If your lettuce has thrown up a tall central stalk and turned bitter, it's bolted — switched from growing leaf to growing flowers. In a temperate garden this is less a fault than a near-certainty for anything sown straight into the December-March heat: the trigger is long daylight combined with rising temperature, and once the stalk appears, the bitterness that comes with it can't be undone.
The temperate summer core is the real driver
Lettuce's sowing window here runs September to March, harvest November to May — the same Egmont-sourced dates used in every NZ zone, so the calendar itself isn't the differentiator. What is different is how long and how reliably hot the middle of that window runs. Temperate summers hold a long December-to-March stretch of consistent warmth, not the shorter, more broken hot spells further south — so a lettuce sown straight through that core doesn't occasionally bolt, it bolts about as a rule. The practical shift this demands is treating the hottest ten or so weeks as a gap in your succession schedule, not a stretch to sow through and hope.
The frost-risk period here runs April to November, which matters more for what it rules out than what it causes: it confirms the growing season's cold end is well clear of the sowing window, so bolting in a temperate lettuce patch is a heat-and-daylight problem start to finish, not a cold-related one.
What's actually pushing it to flower
Ranked by how often each one explains a lettuce running to seed:
- Day length as much as heat. Lettuce switches to flowering in response to lengthening daylight, not temperature alone — which is why a plant can bolt even in a run of days that don't feel unusually hot.
- A sowing landing mid-heat. Anything sown into or left growing through the warm core meets both triggers together and runs up fast.
- Dry soil. Letting the bed dry out accelerates the switch to flowering, and separately sours the flavour even before a stalk appears.
- Crowding or a rough transplant. Any added stress on the plant tips it toward bolting sooner than it would under steady conditions.
- Growing a standard fast variety through summer. Types without any bolt tolerance run up in the heat far quicker than ones bred to resist it.
Bolted, or just bitter?
Worth separating before you act: a bolted lettuce has a firm flower stalk and bitter leaf together. But dry soil alone can turn a lettuce bitter with no stalk at all. If there's no stalk, the fix is watering, not timing — the advice below on bolting itself doesn't apply yet.
Cultivar picks for the hot months
Variety choice does real work here, since the summer window is long enough that a fast standard type simply won't outrun it:
- Coral Red (Kings Seeds NZ) is sold as a slow-bolting variety, built for sowing across the warmer months rather than only the cooler shoulders.
- Drunken Woman Fringed Head (Kings Seeds NZ) carries supplier-noted tolerance to bolting as well as tipburn — a fair pick for a sowing that has to sit through real heat.
- Buttercrunch (Kings Seeds NZ) is described by the supplier as staying free of bitterness, useful insurance against the dry-soil bitterness problem even outside full bolting.
- Devil's Ear (Koanga Institute) is noted as coping well with heat, and Koanga's own instructions call for sowing it under shade through summer — pair that with real shade cloth rather than treating the variety as protection on its own.
- Cos (Kings Seeds NZ) is a straightforward choice for the cooler shoulder months, where the long-window bolting problem isn't really in play.
Working around a long hot core
- Sow the shoulders, skip the peak. Run successions through the cooler parts of the window and stop new sowings before the December-March core rather than fighting it directly.
- Water on a schedule, not by eye. A temperate summer dries beds out fast; mulch and water at the base so the plant never runs short between waterings.
- Put shade cloth up before the heat lands, not after a plant has already started to stress.
- Take heads as soon as they're ready. Holding a finished lettuce in the ground through a hot stretch just to delay harvest is the most common way to lose it to bolting.
- Once a stalk shows, harvest straight away. There's no walking it back — the leaf gets more bitter the longer it stands.
For next season
If this year's sowing bolted earlier than expected, that date is useful information — it's a fair guide to moving your last "safe" pre-summer sowing earlier next year, and your first post-summer one accordingly later. Reserve the slow-bolting cultivars above for anything that has to sit through December-March, and save the standard fast types for spring and autumn, where they'll size up before the heat becomes the deciding factor.
More on this zone: see the full NZ Temperate Planting Calendar →.