Growing kohlrabi 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.
Kohlrabi (Brassicaceae family) takes 50–60 days to maturity and 7–10 days to germinate. Kohlrabi is the odd one out in the cabbage family: the part you eat is the swollen stem, which grows above the ground like a pale green or purple bulb. In Zone 4 it is an easy, reliable crop — frost-tolerant, quick at 50–60 days to maturity, and unbothered by the cool spring. The key is steady water: dry spells make the bulbs woody, and nobody enjoys woody kohlrabi.
Zone 4 timing for kohlrabi
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 | 50–60 days | | Expected harvest | Roughly late June to early July (from 50–60 days after sowing or transplanting) | | Cold tolerance | frost-tolerant |
The Zone 4 planting calendar shows what else shares these weeks.
When to sow kohlrabi in Zone 4
Kohlrabi 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 kohlrabi fit the Zone 4 season?
With 50–60 days to maturity, a crop sown or transplanted around the last frost should be ready in roughly late June to early July — well before the first frost in Sep 15–Oct 1. A second sowing in mid-July matures in the cool weeks of September, when the bulbs stay sweet. 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 bok choy 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 full sun — 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 6–9 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. Kohlrabi wants compost-rich, well-drained soil and full sun. Space plants 6–9 inches apart — crowded plants give small bulbs. The swollen stem forms above the soil, so do not hill soil up around it the way you would with potatoes.
Watering kohlrabi 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, but steadiness matters more than volume: even moisture keeps the bulbs crisp and sweet, while dry spells followed by heavy watering can split them. Mulch holds moisture in and steadies soil temperature. 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 kohlrabi include marigolds, basil, onions, lettuce. Some, like marigolds, are traditionally planted to help confuse or deter pests. Do not follow kohlrabi 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. Flea beetles pepper young leaves with small holes in dry springs. 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
Harvest when the swollen stem is roughly the size of a tennis ball — bigger bulbs turn woody, so do not wait for size. Pull the whole plant, trim the leaves and peel the tough outer skin before eating raw or cooked. Use fresh; kohlrabi keeps a few weeks in the fridge. Count 50–60 days from sowing or transplanting, then check the plant itself — in Zone 4 that usually means roughly late june to early july. Harvest in the morning and handle the crop gently, as bruised produce stores badly.
Frequently asked questions
When should I plant kohlrabi 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 kohlrabi take to grow in Zone 4?
About 50–60 days to maturity, with germination in 7–10 days — usually a harvest in roughly late june to early july. A cold spring can add days, so check the plant itself.
What should I plant near kohlrabi 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 kohlrabi 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.