Perennial multiplying leeks (Allium ampeloprasum), also known as perennial leeks or wild leeks, are hardy, low-maintenance plants that can thrive in New Zealand’s varied climates. They multiply by producing bulbils or offsets and can provide a continuous harvest with minimal effort.

Here’s a guide to planting and growing them successfully in New Zealand:

1. Choosing the Right Time and Location

• Planting Time: Plant in early spring (September–October) or autumn (March–April) for best establishment. In milder regions like Auckland or the Bay of Plenty, planting can extend into early summer.
• Climate Suitability: Perennial leeks are hardy and grow well in most New Zealand climates, from the temperate North Island to cooler South Island regions. They tolerate frost but prefer mild conditions.
• Site Selection:
  • Choose a sunny spot with at least 6 hours of sunlight daily.
  • Ensure well-draining soil, as leeks dislike waterlogged conditions.
  • They tolerate a range of soil types but prefer fertile, loamy soil with a pH of 6.0–7.0.

2. Preparing the Soil

• Soil Preparation:
  • Dig the soil to a depth of 30 cm, removing weeds and rocks.
  • Incorporate organic matter like compost or well-rotted manure to improve fertility and drainage.
  • If soil is heavy clay, add gypsum or sand to improve structure.
• Fertiliser: Apply a balanced organic fertiliser like blood and bone before planting.

3. Planting

• Method:
  • Dig small holes or trenches 10–15 cm deep and 15–20 cm apart. Space rows 30–40 cm apart for easy access.
  • Place a single bulbil, offset, or transplant in each hole, with the root end down.
  • Cover with soil, ensuring the top of the bulbil or plant base is just below the surface.
  • Water gently to settle the soil.
• Tip: For thicker stems, mound soil around the base as they grow (blanching).

4. Care and Maintenance

• Watering:
  • Keep soil consistently moist but not soggy, especially during establishment.
  • Once established, they’re drought-tolerant but benefit from occasional deep watering in dry spells.
• Mulching:
  • Apply a 5–10 cm layer of organic mulch (e.g., straw, bark, or grass clippings) to retain moisture, suppress weeds, and regulate soil temperature.
• Fertilising:
  • Feed annually in spring with compost or a balanced fertiliser to encourage vigorous growth.
  • Avoid high-nitrogen fertilisers, which can promote leafy growth at the expense of bulb development.
• Weeding:
  • Keep the area weed-free, especially in the first year, to reduce competition.
• Pest and Disease Control:
  • Perennial leeks are generally pest-resistant but watch for onion thrips or slugs in humid conditions.
  • Use organic controls like neem oil for thrips or beer traps for slugs.
  • Prevent fungal diseases by ensuring good air circulation and avoiding overhead watering.

5. Harvesting

• When to Harvest:
  • Start harvesting young leaves or whole plants after 6–12 months, depending on growth.
  • For mature leeks, wait until stems are thick (1–2 years).
• How to Harvest:
  • Cut leaves as needed for cooking, leaving the base to regrow.
  • Dig up entire plants or remove offsets for replanting or eating.
  • Harvest bulbils from the base or flower heads to propagate new plants.
• Perennial Cycle: Leave some plants in the ground to multiply naturally. They’ll form clumps that can be divided every 2–3 years.

6. Propagation

• Dividing Clumps:
  • Every 2–3 years, dig up clumps in spring or autumn.
  • Separate offsets and replant immediately, spacing as described above.
• Bulbils:
  • Collect bulbils from flower heads or the base of mature plants.
  • Plant them shallowly (2–3 cm deep) in prepared soil.
• Self-Seeding: If flower heads are left, leeks may self-seed, but this is less reliable in colder regions.

7. Adapting to New Zealand Conditions

• North Island:
  • In warm, humid areas (e.g., Auckland, Waikato), ensure good drainage to prevent rot. Mulch to keep roots cool in summer.
• South Island:
  • In cooler areas (e.g., Canterbury, Otago), protect young plants from heavy frost with cloches or fleece during winter.
  • In windy regions like Wellington, plant in sheltered spots or use windbreaks.
• Coastal Areas:
  • Perennial leeks tolerate salt-laden winds but benefit from organic matter to counteract sandy soils.

8. Tips for Success

• Companion Planting: Grow alongside carrots, celery, or chamomile to deter pests and enhance growth. Avoid planting near beans or peas, which may compete.
• Rotation: Although perennial, rotate planting sites every 4–5 years to prevent soil-borne diseases.
• Storage: Harvested leeks store for 1–2 months in a cool, dry place or can be left in the ground until needed.

9. Common Challenges

• Slow Growth: If growth is sluggish, check soil fertility or drainage. Apply liquid seaweed fertiliser to boost vigor.
• Bolting: In hot summers, plants may bolt (flower prematurely). Pinch off flower heads to redirect energy to bulb growth.
• Overcrowding: Divide clumps regularly to maintain productivity.