When to start cucumber seeds

Direct sowing is usually best

University of Minnesota Extension calls direct seeding the best way to start cucumbers. Use a soil thermometer and sow after the last frost date, once the soil is at least 70°F at a one-inch depth. University of New Hampshire Extension gives the same 70°F soil rule. In much of Minnesota that is late May, but the date depends on your soil, not the calendar.

Sow about half an inch deep. For vining types, UMN sows seeds 2 inches apart and thins to 8 to 12 inches, or plants "hills" of three or four seeds with 5 to 6 feet between hills. Bush types can go in rows or hills only 2 to 3 feet apart. Training vines up a 3 to 4 foot trellis saves space and gives straight fruit.

Starting indoors

If you want a head start, UNH gives 3 to 4 weeks from seed to transplant, setting out 1 to 2 weeks after the last frost. With an average last frost of May 10, you would sow indoors April 19 to April 26 and set plants out May 17 to May 24. UMN's guidance is firm: start no earlier than four weeks before the last frost date, use peat pots you can plant directly, and transplant before the plant outgrows its container without damaging the taproot. Cucumbers set into cold soil grow poorly, so a transplant only helps if the garden is warm.

Germination

Cucumbers need warmth. Harrington's data (published by Oregon State Extension) list a 60°F minimum, with little or no germination below that. Seedlings appeared in about 13 days at 59°F, 6 days at 68°F, 4 days at 77°F and 3 days at 86°F. Iowa State Extension lists 75–80°F with emergence in 3 to 10 days.

Getting an earlier start outdoors

UMN suggests black plastic mulch, laid after you prepare the soil in spring, to warm it so you can plant earlier; cut slits and sow through them. Spun row covers over seedlings raise the air temperature and protect from cold nights and pests. Remove them once flowering begins so bees can pollinate, unless the variety is seedless.

Watering and care

Cucumbers are mostly water, and steady moisture keeps fruit coming. The garden watering calculator turns the common one-inch-per-week guideline into gallons for your bed. Pick fruit often; letting cucumbers grow oversized slows the plant down.

Common mistakes

• Sowing into cold soil. Below 60°F, seed sits in the ground and may fail to come up.
• Starting indoors too early. Big, root-bound cucumber seedlings transplant badly.
• Disturbing roots. Use plantable pots and handle the root ball gently.
• Leaving row covers on during flowering. Most cucumbers need insect pollination.