Choy sum
Brassica rapa var. parachinensis
Also known as: Yu choy, Chinese flowering cabbage, Caisin, Cai xin, Flowering Chinese cabbage
Quick facts
- Category
- leafy greens
- Difficulty
- beginner
- Days to harvest
- 35 to 55 days
- Harvest type
- cut leaves, plant regrows for repeated harvests
- Spacing
- 20 cm between plants
Environment
- Temperature
- 15–28°C
- pH
- 6 to 7
- EC (hydroponic)
- 1.3 to 2.2 mS/cm
- Daily light
- 13 to 19 mol/m²/day
Climate and zones
- USDA zones
- 5 to 10 (winter low around -29°C or warmer)
- Frost tolerance
- tolerates light 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
Choy sum works in:
- deep water culture (rafts)
- NFT channels
- media bed (ebb and flow)
- wicking bed
- soil bed
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 (choy sum works in the media listed below).
| Medium | pH effect | Water retention | Bacterial surface |
|---|---|---|---|
| Rockwool (Mineral wool) | alkaline until pre-soaked | very high | low |
| Expanded clay pebbles (LECA) | neutral / inert | low | high |
| Coco coir (Coconut coir) | slightly acidic | high | moderate |
| Net pot, no medium (Bare-root) | - | - | - |
| 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 | 0.7 |
| vegetative | 3 | 1 | 2 | 1.8 |
Companion-growing notes
- Heavy uptake of 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
One of the fastest and easiest Asian greens for hydroponic production. EC 1.2-2.0 mS/cm. pH 5.5-6.8. Temperature: 15–25°C (cool to moderate; bolts in sustained heat above 28°C, but unlike lettuce, the flowers are part of the harvest so early bolting is less problematic). Moderate light (DLI 12-18 mol/m2/day). From seed to harvest: 4-6 weeks, making it one of the fastest brassica crops. NFT, DWC, raft, and vertical systems all work. Harvest the whole plant or cut the main stem and allow side shoots to develop for a second harvest. The speed and ease make choy sum ideal for commercial hydroponic operations targeting Asian vegetable markets. Succession plant every 2-3 weeks for continuous supply. Nutrient demand is moderate; standard leafy green formulation. Flea beetles and caterpillars are potential pests in open greenhouses. For hydroponic growers near Asian communities, choy sum is a high-demand, fast-turnover crop with consistent market pricing ($4-8/kg). In aquaponics, choy sum grows vigorously in warm, nutrient-rich water.
Verified against: rhs-uk. Last reviewed 2026-05-15.