A head that stays small or loose without ever flowering is a different problem from one that bolts — this is about competition and nutrition during formation, not heat pushing the plant to flower. Overcrowded plants are the most common controllable cause: broccoli spaced too closely trades individual head size for plant numbers, whether that trade was intentional or not. Nitrogen shortfall runs a close second, since broccoli is a genuinely heavy feeder. Mild heat or patchy watering during formation can shrink a head too, though that's a smaller factor in this zone specifically than it would be somewhere warmer.

Ranked, most likely first

  1. Overcrowding. Plants spaced too closely compete for light, water, and nutrients — tighter spacing produces smaller individual heads even on an otherwise healthy plant. Commercial growers use exactly this to produce small bunching heads deliberately; the same trade-off works against you if the spacing wasn't planned.
  2. Nitrogen shortfall. Broccoli's nitrogen appetite is genuinely high, and a short supply shows as pale foliage, stunted growth, and small, loose heads — a severe shortfall can prevent a marketable head forming at all. It needs a steady supply through the season, not one heavy feed.
  3. Heat or inconsistent watering during formation. Broccoli heads develop best in a fairly narrow cool range; warmer conditions during formation can leave a head smaller and looser without pushing it all the way to flower. Patchy watering over the same stretch adds to the effect.

Why spacing and feeding matter more than heat here

Broccoli's calendar is the same in every NZ zone — sow August to November, transplant September to December, harvest December to March, all Egmont-sourced — so there's no timing edge to point to. What genuinely differs by zone is how much heat actually reaches the plant during head formation, and this zone runs cooler summers than a warmer garden by definition. That pushes heat down the list of likely explanations here specifically — a small head in cool/mountain is comparatively more likely to trace back to spacing or a nitrogen shortfall than to warm weather catching the plant mid-formation. Worth checking those two first before assuming the season's temperature is to blame.

Fixing it

  • Measure your actual spacing. There's no rescuing this season's undersized heads, but it's the first thing to correct for the next sowing — giving up some plants per bed for meaningfully larger individual heads.
  • Feed on a schedule, not once. An early feed supports leaf growth, then a moderate follow-up as the floret develops. Pale, stunted growth is the early warning sign — by the time the head itself looks small, the shortfall has already done its damage.
  • Don't overcorrect into heavy feeding either. More nitrogen isn't automatically better for every variety, and pushing hard late in the season can work against tight head formation on some types.
  • Keep watering even through the head-formation window specifically. This is the stretch where patchy moisture does the most damage to final head size, even if the rest of the season was fine.

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 variety-specific claim about head size or nitrogen sensitivity beyond what's already covered. The one genuine counterpoint worth flagging: Winter Green F1 is specifically noted not to be over-fertilised with nitrogen for best production, a reminder that more feed isn't a blanket fix across every type.

If the problem is actually the plant flowering rather than staying small, that's a different mechanism — see Why Is My Broccoli Flowering Too Early in NZ Cool/Mountain?, or the temperate version if you're comparing zones.

Holes in the leaves rather than a small head point to a pest problem instead — see White Butterfly and Cabbage Moth on NZ Brassicas.

Next season

Space plants generously rather than tightly if head size matters more to you than plant count, and get a feeding schedule in place before transplanting rather than reacting once growth already looks pale or stunted. Both are more directly in your control here than the weather during head formation.

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