A lettuce that suddenly sends up a tall central stalk has bolted. It has stopped making leaf and started making flowers. The main triggers are long summer days and heat, and drought, crowding or a rough transplant all speed it along. Once the stalk appears there's no reversing it, and the leaves turn bitter as part of the same change.

The fix is mostly about timing: when you sow, when you pick, and which variety goes in for the warmest weeks.

Bolted, or Only Bitter?

Check for the stalk first. A bolted lettuce has a firm stem rising from the centre, and the leaves taste bitter along with it. A lettuce can also turn bitter with no stalk at all, from dry soil, heat or age. That's a different problem with a different fix, covered in Why Is My Lettuce Bitter? NZ Causes and Fixes.

What Triggers Bolting

These come from extension-service guidance on lettuce in general, not NZ-specific trials. Most likely first:

  1. Long days and heat together. Lettuce is a cool-season crop, and it reacts to lengthening days as much as to temperature. That's why a plant can bolt through a stretch that doesn't feel especially hot.
  2. A sowing that lands in the warmest weeks. Succession sow late enough and the crop meets both triggers at once. It often runs up before it reaches a useful size.
  3. Dry soil. Letting the bed dry out hurries the switch to flowering.
  4. Crowding or transplant shock. Tight spacing, a rough move from tray to bed, or any other stress tips a warm plant over sooner.
  5. The wrong variety for summer. Standard fast types run up in heat far sooner than lettuces bred to be slow-bolting.

The Sowing Window Is the Same Everywhere

Our planting data has one lettuce window for the whole country: sow from September to March and harvest from November to May. The supplier's own growing notes say to succession sow every 2–3 weeks and use shade cloth in summer to prevent bolting.

So the calendar doesn't change by region. What changes is how long the risky part of summer lasts.

By Region: How Long the Risk Lasts

This is our reading of how heat and season length differ around the country, not measured data.

Northland, Auckland and coastal Taranaki (sub-tropical). The heat and humidity are the strongest and longest-lasting here, so the safe sowing stretch is the narrowest. For any summer sowing, shade and steady moisture are close to essential rather than optional.

Most of the country (temperate). The December to March warm spell runs long and reliably hot. Lettuce sown straight through it bolts as a rule, not now and then. Leave a gap in your succession over the hottest weeks rather than relying on shade cloth alone.

Central Otago, Queenstown and the Central Plateau (cool/mountain). Summers are cooler and shorter, so bolting usually comes from a short, sharp hot spell landing on a late sowing. The short season tempts you to keep sowing through the warmest weeks, and that's the sowing most likely to run up.

What to Do Now

  • Stalk already up? Harvest straight away. The leaves only get more bitter from here.
  • Mature heads in the bed? Pick them as soon as they're ready rather than holding them through a hot spell.
  • Hot weather arriving? Get shade cloth over the bed before the plants show stress.
  • Soil drying out? Mulch and water at the base on a regular schedule.

Varieties That Buy Time

For sowings that have to sit through summer, choose a lettuce bred or noted for it. These descriptions are from the suppliers' own listings.

  • Coral Red (Kings Seeds NZ), a Lolla Rossa type described as slow bolting and suitable for growing year-round.
  • Drunken Woman Fringed Head (Kings Seeds NZ), an Italian heirloom with "good tolerance to tipburn and bolting".
  • Devil's Ear (Koanga Institute), noted as standing well in heat. Koanga's own notes say to sow it in summer under shade, so pair it with shade cloth rather than relying on the variety alone.
  • Cos (Kings Seeds NZ), a reliable choice for spring and autumn sowings, when bolting risk is lowest anyway.

Next Season

  • Sow either side of the hottest weeks and pause new sowings through the middle.
  • Save fast standard types for spring and autumn, and use slow-bolting ones for anything that will stand through summer.
  • Note the date of any sowing that bolted early. It tells you how much earlier to make your last summer sowing next year.

For sowing times in your zone, see the planting calendar for the sub-tropical north, most of the country or the cool/mountain south and inland.