A lettuce doesn't need to bolt to turn bitter. Heat, a bed that's dried out between waterings, or a head left standing past its best all push the plant to build up more of the bitter compound in its leaves on their own — bolting just adds to whichever of those is already happening. If your plant has a firm central flower stalk, that's the dominant cause and it has its own article: Why Is My Lettuce Bolting in NZ Cool/Mountain?. No stalk, just an off taste — that's what this one covers.

What's behind it, ranked

  1. Heat. Sustained temperatures in the 70–80°F (21–27°C) range push the plant into stress and more bitter-compound production — this happens well ahead of any flower stalk showing.
  2. A bed that swings between dry and wet. Lettuce roots sit shallow and need steady moisture. Let the soil dry out between waterings and bitterness follows even in mild weather.
  3. A head left too long before picking. The bitter compound builds as the plant matures from a low base when young — picked early, a head tastes noticeably milder than the same plant given another week or two.
  4. Bolting. A seedstalk forming brings bitterness with it as part of the same shift — if that's what you're seeing, it's the dominant cause rather than one of several contributing factors.
  5. Variety. Leaf and butterhead types generally outlast romaine in the heat, and romaine outlasts head-forming types — the more heat-tolerant the variety, the later bitterness tends to show up.

What the cool/mountain season changes

The sowing and harvest dates are the same everywhere in NZ — September to March to sow, November to May to harvest, Egmont-sourced — so there's no separate cool/mountain calendar behind this problem. What differs is how often the heat trigger actually fires. Summers here run cooler and shorter than further north, so heat-driven bitterness tends to show up around the same short, sharp hot spells that put bolting at risk, rather than sitting as constant background pressure through a long warm season. That's a narrower window of risk, not a smaller one — a lettuce standing through even a brief hot stretch here can turn bitter just as fast as one further north.

Watering lapses and letting a head sit too long don't get easier just because summer's milder — those two causes are as much a risk here as anywhere, and if bitterness turns up outside an obvious hot spell, they're the more likely explanation.

The frost season here runs March to November, a month longer than in warmer NZ zones, but it doesn't touch this problem directly — bitterness in lettuce is a heat-and-moisture issue at the warm end of the calendar, not the cold one.

Fixing it

  • Water to a schedule. Don't wait for the soil to look dry — that's already too late. Consistent moisture heads off the most common non-bolting cause.
  • Have shade cloth ready for hot spells, not just summer generally. Around 3–4 hours of direct sun with shade the rest of the day is enough to carry a plant through.
  • Pick while young. A head left "just a bit longer" is exactly how bitterness builds.
  • Already bitter? Wash and refrigerate before giving up on it. A day or two in the fridge lets much of the bitterness fade.
  • Reach for a heat-tolerant variety for any sowing that might meet a hot spell — see the picks below.

Cultivar picks

  • Buttercrunch (Kings Seeds NZ) is described by the supplier as staying free of bitterness — a solid first pick when bitterness itself, not bolting, is the recurring issue.
  • Coral Red (Kings Seeds NZ) is bred and marketed as slow-bolting, which buys time against the bitterness that comes with flowering too.
  • Drunken Woman Fringed Head (Kings Seeds NZ) carries supplier-noted tolerance to bolting and tipburn, useful cover if a hot spell catches a head unexpectedly.
  • Devil's Ear (Koanga Institute) is noted as standing up well in heat; Koanga's own instructions call for sowing it under shade in summer, worth pairing with real shade rather than counting on the variety alone.
  • Cos (Kings Seeds NZ) is a fine choice for the cooler shoulder months, where this problem is least likely to arise anyway.

Next season

Keep an eye on the forecast during the sowing window rather than treating summer as uniformly risky — a short hot spell is the thing to plan around here, not the whole season. Water on a fixed schedule through it, harvest promptly once heads are ready, and keep a heat-tolerant variety in reserve for whichever sowing ends up standing through the next warm stretch.

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