When to Start Preparing for Fall Kimchi Vegetables: Cabbage and Radish Sowing Timeline
The success or failure of your fall kimchi vegetable harvest is largely decided in July and August — weeks before the seeds go in the ground. Napa cabbage (baechu, Brassica rapa subsp. pekinensis) and Korean radish (mu, Raphanus sativus var. longipinnatus) are unforgiving of timing errors: sow too early and they bolt in summer heat; sow too late and they fail to mature before the first frost. This guide provides the definitive sowing timeline for both crops, explains the soil and compost preparation that must happen before sowing, and gives you a week-by-week action plan starting from mid-July.
Table of Contents
- Understanding the Timing Window
- The Soil Preparation Timeline (Start in July)
- Napa Cabbage (Baechu): Sowing and Establishment
- Korean Radish (Mu): Direct Sowing Guide
- Compost Application for Maximum Yield
- Week-by-Week Action Plan: July–September
- Quick Reference Summary
- Frequently Asked Questions
- References
Understanding the Timing Window
Both napa cabbage and Korean radish are cool-season crops that perform best at soil temperatures between 10°C and 20°C. In most Korean climate zones (and similar temperate climates), this window occurs in autumn: September through November. The crops must be established before temperatures regularly drop below 10°C at night.
Working backward from harvest:
- Napa cabbage: requires 70–90 days from transplant to harvest
- Korean radish: requires 55–70 days from direct sowing to harvest
- Target harvest window: mid-October to mid-November (for kimchi-making season)
Therefore:
- Napa cabbage seeds must be started late July to early August (transplanted late August)
- Korean radish must be direct sown by late August to early September
The Soil Preparation Timeline (Start in July)
Proper bed preparation must begin 3–4 weeks before transplanting or sowing. Starting in mid-July gives you ideal timing.
Why July Preparation Matters
Both brassicas and radishes benefit from deep, nutrient-rich, well-drained soil. Compost incorporated 3–4 weeks before planting has time to partially integrate with soil structure and begin releasing nutrients before the root systems of seedlings arrive.
Soil pH Requirement
Both crops strongly prefer a soil pH of 6.0–7.0. Brassicas are particularly sensitive — at pH below 5.5, clubroot disease becomes active. Test your soil pH in July; if needed, add agricultural lime (calcium carbonate) at this stage. Lime requires 4–6 weeks to fully adjust soil pH.
Napa Cabbage (Baechu): Sowing and Establishment
Starting from Seed Indoors (Recommended)
Sow napa cabbage seeds in cell trays 4–5 weeks before planned transplant date. Use a seed-starting medium with 20–30% mature compost for early nutrition.
Temperature requirements for germination: 20–25°C (excellent for late July/early August indoor sowing)
Seedling care:
- Provide strong light (minimum 6 hours direct sun or equivalent grow light)
- Water consistently — do not allow seedlings to dry out
- Fertilize with diluted compost tea at 2-week intervals once first true leaves emerge
Transplanting Timing and Spacing
Transplant when seedlings have 4–5 true leaves (approximately 4–5 weeks after sowing). Plant in the evening or on a cloudy day to reduce transplant stress.
| Spacing | Result |
|---|---|
| 40 × 40 cm | Medium-sized heads suitable for home kimchi |
| 50 × 50 cm | Large heads; requires richer soil and more water |
Korean Radish (Mu): Direct Sowing Guide
Korean radishes are strictly direct-sown — they do not tolerate root disturbance from transplanting. Prepare the bed deeply (30–40 cm) to allow the long, thick root to develop without obstruction.
Sowing depth: 1–2 cm Spacing after thinning: 20–25 cm between plants
Critical step — soil loosening: Use a fork or broadfork to loosen soil 40 cm deep before sowing. Remove all stones and compacted clods. Any obstruction will cause forked, deformed roots.
Compost application for radish: Work 5 cm of mature compost into the top 20 cm of soil. Avoid fresh manure or immature compost — excess nitrogen causes radishes to produce lush tops but small, forked roots.
Compost Application for Maximum Yield
What Type of Compost to Use
Ideal: Fully mature compost (dark, crumbly, earthy-smelling, no recognizable materials). Work 5–7 cm into the top 30 cm of soil.
Acceptable: Semi-mature compost worked into soil 6+ weeks before planting.
Avoid: Fresh compost or fresh kitchen scraps directly added to planting area — excess nitrogen and heat from active decomposition burns young roots.
Application Method
Broadcast 5 cm of compost over the bed surface and fork it into the top 25–30 cm of soil. Water thoroughly to integrate compost. Allow 3–4 weeks for integration before sowing.
Week-by-Week Action Plan: July–September
| Week | Action |
|---|---|
| Week 1 (Mid-July) | Test soil pH; clear old summer crop debris; add lime if needed |
| Week 2 (Late July) | Apply and fork in 5–7 cm mature compost; start baechu seeds in cell trays indoors |
| Week 3 (Early August) | Thin and fertilize baechu seedlings with compost tea; prepare radish bed |
| Week 4 (Mid-August) | Deep-loosen radish bed; baechu seedlings have 3 true leaves |
| Week 5 (Late August) | Transplant baechu seedlings; direct sow mu (radish) seeds |
| Week 6–8 (September) | Thin radish seedlings; monitor baechu for pests; side-dress with compost |
Quick Reference Summary
| Crop | Start Seeds | Transplant/Sow | Days to Harvest | Target Harvest |
|---|---|---|---|---|
| Napa Cabbage (Baechu) | Late July – Early August | Late August | 70–90 days | Mid-October – November |
| Korean Radish (Mu) | Direct sow only | Late August – Early September | 55–70 days | October – November |
Frequently Asked Questions
Q: Can I still have a successful harvest if I start in mid-August instead of July? A: You can, but your margin for error narrows. Starting mid-August for cabbage means transplanting in mid-September, pushing harvest to late November or December when frost risk increases. In warmer climates, this may be acceptable. In colder climates, protect late-season crops with row cover or cold frames.
Q: Do I need to use chemical fertilizers for napa cabbage? A: No. Well-prepared compost-amended soil provides all the nutrients baechu needs. Side-dress with additional compost or compost tea at 4 weeks after transplanting if plants show slow growth or yellowing leaves.
Q: Can I use kitchen compost directly from my electric composter for this? A: The material from an electric composter (like Reencle) is a pre-compost that still needs outdoor curing for 2–4 weeks before it is fully stable. Use it as a soil amendment incorporated at least 6 weeks before planting, or let it cure in an outdoor pile for 4 weeks first.
Q: What pests should I watch for on fall kimchi vegetables? A: The main pests are cabbage white butterfly caterpillars (Pieris rapae) and aphids. Row covers during the seedling stage prevent most butterfly damage. Aphids can be controlled with insecticidal soap spray or by encouraging natural predators (ladybugs, lacewings).
References
RDA (Rural Development Administration, Korea). Vegetable Cultivation Standards: Brassica and Radish. https://www.rda.go.kr/
University of California Cooperative Extension. Cool-Season Vegetables: Cabbage and Radish. https://ucanr.edu/
Cornell Cooperative Extension. Brassica Production Guide. https://cce.cornell.edu/
Brady, N.C., & Weil, R.R. (2008). The Nature and Properties of Soils (14th ed.). Pearson Education.
National Institute of Agricultural Sciences, Korea. Soil Management for Vegetable Production. https://www.nias.go.kr/
