Growing Courgettes in NZ Cool/Mountain Zone — Timing and Common Problems

The cool/mountain zone is the hardest place in NZ to grow courgettes well. The season is compressed, frosts arrive early and leave late, and courgettes are frost-tender at both ends. But the payoff is worth it: grown in good summer conditions, courgettes in this zone are prolific. You just need to get the timing exactly right and avoid the specific mistakes that kill them here.

The Season Window

In the cool/mountain zone (Canterbury, Otago, Central Otago, Manawatū hill country, Wairarapa inland), your reliable frost-free window is roughly late November to late March — about 16 weeks. That's workable but tight for a crop that needs 8–10 weeks from transplant to first harvest.

This means one thing: you must raise plants indoors first. Direct sowing courgettes in cool/mountain doesn't give you enough season. Anyone telling you to direct sow in October is writing advice for a warmer climate.

Sowing Timeline

Action	Cool/Mountain Timing	Notes
Sow indoors	Late September – mid-October	Don't sow earlier — seedlings get leggy if held too long
Transplant outdoors	After last frost: late November in most areas, December in high country	Soil must be above 16°C
First harvest	Late January – early February
Season end	Late March – April (first frost)

Soil temperature matters more than calendar date. If you transplant into cold soil, courgettes sit still, exhaust themselves, and become susceptible to disease. Buy a soil thermometer — they're cheap and will save you weeks of frustration.

How to Raise Seedlings Indoors

1. Sow two seeds per 10cm pot in mid-late September, thin to one after germination
2. Use seed-raising mix, not garden soil — courgettes need excellent drainage at the seedling stage
3. Germination needs warmth: 20–25°C. A hot water cupboard works. Move to a bright windowsill once seeds sprout.
4. Grow on in a sheltered, sunny spot — a cold greenhouse or cloche is ideal, not a cold room
5. Harden off for 7–10 days before transplanting: move plants outdoors during the day, back inside at night. This step is skipped more than any other and accounts for most early transplant failures in this zone.

Transplanting

• Wait until night temperatures are reliably above 10°C
• Choose the warmest, most sheltered spot in your garden — a north-facing bed against a fence or wall is ideal in cool/mountain zones
• Bury the stem slightly deeper than it was in the pot — courgettes can root from the stem
• Water in well, then do not water again for 3–4 days to encourage downward root growth
• Use a cloche or frost cloth for the first 2 weeks post-transplant — even if frosts are unlikely, the warmth boost accelerates establishment significantly

Spacing: 90cm–1m between plants. They get large. Crowding them causes poor air circulation, which in cool/mountain zones (where summer mornings are often cool and dewy) leads directly to powdery mildew.

The Three Problems That Kill Courgettes in This Zone

1. Powdery Mildew (the big one)

Cool nights, dewy mornings, and occasional cloudy spells make powdery mildew almost inevitable in cool/mountain zones by late February. The white powdery coating isn't immediately fatal but progressively weakens the plant, reducing yield in the final weeks of your short season.

Prevention is far better than cure:
- Maximise air circulation — don't crowd plants and remove lower leaves once the plant is established
- Water at soil level only, never overhead, and water in the morning
- A preventative fortnightly spray of diluted milk (1 part full-fat milk, 9 parts water) from January onward genuinely works and has research support

Once you see it: Remove affected leaves immediately. Apply a copper-based fungicide if it's spreading fast. The plant can continue producing through mild infection — don't pull it out prematurely.

2. Poor Fruit Set (flowers dropping without fruit)

Courgettes produce separate male and female flowers. Early in the season, plants often produce only male flowers first — this is normal. Female flowers (identifiable by the tiny courgette behind the petals) arrive a week or two later.

If female flowers are forming but dropping without fruit, the problem is almost always poor pollination — usually because it's cold and bees aren't flying.

In cool/mountain zones, this happens regularly in December and early January. Fix it by hand-pollinating: use a small paintbrush or simply pick a male flower and rub its centre against the female flower's centre. Do this on sunny mornings before 10am.

3. Transplant Shock from Cold Soil

Already mentioned, but worth repeating because it's the most common cause of failure. Courgettes transplanted into soil below 16°C don't establish — they sit there looking pale and getting weaker. If they survive, they're two to three weeks behind where they should be, which in a 16-week season is significant.

If you transplanted too early and your plants look stunned, cover with frost cloth to warm the soil and reduce stress. Don't fertilise a stressed plant — wait until they show new growth.

Feeding and Water

Once established and growing well (usually 2–3 weeks post-transplant), courgettes are hungry:
- Apply a liquid fertiliser every 2 weeks — seaweed plus fish emulsion is ideal
- Keep moisture consistent; irregular watering causes poor fruit development
- Mulch heavily to retain moisture and keep soil warm

Variety Notes for Cool/Mountain

Standard varieties work fine, but for this zone prefer:
- 'Defender' F1 — mildew-resistant, good early yield, available from Kings Seeds
- 'Black Forest' F1 — climbing type, good for small beds, reasonable mildew tolerance
- Avoid very late-maturing Italian heirloom varieties — they won't finish in this zone's short season

---

Courgettes are just one of the summer crops you can squeeze into this zone. See the full cool/mountain summer sowing calendar →