Companion Planting for NZ Vegetable Gardens

Companion planting advice on the internet is mostly folklore dressed up as science. "Plant basil next to tomatoes" appears on thousands of sites, almost none of which cite evidence for why, or whether it actually works in NZ conditions. This article is the honest version: what the research supports, what's plausible but unproven, and what's worth trying in an NZ vegetable garden.

What Companion Planting Can Actually Do

Companion planting works through a small number of real mechanisms. If a claimed combination doesn't fit one of these, treat it with scepticism:

1. Physical pest barrier. A tall, dense plant can physically block flying insects from reaching a target crop. This is real and reliable.

2. Attracting beneficial insects. Flowers that provide nectar and pollen attract hoverflies, parasitic wasps, and lacewings — all of which eat or parasitise common vegetable pests. This is the most evidence-supported mechanism.

3. Repelling pests through scent. Some aromatic plants deter specific pests. The effect is typically partial — it reduces rather than eliminates pest pressure, and needs to be at reasonably high density to work.

4. Trap cropping. A sacrificial plant that pests prefer more than your main crop, planted nearby to draw them away. This works but requires you to manage the trap plant — if you let the pests breed unchecked on it, you've made the problem worse.

5. Nitrogen fixation. Legumes (peas, beans, clover) fix atmospheric nitrogen through root bacteria. Growing them near nitrogen-hungry crops, or better — growing them first and leaving the roots in the ground — genuinely improves soil for subsequent crops.

What doesn't have reliable evidence: Biodynamic planting calendars, the idea that plants "communicate" through the soil to warn each other of pests, and most specific "plant A next to B" rules that can't be explained through the above mechanisms.

Combinations Worth Using in NZ

Brassicas + French Marigolds

What it does: French marigolds (Tagetes patula, not African marigolds) attract hoverflies whose larvae eat aphids. They also deter whitefly to some degree through scent.

NZ relevance: Aphids and whitefly are major brassica problems across all NZ zones. Marigolds that flower late summer–autumn align well with the late-season brassica pest peak.

How to do it: Plant a dense row of French marigolds along the downwind edge of brassica beds. A few scattered plants has less effect than a solid border.

Beans + Corn (Three Sisters variant)

What it does: Beans fix nitrogen, corn provides climbing structure for beans, and traditionally squash is added as a living mulch. In NZ the third sister is usually courgette or pumpkin rather than traditional squash.

NZ relevance: Works well in warmer zones (NZ sub-tropical, NZ temperate in summer). Cool/mountain zones struggle to grow corn and beans simultaneously due to the short season.

How to do it: Plant corn first — let it reach 20cm before sowing beans at its base. Add a courgette plant 60cm away from each corn plant. Requires a full-sun, warm site with consistent moisture.

Carrots + Onions / Spring Onions

What it does: Carrot fly is attracted by the smell of carrots; onion fly by onions. The theory is that each crop's scent masks the other from its respective pest. Evidence is mixed but the combination is low-cost and often recommended by experienced growers.

NZ relevance: Carrot psyllid is the more damaging NZ carrot pest (not carrot fly, which is a UK problem) and onions don't repel it. However, this combination is still worth doing — growing them in the same bed works fine, and the intercropping makes efficient use of space regardless of the pest logic.

Brassicas + Nasturtiums (Trap Crop)

What it does: Nasturtiums are more attractive to aphids and cabbage white butterfly caterpillars than most brassicas. They draw pests away from the main crop.

NZ relevance: Cabbage white butterfly is one of the biggest brassica pests in NZ. Nasturtiums are easy to grow in all NZ zones and seed themselves year after year.

Important: This only works if you monitor and destroy the nasturtiums before pests move on or breed. Check twice a week and squash caterpillars, remove aphid-heavy shoots. If you plant nasturtiums and ignore them, you may increase pest pressure rather than reduce it.

Tomatoes + Basil

What it's claimed to do: Repel aphids and whitefly, improve tomato flavour.

Honest verdict: The flavour claim has no credible evidence. The pest repelling effect, if it exists, is minor. However: basil and tomatoes have the same growing requirements (warmth, full sun, similar watering), so growing them together is sensible regardless of any companion planting benefit. Basil is also useful after harvest — fresh tomatoes and basil are inseparable in the kitchen.

Plant them together for practical reasons, not pest control ones.

Flowers for Beneficial Insects — The Most Reliable Strategy

The single most reliable companion planting approach for NZ vegetable gardens is growing nectar-rich flowers to support beneficial insects. This isn't crop-specific pairing — it's a whole-garden strategy.

Best NZ-suitable flowers for beneficials:
- Phacelia — one of the best hoverfly attractors, fast-growing annual, flowers in 6–8 weeks from seed
- Alyssum — self-seeds, long flowering, excellent for hoverflies and parasitic wasps
- Borage — attractive to bumblebees, which are important pollinators in NZ vegetable gardens, especially in cool conditions when honey bees aren't flying
- Coriander bolted to flower — let some coriander bolt in summer; the umbrella flowers are excellent for parasitic wasps
- Dill in flower — similar to coriander, a great parasitic wasp plant

Plant these throughout and around beds, not just in one corner. You're trying to ensure that wherever a pest builds up, there's a population of beneficial insects nearby.

What Doesn't Work (Despite Common Claims)

Garlic planted near roses — garlic is a good plant and works well in many beds, but the claim that it deters aphids on roses is not supported. If aphids are a problem on your roses, garlic isn't the solution.

Planting by moon phases — no peer-reviewed evidence supports this practice. Gardening by the moon calendar is a choice, not a technique.

Rue as a general repellent — rue is mildly toxic to humans and pets, can cause skin burns, and the repellent claims are largely anecdotal. Not worth the trouble.

The Practical Approach

Don't overthink this. The most effective companion planting strategy for an NZ vegetable garden:

1. Grow a permanent border of alyssum around vegetable beds
2. Plant phacelia or borage in any gap in the bed that opens up
3. Let some herbs (coriander, dill, fennel) flower rather than pulling them when they bolt
4. Use French marigolds densely alongside brassicas
5. Practice trap cropping with nasturtiums if cabbage white is a problem — but monitor them

Everything else is secondary. A garden full of flowering beneficial-insect plants, with reasonable crop rotation and healthy soil, will have fewer pest problems than any specific pairing can achieve.

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What to grow alongside your companions depends on your zone and season. Check your zone's planting calendar →