Iceberg Lettuce Takes 70 to 80 Days: Count Back From Your First 75°F Highs to Get a Head That Closes

Iceberg lettuce takes roughly 70 to 80 days from sowing to a firm head, and it has to spend that whole stretch in cool weather. Iowa State Extension puts harvest at 70 to 80 days after sowing, and Botanical Interests lists ‘Great Lakes 118’ at 68 to 82. Illinois Extension calls crisphead lettuce “extremely sensitive to heat” and says it “must mature before the first hot spell of summer.” Leaf lettuce, by Illinois’s count, is ready in 50 to 60 days, and it still gives you a salad if a heat wave arrives early. A struggling iceberg gives you a bolted stalk, a loose rosette or a head that’s brown on the inside.

That is why University of California sources call crisphead “difficult to grow in home gardens.” This spoke of our zone-by-zone lettuce growing guide covers what iceberg needs that other lettuce doesn’t: the clock, what heat does to a forming head, how to count back from your own heat date to a sowing date, spacing, cultivars, and what a failed head is telling you.

How Many Days Does Iceberg Lettuce Take to Form a Head?

Plan on 70 to 80 days from sowing, about 10 to 12 weeks, but read every seed packet by asking where its clock starts. Catalogs and extension pages don’t count from the same day.

The takeaway: a “57-day” iceberg isn’t two weeks faster than a 70-day one when the 57 starts at transplanting. Add the 3 to 6 weeks it takes to grow a transplant (UC Davis says 3 to 4 weeks, Utah State 5 to 6) and the two catalogs describe a similar plant.

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The type-by-type gap matters more than any one catalog figure. Crisphead needs about 20 more days than leaf lettuce, and those extra days all fall in the stretch where the weather is warming up.

Why Does Iceberg Need Cool Weather the Whole Way?

Heat does three separate things to an iceberg: it flips the plant from leaves to a flower stalk, it stops the head from closing, and it starves the inside of the head of calcium. Each one has its own mechanism, and each one ruins a different crop.

Heat triggers the flower stalk. A 2022 BMC Genomics study induced bolting in lettuce seedlings with 14 hours of light at 33°C (91°F) and 10 hours of dark at 25°C (77°F), against a control of 20°C (68°F) days and 13°C (55°F) nights. The authors describe the rapid stem elongation as irreversible. Day and night temperature changed together, so the study can’t say which one matters more, but the dark period in the hot treatment was a warm 77°F night. Illinois Extension adds that seedstalks come from “a combination of long days, warm temperatures and age.” Age is the sneaky one: an iceberg that sits for months in cold soil is still getting older.

Heat can stop the head closing at all. A 1996 Tropical Agriculture trial grew six iceberg cultivars in warm tropical conditions. With root zones at 24 to 30°C (75–86°F), no cultivar formed heads, and the plants bolted about 25 days after transplanting. Cooling the root zone to 15°C (59°F) around the clock produced compact heads in only two of the six. Cooling for 20 hours a day instead of 24 gave irregular heads or bolting. Root-zone cooling is a laboratory technique, not a garden tip. What it shows is how narrow the margin is, and that the cultivar matters.

Heat starves the center of the head. Tipburn, the brown, rotting leaf edges inside a head, is a calcium shortage in fast-growing tissue even when the soil has plenty of calcium, according to UC IPM. Calcium travels with the water the plant transpires, and “head formation reduces transpiration in the inner leaves of iceberg lettuce,” so the innermost leaves get the least. UC Davis’s home-garden sheet lists high temperatures and high soil fertility as aggravating factors and notes tipburn is “not visible from outside of head.” You find out when you cut it. UC IPM adds that foliar calcium sprays don’t work in head lettuce, because the spray can’t reach the interior in time.

Is Cold Weather the Cure? The Open-Head Problem

No. “Cool” is a range, and the sources give slightly different ones.

None of these sources puts the good range above about 75°F, and only the seed company and the UC factsheet give night temperatures. There’s no official bolting switch, but 75°F highs are a workable warning line. I’d treat it as a rule of thumb rather than a measured threshold.

