Kohlrabi
Brassica oleracea var. gongylodes
Also known as: German turnip, Turnip cabbage, Stem turnip
Quick facts
- Category
- roots bulbs
- Difficulty
- beginner
- Days to harvest
- 50 to 75 days
- Harvest type
- single harvest then replant
- Spacing
- 25 cm between plants
Environment
- Temperature
- 8–24°C
- pH
- 6 to 7
- EC (hydroponic)
- 1.5 to 2.5 mS/cm
- Daily light
- 14 to 20 mol/m²/day
Climate and zones
- USDA zones
- 3 to 10 (winter low around -40°C or warmer)
- Frost tolerance
- frost hardy (handles regular frost)
- Season
- cool (spring and fall crops)
Viable growing environments:
- outdoor year-round (in zone)
- outdoor in growing season (annual)
- unheated greenhouse / hoop house
- heated greenhouse
- indoor (heated home)
- indoor hydroponics under grow lights
USDA zone bounds reflect outdoor year-round survival. Anywhere outside the bounded zone range, this crop still grows as an annual in the warm months (outdoor_seasonal), under cover (greenhouse), or indoors under lights.
Growing systems
Kohlrabi works in:
- media bed (ebb and flow)
- wicking bed
- soil bed
- drip / Dutch buckets
Growing media
The substrate the roots sit in. Choice depends on the system (clay pebbles don't fit NFT channels; rockwool isn't used in media beds) and the crop (kohlrabi works in the media listed below).
| Medium | pH effect | Water retention | Bacterial surface |
|---|---|---|---|
| Expanded clay pebbles (LECA) | neutral / inert | low | high |
| Coco coir (Coconut coir) | slightly acidic | high | moderate |
| Soil-based mix (Potting soil) | varies by source | high | high |
Bacterial surface area matters for aquaponics: clay pebbles, lava rock, and pumice double as biofilter substrate. Low-surface media (rockwool, perlite, pea gravel) work in hydroponics but need a separate biofilter in aquaponics.
Nutrient demand by stage
NPK ratios are relative weights at each growth stage; the nutrient mix calculator scales them to absolute grams or ml. EC targets shift through the plant's life: seedlings need a much lighter solution than fruiting adults.
| Stage | N | P | K | EC target (mS/cm) |
|---|---|---|---|---|
| seedling | 1 | 1 | 1 | 0.8 |
| vegetative | 2 | 1 | 3 | 2 |
Companion-growing notes
- Heavy uptake of potassium, calcium. Co-grown crops with the same demand will end up deficient even at "correct" EC. Plan around this in shared reservoirs.
Aquaponics suitability
Compatible with typical aquaponics nutrient profiles. Fish waste provides enough nitrogen for healthy growth; supplemental potassium, calcium, and iron may still be needed depending on fish stocking density.
Care notes
A fast, easy hydroponic crop that's underappreciated outside German and Asian cuisine. Media beds or containers with 10 cm depth (the bulb forms at the surface but roots extend downward). EC 1.5-2.5 mS/cm. pH 6.0-7.0. Temperature: 12–24°C (cool-season; above 28°C the bulb becomes woody and strong-flavored). Moderate light (DLI 14-20 mol/m2/day). From transplant to harvest: 45-60 days for standard varieties, 35-45 for mini varieties ('Kolibri', 'Quickstar'). Harvest when the bulb is 6–10 cm diameter (standard) or 4–6 cm (mini types). Overmaturity causes woodiness and cracking. Mini kohlrabi is gaining popularity with commercial hydroponic growers because it matures faster, produces consistently tender bulbs, and sells well at farmers' markets ($3-5 each) as a trendy, Instagram-friendly vegetable. The leaves are a bonus harvest. Pest and disease issues are minimal compared to heading brassicas. A fun, fast crop for growers who want something different from the standard lettuce-and-tomato lineup.
Verified against: rhs-uk. Last reviewed 2026-05-15.