Growing kohlrabi in USDA Zone 9 means gardening in one of the warmest zones in the country: average winter lows of 20 to 30°F, an average last spring frost of Jan 30–Feb 15 and an average first autumn frost of Dec 1–15, giving a frost-free window of roughly 290–320 days. Frost dates vary by microclimate, elevation and year — check your local extension service.
Kohlrabi — the odd, swollen-stemmed brassica that tastes like a mild cross between cabbage and broccoli — is a cool-season crop that does well in Zone 9's mild winter. It matures in 50–60 days, it tolerates a light frost, and it gives you a crisp, versatile vegetable in the months when warm-season crops are finished. Many Zone 9 gardeners overlook it, which is a shame, because it is one of the easiest brassicas to grow here.
When to plant kohlrabi in Zone 9
In Zone 9, start indoors in August to September for autumn transplanting, or direct sow September to October and transplant out September to October for autumn crops; January to February for a spring crop.
The table below puts the whole season on one page, worked back from the Zone 9 frost averages and the crop's published days to maturity (50–60 days). Use the frost date calculator to check the averages for your own area.
| Stage | Zone 9 window | Notes |
|---|---|---|
| Start indoors | Start indoors in August to September for autumn transplanting, or direct sow September to October | Germination takes 7–10 days |
| Direct sow outdoors | September to February, in the cool months | — |
| Transplant outdoors | September to October for autumn crops; January to February for a spring crop | — |
| Days to maturity | 50–60 days | — |
| Cold tolerance | frost-tolerant | — |
Starting and planting
Kohlrabi can be started indoors or direct-sown in Zone 9. For an autumn crop, start seed indoors in August to September and transplant in September to October, or direct-sow in September to October. For a spring crop, sow or transplant in January to February. Thin seedlings to the final spacing once they are up. Kohlrabi needs steady growth — plants that stall from drought or crowding produce small, woody bulbs, so do not let them check.
Spacing, sun and soil
Space plants 6–9 inches apart. Crowded plants compete for light, water and nutrients, stay small, and invite disease through poor airflow, so thin seedlings to the final spacing once they are established. Use the plant spacing calculator to work out how many plants fit in your bed at that spacing before you sow.
This crop needs full sun — at least six hours of direct sun a day where possible. In Zone 9, a south-facing bed or an open spot away from trees and fences gives the strongest growth; part-shade types do better with morning sun and afternoon shade.
Soil should be deep, fertile and free-draining, with a pH of 6.0–7.5. Test your soil through your local extension service before amending it, and dig in plenty of compost, which improves drainage in heavy soil and holds moisture in light soil.
Watering
Water needs are moderate: aim for steady, even moisture rather than feast-and-famine watering. Keep the soil evenly moist throughout the growing period — irregular watering is the commonest cause of woody, cracked kohlrabi. Water at the base in the morning and mulch to hold moisture steady. In a dry spell, water deeply rather than sprinkling a little each day.
Companion planting
Good companions for kohlrabi include marigolds, basil, onions and lettuce. The companion planting finder can suggest combinations for your whole bed. Avoid planting kohlrabi where others of the same family (Brassicaceae) grew last year: rotate crops each season to reduce soil-borne disease and pest build-up.
Common pests and problems
Watch for cabbage worms, aphids, flea beetles and slugs. Inspect plants weekly — look under leaves, where small pests hide, and at the base of stems. Hand-pick larger pests, wash aphids off with a strong jet of water, and remove badly affected leaves rather than leaving them to spread a problem. Beyond pests, the main problems are weather-related: bolting in a hot spell, slow growth in cold soil, and damping off in seedlings kept too wet.
Harvest and storage
Harvest at 50–60 days from sowing, when the swollen stem is about the size of a tennis ball — that is when it is at its crispest and sweetest. Left too long, kohlrabi turns woody and tough, so check plants regularly and pick promptly. Pull or cut the whole plant, trim off the leaves (which are edible too, cooked like other greens), and peel the bulb before eating it raw or cooked.
Succession-sow every two to three weeks from September to February so you always have young bulbs coming. In the coldest nights a little row cover protects young plants, but established kohlrabi handles Zone 9's winter lows of 20–30°F well.
Frequently asked questions
When should I plant kohlrabi in Zone 9?
Sow or transplant according to the table above, working from the average last frost of Jan 30–Feb 15 and the average first frost of Dec 1–15. These are zone averages — check your local extension service before planting out.
Can kohlrabi survive a frost in Zone 9?
Cold tolerance for this crop is listed as frost-tolerant. Hardy crops can take a light frost, frost-tolerant crops can take a light frost once established, and tender crops are killed or badly damaged by frost, so use row cover on cold nights.
How long do kohlrabi take to grow in Zone 9?
Germination takes 7–10 days and maturity takes 50–60 days from sowing or transplanting, according to standard published ranges.
When is kohlrabi ready — how big should the bulb be?
About the size of a tennis ball, usually 50–60 days after sowing. That is when kohlrabi is at its crispest and sweetest. Left much longer it turns woody and tough, so harvest promptly and succession-sow every few weeks for a steady supply rather than letting a whole bed over-mature at once.
See the October planting guide for Zone 9 for what else to sow that month, and when to plant rutabaga in Zone 9 for a related crop.