The opposite failure is documented too. Dutch researchers growing winter iceberg under glass found that heads often closed and then opened again. In one experiment, ‘Cristallo’ grown with a cold root zone (6 or 10°C, about 43–50°F) until heading began and a warmer one afterward produced about 14% open heads with a modest temperature jump, about 24% with a bigger one and about 36% with the biggest (6°C to 18°C). Keeping roots above 14°C (57°F) throughout produced few open heads. The cultivar mattered about as much as the temperature swing: open-head rates ran from 9.4% for ‘Marbello’ to 37.7% for ‘Starlina’ in the same glasshouse.

That is a winter greenhouse study, so it doesn’t transfer neatly to a spring bed. My reading, and it’s an inference rather than a finding: a hardened transplant that sits in cold soil and then meets a sudden warm spell is the pattern most likely to leave you with a head that never closes. The authors note that an iceberg’s growing point is “very close to soil level,” so soil temperature reaches it directly.

Iceberg vs Butterhead vs Loose-Leaf: What Can You Ignore?

Loose-leaf lets you ignore nearly all of this, because you’re harvesting leaves rather than waiting for a head to close. UC Davis says leafy types are more tolerant of heat. A butterhead forms a loose head that “will never become compact,” by the same source, and develops bitterness readily when it’s too hot.

If your cool window is shorter than about 10 weeks, grow leaf or butterhead lettuce. Our guide to butterhead vs romaine and which bolts first covers the other heading types, and the lettuce types guide covers summer crisp (Batavian) lettuces, which have heat tolerance bred in.

How Do You Time Iceberg Backward From the Heat?

Take the date your daytime highs usually settle above about 75°F and subtract 70 to 80 days. The result is your latest sowing date, not your target. The table below is our own arithmetic on Iowa State’s 70 to 80 days, so check it against your local climate normals.

Treat each date as the latest you can be and still make it. Early-spring cold slows growth, and I found no source that quantifies by how much, so aim a week or two earlier if your weather allows. Illinois Extension’s advice is to start spring transplants indoors or in a cold frame and set them out “as early in the spring as the weather settles.” Transplants should have 4 to 6 mature leaves first, according to Utah State and UC.

Direct seeding in warm soil backfires. UC Davis warns that seed sown during high temperatures may go dormant, and our guide to growing lettuce from seed and the 80°F germination ceiling explains the mechanism. For a fall crop, Illinois says plants started in late summer to mature in cooler fall weather “have a much better chance of success.” Our late-summer lettuce sowing guide covers the seed-chilling trick for that start.

How Far Apart Should You Plant Iceberg Lettuce?

Space iceberg about 12 inches apart. That’s the one number inside every extension range we checked.

Crowding costs you twice. Holmes Seed says too little spacing prevents proper head formation and cuts airflow, and that dense heads trap moisture, which invites bottom rot, downy mildew and soft rot. Because lettuce is shallow-rooted, UC Davis recommends frequent, light waterings and morning overhead watering, an organic mulch rather than black plastic once the soil warms, and as little nitrogen as you can get away with. Utah State’s tipburn advice is the same in one line: keep soil moisture even.

Which Iceberg Cultivars Give a Home Garden the Best Odds?

Pick a cultivar sold as slow to bolt, because timing can only do so much. Seed catalogs make strong claims, so treat the notes below as the sellers’ and not as trial data.

I found no published open-head or tipburn rates for home-garden cultivars. The Dutch numbers above came from glasshouse cultivars, but their lesson holds: cultivar choice alone swung open-head rates about fourfold.

Why Isn’t My Iceberg Lettuce Forming a Head?

Match the symptom to the table before you change anything. The first four rows are heat-and-timing failures, the rest are rots.

When a head is dead on arrival, our lettuce problems guide covers the pests and diseases that overlap with these.

How Do You Know When to Cut an Iceberg Head?

Cut it when it feels firm but not hard. UC Davis says to harvest heading types when firm and warns that hard heads lose flavor and other quality traits. A crisphead is ready “when leaves overlap to form a head similar to those available in groceries,” in the Virginia and Maryland extension wording. Squeeze gently: if it gives like a soft ball, give it another week, and if it feels like a stone, cut it now.

