises for no reason or a deficiency appears.
The lower, old leaves turn yellow at some point. That’s normal, I cut them off. That way more air reaches the bulb and pests find fewer hiding places.
| Your water | EC without nutrient | What you do |
|---|---|---|
| soft | below 0.3 | feed normally, measure the pH more often at first |
| medium | 0.3 to 0.5 | feed by EC, a little less room is left for nutrient |
| hard | above 0.5 | mix half and half with rain or RO water |
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5Harvest
about 10 to 12 weeks after sowing
How big the bulb can get depends on the variety. Harvest early varieties while they’re about the size of a tennis ball. Big varieties like Superschmelz stay tender even as a thick bulb. I like to let my kohlrabi grow nice and big.
I cut the bulb off directly above the ring. Cube and roots go, I clean the net pot, and a new young plant goes in. I eat the young leaves too, they taste like mild kale.
Light and location
Indoors kohlrabi needs a lamp. 200 to 300 PPFD at leaf height is enough for it, about as much as lettuce. With a strong lamp 14 hours of light a day are enough, with a weaker one leave it on for 16 hours, but never longer.
If light is missing, the stems get long and the bulb stays small. Hang the lamp at the side of the tower, so all levels get light. A small fan provides air movement and makes the plants stronger.
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Outdoors
Outdoors kohlrabi grows best in spring and autumn. In great heat the risk that the bulb gets woody rises, and the pests are especially active then. That’s why I plant brassicas very early in spring or only from August. In autumn the kohlrabi stays outside, because what was outdoors doesn’t come into my house.
Pests on kohlrabi: what I look out for
Brassicas are popular with pests. Indoors you have little of that, outdoors you need to protect kohlrabi.
No brassicas without a net: I screw a ring with a 40 cm diameter onto the lid and stretch a fine-mesh net with a zip over it. That keeps the cabbage white away. Flea beetles crawl through any gap though, so I plant brassicas at times when they’re less active.
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Common problems and what helps
The bulb gets woody
Usually it stood too long or it was too hot. Harvest in time and choose a variety known to be tender for summer. Lack of water also makes the bulb fibrous, never let the tank run empty.
The bulb splits
That happens with strong fluctuations: first too little, then suddenly a lot of water or nutrient. Top up regularly with ready-mixed nutrient solution and raise the EC in small steps.
Kohlrabi shoots up
If it forms a flower stalk instead of a bulb, it was too cold for too long when young. Keep seedlings above 15 °C, especially on the windowsill in winter.
Brown spots in the bulb
Brown or hollow spots inside indicate boron deficiency. Brassicas need boron, a complete nutrient with trace elements covers that. Check the pH too, if it’s too high the plant takes up boron less well.
Brown leaf edges inside
Dry edges on the young heart leaves are usually calcium deficiency due to too little evaporation. Provide air movement with a fan and keep humidity below 60 percent.
Pale or yellow leaves
Measure the EC first. If it’s too low, nutrients are missing. The lowest, old leaves turning yellow, on the other hand, is normal.
Fancy more kohlrabi? You’ll find the HydroFlex Tower 9+ with 37 litre tank in my shop ›
FAQ about kohlrabi in hydroponics
Can you grow kohlrabi in a hydro tower?
Does kohlrabi need light or darkness to germinate?
At what temperature does kohlrabi germinate?
How long does kohlrabi take to germinate?
How many seeds go into one cube?
What pH value does kohlrabi need?
What EC value does kohlrabi need?
How much nutrient does kohlrabi need?
When does kohlrabi go into the tower?
Does kohlrabi need a support ring?
How long does it take until harvest?
When is kohlrabi ready to harvest?
Why is my kohlrabi getting woody?
Why is my kohlrabi splitting?
Why is my kohlrabi bolting?
Can you eat kohlrabi leaves?
How much light does kohlrabi need indoors?
Which pump times do I use for kohlrabi?
How do I protect kohlrabi from the cabbage white?
How warm should the nutrient solution be?
You’ll find more vegetables without soil under vegetable growing in the hydro tower. You’ll find the right HydroFlex Tower for your space in my shop.
Continue with your own growing
168 pages, simply explained for beginners and with videos on the topics (in German). Exactly how I grow myself: from sowing to harvest.
More about the book ›A few questions about space, location and what you want to harvest, and you get a recommendation.
To the tower advisor ›Questions about your setup? Message me on WhatsApp.
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