When to Plant Cucumbers

Soil temperature decides, not the calendar

Clemson notes that cucumber seed will not germinate in soil colder than 50°F. The extension services set the planting threshold at different points: Colorado State, NC State and Iowa State plant at 60°F, Utah State waits for 65°F and Clemson for 70°F. Utah State says seeds germinate in 5 to 10 days with soil between 65 and 85°F.

Measure with a soil thermometer pushed to seed depth in mid-morning, and look for several days in a row at or above 60°F. In most of the country this happens 1 to 3 weeks after the average last frost, which is why the dates for your zone above begin after your frost date. Colorado State’s rule is to wait at least a week past the frost date and longer if the weather is chilly. NC State says transplants should go out no earlier than two weeks after the last frost.

Iowa State gives a useful regional example: cucumbers go in around mid-May in central Iowa, a week earlier in the southern part of the state and a week later in the north. The same logic applies anywhere. Your local frost date sets the outer limit, and soil warmth sets the actual day.

Planting earlier with plastic and covers

You can gain two to three weeks in spring. Lay black plastic mulch to warm the soil, then plant through it and cover the bed with row covers or plastic tunnels. Utah State says this combination lets you plant 2 to 3 weeks before the last frost date. Watch the temperature under the covers: vent them when air temperatures pass 80°F, and take them off once plants flower or when it gets above 90°F inside, because heat reduces fruit set and bees need access to the flowers.

Transplants are the other route to an early harvest, since they bear about two weeks before seeded plants. Start seeds indoors only 2 to 4 weeks ahead, in peat pots or other plantable containers. Iowa State plants them out at the one- or two-true-leaf stage, after a few days of hardening off.

Timing a fall crop

Spring vines often slow down or quit by late summer. A midsummer sowing gives fresh plants that bear until frost. To find your last sowing date, the University of Nebraska counts back from the first expected fall frost the variety’s days to harvest, plus about 14 days because growth slows in cooler, shorter days, plus another 14 days because frost-tender crops need to mature at least two weeks before frost. For a typical 50- to 60-day cucumber, that means sowing about 11 to 13 weeks before the first frost. Iowa State puts the last practical sowing date for central Iowa in July.

Late plantings have a bonus. Iowa State notes that cucumbers planted after mid-June there get less cucumber beetle damage because the spring generation of beetles has passed. On the other hand, Nebraska warns that beetle and squash bug numbers climb again in August and can wipe out young seedlings, so cover fall sowings with row cover until they bloom.

The South and the desert Southwest

In the Southeast, cucumbers fit two windows. Clemson plants spring cucumbers from April 15 to June 5 in the South Carolina Piedmont and March 15 to May 15 on the coast, with fall plantings from August 1 to September 30 in the Piedmont and through August on the coast. Early spring planting also helps the crop finish before leaf diseases peak in midsummer.

In the low desert around Phoenix, the University of Arizona schedules cucumbers from mid-February to mid-April and again from mid-August to mid-September, skipping the summer months when heat cuts fruit set.

Warm-winter zones 9 to 11

Where frost is rare, cucumbers become a cool-season crop. UF/IFAS recommends March to April and August to September in North Florida, January to March and September in Central Florida, and September through February in South Florida. In zone 9 elsewhere, follow the same pattern: a late-winter to spring planting and a late-summer to early-fall planting, with the hottest months left empty.