Don’t judge a home head against the grocery store. Commercial growers cut at about 2 pounds, per Oregon State, while the glasshouse heads in the Dutch trials weighed 480 to 557 grams (roughly 1.1 to 1.2 pounds). Cut at the base, and don’t expect regrowth: our cut-and-come-again guide is for leaf types, not heads.

The other problem is timing. UC Davis puts it bluntly: “Thirty heads of iceberg lettuce harvested at once can present a major storage problem!” Botanical Interests suggests sowing every 3 weeks for continuous harvests. Oregon State says commercial head lettuce keeps 2 to 3 weeks at 32°F after precooling. UC Davis says lettuce in general keeps about 2 weeks in a fridge, picked dry and early in the day, unwashed, in a plastic bag in the crisper.

Frequently Asked Questions

How long does iceberg lettuce take to grow?

About 70 to 80 days from sowing, according to Iowa State Extension, with catalog figures running from 68 to 82 days. Some catalogs count from transplanting instead of sowing, so check which one a packet means.

Why is my iceberg lettuce not forming a head?

Heat is the most common cause: heading slows above roughly 75°F and the plant may bolt instead. Crowding, water stress and a cold-then-warm swing can also leave a loose head. The symptom table above sorts these out.

Can you grow iceberg lettuce in summer?

Rarely. Illinois Extension says crisphead lettuce must mature before the first hot spell of summer and does better when started in late summer for a fall crop. If your summers are hot, leaf types tolerate more heat, so grow those instead.

How far apart do you plant iceberg lettuce?

Plant it about 12 inches apart. Extension ranges run from 8 to 15 inches in the row, and 12 inches falls inside every one of them. Rows 14 to 18 inches apart are the most common recommendation.

Can you eat lettuce that has bolted?

You can, but it won’t taste good. Bolted lettuce turns bitter, according to the UC factsheet and Holmes Seed, and the head stops developing. Pull it and replant for cooler weather.

Is iceberg lettuce harder to grow than other lettuce?

Yes. UC Davis and Illinois Extension both rank crisphead as the hardest lettuce type for home gardens, because it needs a long, cool season and a closed head to count as a harvest. Leaf and butterhead lettuce forgive more mistakes.

Sources

1. Haynes, C. Let Us Grow Lettuce! June 17, 2009. Iowa State University Extension and Outreach
2. Illinois Extension. Lettuce (Home Vegetable Gardening). University of Illinois Extension
3. UC Davis Vegetable Research and Information Center. Lettuce (Lactuca sativa), Home Vegetable Gardening. University of California
4. UC Agriculture and Natural Resources. Know What You Grow! Lettuce (factsheet, 2021). University of California
5. UC IPM. Tipburn, Lettuce Pest Management Guidelines. University of California Statewide IPM Program
6. Drost, D. How to Grow Lettuce in Your Garden, April 2020. Utah State University Extension
7. Utah State University Extension. Other Production Problems, Leafy Greens (Utah Vegetable Production and Pest Management Guide)
8. Relf, D., McDaniel, A. and Olsen, E. Leafy Green Vegetables (Publication 426-408). Virginia Cooperative Extension
9. University of Maryland Extension. Growing Lettuce in a Home Garden (updated March 25, 2026)
10. Oregon State University Department of Horticulture. Lettuce (Oregon Vegetables)
11. Hao, J. et al. Molecular basis of high temperature-induced bolting in lettuce revealed by multi-omics analysis. BMC Genomics 23:580 (2022)
12. Lee, S.K. and Cheong, S.C. Inducing head formation of iceberg lettuce (Lactuca sativa L.) in the tropics through root-zone temperature control. Tropical Agriculture 73(1) (1996)
13. Maaswinkel, R.H.M. and Welles, G.W.H. Factors affecting head formation of iceberg lettuce (Lactuca sativa L.). Netherlands Journal of Agricultural Science 35:37-42 (1987)
14. Holmes Seed. Why Is Head Lettuce Considered Difficult to Grow? Holmes Seed Company
15. Botanical Interests. Great Lakes 118 Crisphead Lettuce Seeds. Botanical Interests
16. Johnny’s Selected Seeds. Crispino Organic Lettuce Seed. Johnny’s Selected Seeds