This is a different problem from a plant that bolts to flower — a head that just stays undersized or loose points to competition and nutrition during formation, not heat forcing the plant to flower outright. Tight spacing is the most common fixable cause: broccoli planted too close together trades individual head size for plant numbers whether you meant it to or not. A short nitrogen supply runs a close second, since broccoli's appetite for it is genuinely high. Warm weather during head formation adds a third factor worth taking seriously in this zone specifically.

Ranked, most likely first

  1. Tight spacing. Crowded plants compete for light, water, and root room, and less space per plant means a smaller individual head — full stop, even on an otherwise well-cared-for planting. Growers use this trade-off deliberately to produce small bunching heads; it works against you the same way if the spacing wasn't planned.
  2. Nitrogen shortfall. Broccoli needs a lot of nitrogen to build a proper head, and running short shows up as pale leaves, stalled growth, and a small or loose head — in a bad case, no real head forms at all. It's a season-long need, not a one-off feed.
  3. Warm weather during formation. Broccoli heads form best in cool conditions. A run of warmer days while the head is developing — short of the sustained heat that pushes it into flower outright — can still leave it noticeably smaller and looser than it should be.

Why heat carries more weight here

Broccoli's timing doesn't change by zone — sow August to November, transplant September to December, harvest December to March, all Egmont-sourced — so there's nothing to point to in the calendar itself. What does shift by zone is how much warmth actually reaches the plant while a head is forming, and temperate summers run hotter than a cooler garden's by definition. That puts more of this zone's December-to-March harvest window into genuine heat-during-formation risk on top of the spacing and nitrogen factors that apply everywhere — worth checking alongside those two rather than assuming it's purely a feeding or crowding issue if the timing lines up with a hot spell.

Fixing it

  • Go back and measure your spacing. This season's heads won't grow any bigger, but it's the clearest lever for the next sowing — fewer plants per bed in exchange for noticeably larger individual heads.
  • Put broccoli on a proper feeding schedule. An early feed drives leaf growth, then a lighter follow-up as the floret develops — not a single heavy application. Pale, stalled growth is the tell to watch for before the head itself ever looks small.
  • Resist the urge to over-feed as a fix. More nitrogen doesn't automatically mean a bigger head on every variety, and heavy late-season feeding can actually work against tight head formation on some types.
  • Water evenly right through head formation. This is the window where patchy moisture does the most visible damage to final head size, and it matters more here given how much heat the season can also throw at the plant.

Cultivar notes

Belstar F1, McLaren F1, Purple Sprouting, Romanesco, and de Cicco are all raised in trays and transplanted per the sourced notes, without a specific claim tying any of them to head size or nitrogen sensitivity beyond the general advice above. Worth flagging regardless: Winter Green F1 is specifically noted not to be over-fertilised with nitrogen for best results, which is a useful check against assuming more feed always helps.

The temperate-side pattern for a head that bolts rather than stays small is different again — see Why Is My Broccoli Flowering Too Early in NZ Temperate?, or the cool/mountain version for how that plays out where heat matters less.

In a cooler garden, this same small-head problem leans more on spacing and nitrogen since summer heat is less of a factor there: Why Are My Broccoli Heads Staying Small in NZ Cool/Mountain?

Holes in the leaves instead of an undersized head point to a pest problem — see White Butterfly and Cabbage Moth on NZ Brassicas.

Next season

Give plants real room rather than crowding them in, and have a feeding plan ready before transplanting instead of reacting once growth already looks pale. If a hot stretch lines up with head formation, that's an extra reason to stay on top of watering here specifically, since the heat load through this zone's harvest window adds to whatever spacing and feeding are already doing.

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