How to Grow Kohlrabi from Seed to Harvest
Find a video version of this growing guide on my YouTube channel HERE.
I feel like this is such an underrated veggie crop and more people should try growing it. Kohlrabi is part of the brassicas family just like broccoli and cabbage. It was bread by humans over the years to grow a big edible above the surface bulb instead of a flowering head like broccoli. I find it way easier to grow than broccoli. Every part of the plant is edible, but what is most desired is the bulb.
Uses for kohlrabi - Raw kohlrabi can be grated and used in salads, shredded into slaws, added to soups, steamed, roasted. It has a mild broccoli/cabbage like flavor. Crunchy & juicy. Greens can be eaten like collard greens or feed to your chickens.
Cultivar Selection
⦁ Differences between color - green, white, or purple skin. All have a white interior.
⦁ Heat tolerance - some say purple color veggies tolerate more heat than the normal cultivars so I grow the purple.
⦁ Size - standard size is ready to harvest when they are 3 inches in diameter but there are some giant kohlrabi cultivars.
Sowing Seeds
⦁ Direct Sowing is preferred but can start in seed trays if your careful. It is not technically a root crop because it forms a bulb above ground, but you have to be careful not to disturb its tap root.
When to Sow Seeds
General guidelines are to direct sow seeds right after your last spring frost date or 3 months before your first winter frost date.
⦁ Check out www.plantmaps.com to get find your average last spring frost date.
⦁ Growing any kind of brassicas is challenging for warmer zones +8. They require 4 months in a row of cool temperatures. For zones +8 that is fall, winter, and spring. If they are growing when there is too much heat, they get stunted in growth.
⦁ If your in a warm zone, grow during its ideal temperature range AND grow the heat tolerant cultivars for better success. Ideal temps is 65°F to 75 °F. EXAMPLE - in my garden www.plantmaps.com shows that November thru March have average max temperature below 80F. I direct sow seeds outside October thru November and usually harvest January thru February. Goal is to harvest before temps exceed 80F starting in April.
⦁ SITE SELECTION - Pick a spot that has direct sunlight. This is going to be North to South orientation. Mine are growing in a spot that gets very bright morning sun with some afternoon shade.
⦁ SOIL - prefers lots of organic matter. If grown in sandy soil you must fertilize to keep nutrients consistent and stay on top of watering.
⦁ DIRECT SOWING - Make an indent every 1 foot. Sprinkle seeds over the surface, press down to make contact with the soil, keep watered for good germination. After they have germinated, thin out to 1 foot apart.
⦁ TRANSPLANTS - If your going to start seeds for transplants - Start them in doors in 72 cell seed trays. They are ready when the root system fills up the cell.
Zones 1 to 7 - Goal is to transplant 2 weeks before your last spring frost. So back track a total of 10 weeks before your last spring frost date.
Zones +8 - If your in warmer zones like me, and have a target date to transplant them outside in October, then back track 10 weeks which lands you at end of July or August. It is too hot outside, so you must start seeds in doors. Plant them 1 foot apart.
⦁ They don't like loose soil. Before direct sowing seeds, just push down on the soil to slightly compact it down. If your planting transplants, gently push the soil down around the stem after planting.
Water Requirements
They need at least 1 inch of water each week during the growing season from rainfall or irrigation. Always soak the soil thoroughly when watering. In Florida we grow this during fall, winter, and spring which is our dry season so just keep an eye on them. If the plants get overheated or dry out, they will produce chemicals that have a bitter flavor. The bulbs will also become fibrous and can crack.
Growth Habit
They get bushy. The more leaves the better. More leaves = more energy production = bigger roots. Do not prune unless leaves are diseased or damaged.
Fertilizing
A good application of something high in nitrogen (blood meal) at the beginning is essential to promote a lot of rapid leafy growth. ONLY initially. Then half way thru the growing period, switch to fertilizers higher in potassium and phosphorus which encourage large bulb formation.
Pests
Not too many pest issues when grown during the correct time of year.
⦁ Nematodes - microscopic worms that attack the roots cutting the plant off from water and nutrients. They especially love warm, wet, sandy soils. Add organic material into your soil. Rotate crops. Do not plant nematode sensitive crops in the same spot year after year. If needed, solarize during the summer with a clear plastic tarp. Add crab meal to the soil. Grow lots of marigolds and use them in your compost or "chop and drop" marigolds into your soil.
⦁ Be prepared if you get worms. The leaves will look munched on or you will find small orange/green/black balls on the leaves (worm eggs and poop). You probably wont find the worms because they tend to be nocturnal. Spray with BT (bacillus thurigensis) or Spinosad. Both are considered organic.
⦁ flea beetles - chew small holes in leaves. cover with row covers.
Diseases
⦁ Rotting roots - Moist conditions are usually the culprit. Grow in containers or raised beds helps. Allow the soil to dry out in between watering.
⦁ Club root - affects a lot of brassicas. The roots at the base look misshapen/swollen and has a club shape. Remove the entire plant with roots immediately because there is no cure and they will not recover. They are now a host for the pathogen and will spread it to other plants. Don't compost with infected plants.
⦁ Leaf diseases such as mildew, blight, or leaf spots. Spray with 1C hydrogen peroxide (3%) per gallon of water.
Harvest
Harvest when they are 3 inches in diameter. They might get too woody or bitter once they are bigger than 3 inches.
Storage
⦁ Store in the fridge for 2 weeks. Cut off the leafy greens immediately or they will continue to suck moisture out of the bulb making it limp. Wash them really good. Can soak in water with vinegar to kill any bacteria. Allow them to dry 100% before storing in a with a piece of paper towel to absorb extra moisture.
⦁ Can store for months in a cellar if you can maintain 36F and 95% humidity.
⦁ It can be diced, then par boiled or blanched to be frozen.