Coriander bolts fast because it is a short-lived, cool-season, long-day annual: as soon as it senses heat, lengthening days, or stress, it stops making leaf and rushes to flower and set seed. In a cool or mountain garden the trigger is often not steady heat — that arrives later here than further north — but instability: a sharp swing between cold nights and warm days, a spell of dry soil, or a disturbed taproot after transplanting. Bolting can't be reversed once the flower stalk is up, so this is about prevention through variety, timing and steady moisture.
The most common reasons
Ranked by how often they explain coriander that runs to seed almost as soon as it's up:
- Heat. Coriander bolts in warm weather; above roughly 24–27°C it switches from leaf to flower, and a plant sown into midsummer heat can set seed within four to six weeks. Summer sowings often bolt before producing much usable leaf.
- Long days. Coriander is a long-day plant. As daylight lengthens through late spring and early summer it is pushed toward flowering even when temperatures are still fairly mild — which is why even slow-bolt strains stop holding once days are long.
- Dry soil and stress. Letting plants dry out makes them bolt very quickly, and the leaves turn tough and almost tasteless. Any check to steady growth reads to the plant as a reason to set seed.
- Sudden temperature swings. Coriander is unreliable when the weather is very hot, very cold, or lurches abruptly between the two — exactly the cold-night, warm-day pattern common at altitude.
- A disturbed taproot. Coriander forms a long taproot and does not transplant well; disturbing that root, or letting it bottom out in a small cell, brings bolting on faster. This is why it's usually direct-sown.
- Variety. Some strains are bred to hold leaf longer; a bolt-prone strain runs to seed sooner than a slow-bolt one under the same conditions.
Check your timing first
The local sowing window for coriander is July to November, with leaf harvest running September to January. It's direct-sown in the calendar data, not raised as transplants, and the supplier guide lists a second, smaller autumn sowing too.
That window rewards judgment. A sowing left too late still meets the long days of early summer before the plant has bulked up, so it flowers instead of filling out. Rather than one big patch, sow a short row every two to three weeks through the cool part of the season, so a fresh batch is always coming on and an early bolt never leaves you without leaf.
What altitude and frost change here
The cool/mountain frost-risk period runs March to November, leaving only a short genuinely warm stretch mid-summer. That shape cuts two ways for coriander.
On the good side, the pure heat trigger arrives later than in warmer regions, so spring and autumn leaf crops can run a little longer before summer forces them to flower. On the harder side, the distinctive local trigger is instability. Big day-to-night temperature swings and late spring or early autumn cold snaps are precisely the kind of sudden change that unsettles coriander and tips it toward seed. The practical read: keep to spring and autumn sowings, and protect young plants from sharp cold rather than worrying only about heat.
This is a climate inference from coriander's known physiology plus the local planting window and frost pattern, not a separate regional trial — but the takeaway is sound: steadiness matters here as much as temperature.
Choosing coriander and sowing it right
Start with a slow-bolt strain. These hold in leaf noticeably longer than standard coriander; Yates sells one bred for the purpose. None are bolt-proof once real heat and long days arrive, but they buy weeks.
For NZ-available seed, Kings Seeds NZ lists Picante, a direct-sown coriander that suits the don't-disturb-the-taproot rule. Koanga's heritage NZH is instead raised in trays and pricked out young before transplanting; if you go that route, move seedlings while small so the taproot isn't checked. Either way, sow into the cool shoulders of the season and keep the soil evenly moist.
What to do now
- Water steadily. Never let plants dry out; water at the base and mulch so the root zone stays even rather than swinging from dry to soaked.
- Give part shade as it warms. Grow coriander in full sun while it's cold, but shade it lightly once summer heat builds to slow the switch to flowering.
- Start the next succession. If a batch has bolted, get the next short row in now so leaf keeps coming.
- Let a bolted plant finish. Bolting can't be reversed. Leave the flowers for bees, then let the seed heads brown to collect coriander seed for cooking.
What to change next season
Sow slow-bolt seed direct, in small succession batches through spring and again from late summer into autumn, with a break over the coldest weeks. Keep moisture steady and add part shade as days lengthen. Aim for a rolling supply of young plants rather than one long-lived crop — with coriander in this climate, a fresh batch on the way beats fighting an established plant's instinct to seed.
More on this zone: see the full NZ Cool/Mountain Planting Calendar →.