When to start cabbage seeds

How many weeks before the last frost?

University of New Hampshire Extension gives cabbage 4 to 6 weeks from seed to transplant and says it is safe to set out 4 weeks before the last frost, so seeds go in about 8 to 10 weeks before your last frost date. With an average last frost of May 10, you would sow indoors March 1 to March 15 and set plants out April 12.

University of Minnesota Extension explains the reason for the indoor start: if you wait until the weather settles and sow later, spring cabbage matures in the hottest part of summer and quality suffers.

Germination

Cabbage germinates over a wide temperature range. Harrington's data (published by Oregon State Extension) give a 40°F minimum and an 85°F optimum. Seedlings appeared in about 15 days at 50°F, 9 days at 59°F, 6 days at 68°F and 4 days at 77°F. Iowa State Extension lists 65–70°F with emergence in 4 to 7 days.

Sow a quarter inch deep (UMN). Iowa State notes cabbage seed germinates in darkness, so cover it.

Keep seedlings cool

UMN says there is no need for a heat mat. Ordinary room temperatures of 60°F to 70°F and bright overhead light give the best plants. Higher temperatures cause fast growth and tall, weak seedlings that are hard to handle without damage.

Begin feeding when the first true leaf appears, using a half-strength starter solution once a week, then twice a week after two true leaves (UMN). When plants have about five true leaves, roughly three weeks after sowing according to UMN, reduce watering and start hardening off.

Hardening off and transplanting

Put plants outside where they get a couple of hours of sun and wind protection, and expose them to more sun over a week or two. UMN advises bringing them indoors if night temperatures drop below 40°F during this period. Transplant 15 to 18 inches apart at the same depth they grew in the pot, and water in with a starter solution high in phosphorus.

Fall cabbage and direct sowing

For a fall crop, UMN direct seeds cabbage in early July in Minnesota. Cabbage takes 60 to 100 days to mature, and direct-seeded plants can take up to three weeks longer than transplants. Sow three seeds every 15 to 18 inches, a quarter to half inch deep, thin to one, and keep seedlings watered through summer heat. Elsewhere, use the fall planting calculator to count back from your first fall frost.

Common mistakes

• Growing seedlings too warm. Warmth gives leggy, fragile transplants.
• Starting in late spring. Heads that size up in midsummer heat are lower quality.
• Uneven moisture. UMN notes cabbage needs to take up water and nutrients steadily as it grows.
• Same spot every year. Rotate cabbage-family crops (broccoli, cauliflower, kale and others).