Heat is the most likely cause. Once a properly formed head hits sustained warm weather, it races to open small yellow flowers within days, and once that's underway there's no bringing it back. If the head was always small before it flowered, that points to something earlier instead — a seedling held too long before transplanting, primed toward flowering by cold while it was still too small to form a proper head. Either way, once you see flower buds opening between the beads, harvest immediately — it's still edible cut right away, but won't hold or improve from there.

Ranked, most likely first

  1. Heat, once the head has properly formed. Broccoli grows best in cool conditions, and sustained warm weather pushes a formed head to flower within days. This is the dominant cause and what most gardeners mean by broccoli "bolting."
  2. Premature vernalization — "buttoning" — from an overgrown seedling. A young plant left in its tray past the point it should have gone out, or otherwise stressed, can be primed toward flowering by cool temperatures while still too small for a proper head, producing a small head that flowers well ahead of schedule.
  3. Water or nutrient stress. Drought or inconsistent watering can speed up either route above once it's already under way, though it's rarely the sole trigger on its own.

Why heat is the sharper risk here

Broccoli's calendar doesn't shift by zone — sow August to November, transplant September to December, harvest December to March, all Egmont-sourced — so timing isn't the differentiator. Heat is: temperate's summer runs warmer than a cooler zone's by definition, and that heat lands squarely inside this harvest window. A head sitting in the ground through a genuinely hot December or January stretch here is running real bolting risk, more so than the same head further inland or at altitude. That's the reason a heat-tolerant variety and steady watering through any hot spell carry more weight in this zone specifically than they would somewhere cooler.

Fixing it

  • Cut the moment flower buds appear. Small yellow flowers between the beads mean the head is committed — harvest now rather than waiting for it to fill out further.
  • There's no reversing heat-driven bolting once it's started — only heading it off next time. Pick a heat-tolerant variety for a warm-season sowing, mulch to keep the root zone cooler, and water deeply and consistently through hot stretches instead of letting the plant dry out.
  • If the head was undersized from the start, check your transplant timing. Move seedlings out at the recommended age rather than holding trays longer than needed — an overgrown, pot-bound seedling is more likely to have already been primed toward flowering before it's even in the ground.
  • Keep watering consistent across the whole season, not just when it's obviously hot — uneven moisture stresses the plant regardless of temperature and can speed up either route to early flowering.

Cultivar notes

Kings Seeds NZ's own descriptions split cleanly on this exact problem. McLaren F1 is described as a warm-season broccoli with "good vigour and heat tolerance," and Summer Green F1 as "heat tolerant and very uniform" — both genuine picks for this zone's warmer stretches. Belstar F1 "performs best under cooler conditions" and Winter Green F1 is described as cold tolerant, better suited to a cooler sowing than the height of a temperate summer. Waltham is explicitly noted as heat sensitive, worth avoiding for a hot-season sowing here specifically. Sprouting Tasty Stems F1 and Sprouting Summer Purple both carry spring-through-summer sow windows without a heat note either way.

Holes rather than flowering too early is a different problem — see White Butterfly and Cabbage Moth on NZ Brassicas if that's what's actually showing up on your plants.

Next season

Match the variety to the sowing — a heat-tolerant pick like McLaren F1 or Summer Green F1 for a warm-season sowing, saving Belstar F1 or Winter Green F1 for cooler timing. Transplant on schedule rather than letting seedlings sit, since an overgrown tray-bound plant is already at a disadvantage before it's even in the ground.

More on this zone: see the full NZ Temperate Planting Calendar →.