A lettuce that suddenly sends up a tall central stalk and starts tasting bitter has bolted — switched from making leaf to making flowers. The trigger is long summer daylight combined with heat, and drought, crowding or a rough transplant all speed it along. Once the stalk shows, the bitterness is already underway and there's no reversing it.

Why lettuce bolts, most likely cause first

  1. Long daylight and heat together. Lettuce is a cool-season crop, and it responds to lengthening days as much as it does to temperature. That's why a plant can bolt even through a stretch that doesn't feel especially hot — the day length alone is part of the trigger.
  2. A succession sowing landing in the warm end of the window. Sow late enough and a crop meets both triggers at once, running up before it's reached a useful size.
  3. Drought stress. Letting the soil dry out pushes bolting along, and separately turns the leaf bitter even on a plant that hasn't fully run up yet.
  4. Crowding or transplant shock. Tight spacing, a rough move from tray to bed, or any other stress tips an already-warm plant toward bolting sooner.
  5. Variety choice. Standard fast types run up in heat far sooner than lettuces bred for slow bolting.

Bolting, or just bitter?

These are related but not identical, and it's worth telling them apart before you act. Bolting is the plant switching to flower — a firm central stalk, and the leaf turning bitter as a side effect of that switch. But lettuce can also turn bitter from drought stress alone, with no stalk in sight yet. If you're seeing bitterness without a flower stalk, the fix is watering, not timing — keep reading, but don't assume the plant has bolted just because the leaf tastes off.

What the cool/mountain calendar means here

Lettuce's sowing window runs September through March here, with harvest November through May — the same Egmont-sourced dates that apply in every NZ zone, so there's no separate cool/mountain calendar to learn. The window's own growing notes already flag the risk directly: succession sow every 2–3 weeks, and use shade cloth in summer to prevent bolting.

What does differ locally is how often that risk actually bites. Cool/mountain summers run cooler and shorter than further north, so bolting here is usually a short, sharp hot spell landing on a late succession sowing — not the sustained heat that makes it close to inevitable elsewhere. The catch is that a short growing season tempts gardeners to keep sowing right through the warmest weeks to make the most of it, and that's exactly the sowing most likely to run up. The frost-risk period here (March–November) also bookends the safe sowing window at both ends — sow too close to either edge and you're either fighting cold or heat, not the calm middle of the season lettuce actually wants.

Cultivars that buy you time

For a summer sowing where bolting is the real risk, cultivar choice matters more than usual:

  • Coral Red (Kings Seeds NZ) is bred and marketed as slow-bolting, suited to sowing right through the warmer months rather than just the shoulders.
  • Drunken Woman Fringed Head (Kings Seeds NZ) carries a supplier-noted tolerance to both tipburn and bolting — a solid pick for a late succession.
  • Buttercrunch (Kings Seeds NZ) is described by the supplier as staying free of bitterness, which is useful cover against the bitterness-without-full-bolting problem above.
  • Devil's Ear (Koanga Institute) is noted as standing up well in heat, and Koanga's own growing notes suggest sowing it under shade through summer — worth pairing with actual shade cloth rather than relying on the variety alone.
  • Cos (Kings Seeds NZ) is a reliable general-purpose pick for the cooler shoulders of the window, where bolting risk is lowest anyway.

Fixing it now

  • Keep succession sowing through the cooler stretches, and ease off before the hottest weeks. Don't sow straight through peak summer just to keep the rhythm going.
  • Water consistently. Mulch, and water the soil rather than the leaves — a dry spell does double damage here, speeding bolting and souring the flavour independently.
  • Get shade cloth over the bed once real heat lands, as the window notes themselves recommend.
  • Harvest the moment a head is ready. Don't leave a mature lettuce standing through a hot spell hoping to hold it a few more days.
  • If the stalk's already showing, cut your losses. Harvest immediately — the leaf only gets more bitter from here, and there's no treatment that reverses it.

Next season

Lean on the slow-bolting and heat-tolerant cultivars above for anything sown into the warmer half of the window, and keep the fast standard types for the cooler shoulders where they'll actually finish before running up. If a particular sowing bolted early this year, note the date — it's a reasonable guide to how much earlier to move your last "safe" summer sowing next time.

More on this zone: see the full NZ Cool/Mountain Planting Calendar →.