Growing bok choy in USDA Zone 4 means working with a short, cool summer. Zone 4 has average winter lows of -30 to -20°F, an average last spring frost of May 1–15 and an average first autumn frost of Sep 15–Oct 1 — roughly 120–150 days frost-free. These are zone averages, not promises — check your local extension service first.
Bok Choy (Brassicaceae family) takes 45–60 days to maturity and 7–10 days to germinate. Bok choy is a cool-season crop that suits Zone 4 well: it goes in weeks before the last frost, grows through the mild weeks of late spring, and finishes before summer heat arrives. At 45–60 days to maturity it is ready by early summer. It tolerates part shade, which makes it useful in beds that lose the sun by afternoon — though in Zone 4 full sun is still fine, and often better, in the cool spring.
Zone 4 timing for bok choy
Use the frost date calculator and the harvest date calculator to confirm your dates.
| Average last spring frost | May 1–15 (range — check your local extension) | | Average first autumn frost | Sep 15–Oct 1 (range — check your local extension) | | Frost-free window | roughly 120–150 days | | Start indoors | 4–6 weeks before the average last frost, roughly late March to early April | | Direct sow outdoors | 2–4 weeks before the average last frost, roughly mid-April | | Transplant outdoors | After hardening off, roughly late April to early May | | Days to germination | 7–10 days | | Days to maturity | 45–60 days | | Expected harvest | Roughly late May to early July (from 45–60 days after sowing or transplanting) | | Cold tolerance | hardy |
The Zone 4 planting calendar shows what else shares these weeks.
When to sow bok choy in Zone 4
Bok choy can be started indoors in late March to early April, four to six weeks before the average last frost, or direct-sown in mid-April, two to four weeks before the last frost. Start indoors for the earliest harvest; direct-sown plants catch up quickly in warm soil. Harden transplants off for a week before setting them out in late April to early May.
Germination takes 7–10 days if conditions are right. In Zone 4, April and May soil can still be cold, which slows germination and can rot seed, so use the frost date calculator to pin down your own average dates.
Will bok choy fit the Zone 4 season?
With 45–60 days to maturity, a crop sown or transplanted around the last frost should be ready in roughly late May to early July — well before the first frost in Sep 15–Oct 1 and before the worst summer heat. A fall sowing in late July matures in the cool weeks of September. The same crop in Zone 3 faces a shorter season, so Zone 4 gives you a few extra weeks. It also pairs well in the garden plan with kohlrabi in Zone 4, which shares a similar sowing window in Zone 4. The April Zone 4 calendar shows the other jobs that share these weeks, so you can plan the whole bed at once.
Soil, spacing and sun
It needs part shade — at least six hours of direct sun a day, and more is better in a short season, where shade can cost you a harvest.
Space plants 8–12 inches apart — crowded plants compete for light, water and air. Work out how many plants fit in your bed with the plant spacing calculator. Soil pH should be in the range 6.0–7.5; test your soil rather than guessing. Work in compost before planting and avoid working soil when it is wet. Bok choy wants compost-rich, moist soil; dry soil gives small heads that bolt early. It tolerates part shade, so a bed that gets morning sun and afternoon shade works well in a hot summer. Space plants 8–12 inches apart so heads have room to form.
Watering bok choy in Zone 4
Water needs are moderate. Water deeply when the top few centimetres of soil are dry, rather than sprinkling a little every day, so roots grow down where the soil stays moist. Mulch holds moisture in and steadies soil temperature.
Water needs are moderate. Water deeply when the top few centimetres of soil are dry, and keep the soil steadily moist — dry spells push the plant to bolt. Mulch holds moisture in and keeps roots cool. Water in the morning at the base, not overhead, so leaves dry during the day. Ease off watering as the first frost approaches so plants harden.
Companion plants
Good companions for bok choy include marigolds, basil, onions, lettuce. Some, like marigolds, are traditionally planted to help confuse or deter pests. Do not follow bok choy with another Brassicaceae family crop in the same bed next year. Check a pairing with the companion planting finder before you commit bed space to it.
Common pests and problems
The common pests to watch for are cabbage worms, aphids, flea beetles, slugs. Check plants twice a week — a problem spotted early can be dealt with; two weeks later it may have cost you the crop. Cabbage worms are the pest to watch first — look for green caterpillars and ragged holes, and check leaf undersides for eggs. Hand-pick caterpillars, and cover young plants with fine insect netting if worms are a yearly problem. Slugs favour the low, leafy heads in damp weather. Start simple: hand-pick larger pests, wash aphids off with a strong jet of water, and remove badly affected leaves. Row cover keeps many flying pests off young plants and doubles as frost protection in May and June.
Harvest and storage
Cut the whole head at the base when it feels firm and full, or pick outer leaves as you need them and let the centre keep growing. Baby bok choy can be picked young, at about half size, for the most tender stems. Harvest in the morning and use the same day or store briefly in the fridge. Count 45–60 days from sowing or transplanting, then check the plant itself — in Zone 4 that usually means roughly late may to early july. Harvest in the morning and handle the crop gently, as bruised produce stores badly.
Frequently asked questions
When should I plant bok choy in Zone 4?
Start from the average last frost of May 1–15. 2–4 weeks before the average last frost, roughly mid-April Confirm your dates with the frost date calculator.
How long does bok choy take to grow in Zone 4?
About 45–60 days to maturity, with germination in 7–10 days — usually a harvest in roughly late may to early july. A cold spring can add days, so check the plant itself.
What should I plant near bok choy in Zone 4?
Good companions include marigolds, basil, onions, lettuce. Avoid another Brassicaceae family crop in the same bed two years running.
Can I grow bok choy in Zone 4 if my spring is late?
Yes, but start from the frost dates, not the calendar. The same crop in Zone 3 faces an even shorter season, so Zone 4 is the easier of the two.