Does a border of marigolds really keep pests away?

For some things. A Thai study that alternated marigold and soybean rows 1:1 found fewer aphids and more natural enemies. But if you want fewer nematodes in the soil, marigolds have to be grown as a cover crop about two months before the vegetables on the same spot. Planting them alongside the crop is not enough.

How far can I trust charts of plants that like or hate each other?

Most pairings have never been tested. Use them as ideas, not rules. Look for reasons you can check instead: deep and shallow roots compete less, tall plants shade short ones, and flowers feed beneficial insects.

What do I have to do once a trap crop is in?

Check it often and pick off eggs and caterpillars. If the trap crop gets heavily infested, pull it, bag it and throw it out rather than composting it. Otherwise the pests breed on the trap crop and move back onto your vegetables.

Companion planting: what the evidence supports and what is folklore

Companion planting means growing different plants close together for some benefit: fewer pests on the crop, or better use of space. Part of the idea is backed by research and part of it is pure folklore, and the two are usually mixed together. This lesson separates them and turns what works into layouts you can use in a Thai vegetable bed.

It follows on from the pest section in Start a small pesticide-free vegetable bed; read that first if you haven’t.

Where the companion charts come from

The internet is full of charts saying which vegetables “love” or “hate” each other. A University of Arizona Cooperative Extension article, summarising a review by Dr. Linda Chalker-Scott of Washington State University, puts it bluntly: the phrase “companion plant” is too vague to be useful to plant scientists, and traditional companion charts have entertainment value, not scientific value. Some pairings come from looking at crystallisation patterns of plant extracts, or from linking plants to elements and zodiac signs. None of that has been tested.

That doesn’t make companion planting useless. WSU Extension’s 2023 publication calls it a legitimate practice when it rests on ecology, on how plants actually help each other, rather than on plants “liking” one another.

Claim	Evidence
Trap crops draw pests off the main crop	Supported by research; the trap crop needs managing
Flowers feed beneficial insects	Supported, but uses space and can lower main-crop yield
Marigolds reduce soil nematodes	Works as a cover crop grown first; interplanting is risky
Strong scents repel pests	Works in the lab; not confirmed in field studies
Beans hand nitrogen straight to neighbours	Not how it works; legumes enrich the soil over time through microbes, not directly
These two plants “like” or “hate” each other	No evidence

Trap crops: send the pests somewhere else

A trap crop is a plant the pest prefers to your main crop, grown so the pest goes there instead. A well-supported example is collards, a large-leaved, non-heading brassica, drawing diamondback moth away from cabbage. The University of Saskatchewan suggests planting collards around all four sides of a brassica bed, and planting them early because they grow more slowly.

In Thailand, the Department of Agriculture ran a project in 2010-2012 (B.E. 2553-2555) studying trap crops and host plants for natural enemies in organic vegetable systems in the central region, so this is being studied locally too.

What to remember: a trap crop doesn’t kill anything. It concentrates the pests in one place. Leave it alone and they breed there and move back. Check it, pick off eggs and caterpillars, and if it’s heavily hit, pull it and bag it for the bin, not the compost. WSU’s publication also notes that scent-based trap crops don’t attract insects from more than about 5 metres away, which suits a small home bed.

A trap crop doesn't kill pests, it gathers them; you still remove them

Flowers that feed the good insects

Many natural enemies, such as hoverflies, lady beetles and predatory bugs, feed on nectar and pollen as adults. Grow flowers near the bed and they stay around longer, while their larvae eat aphids and caterpillars. WSU calls these insectary plants.

A Thai study from Valaya Alongkorn Rajabhat University, run in Pathum Thani, alternated marigold and soybean rows. The 1:1 alternation had the fewest aphids and the most natural enemies, including lady beetles, spiders, dragonflies and black ants.

The price is space. NCAT cites a review of 552 experiments that found these plantings reduced pests but also reduced yield of the main crop.

Worked example: a 1.2 x 3 m bed

Run rows along the 3 m length, 30 cm apart. Across 1.2 m that gives 120 / 30 = 4 rows.
Alternating marigolds 1:1 as in the study leaves 2 rows of vegetables: you lose half the crop space.
A cheaper layout is marigolds only along the two short ends, 4 plants per end about 30 cm apart, 8 plants in total. You keep all 4 vegetable rows.
Layout 3 is not what the study tested, so keep notes and compare aphid numbers with last season.

Marigolds and nematodes: plant first, not together

The belief that marigolds tucked between tomatoes clear out root-knot nematodes is only half right. UF/IFAS (University of Florida) explains:

Grow marigolds at least two months before the vegetable, on the same spot where the vegetable will go.
Interplanting marigolds with a nematode-susceptible crop is very risky and may damage the crop.
French marigold (Tagetes patula) works against the widest range of nematodes, but cultivars differ.
Marigolds reduce numbers but can’t eradicate nematodes. You need to grow them ahead of the crop every season and keep weeds down, because nematodes can breed on weeds.

Spacing: the part nobody talks about

Plants close together compete for water, nutrients and light. That is the most concrete reason to pair plants. WSU suggests:

Pair deep-rooted crops such as carrots or radishes with shallow-rooted ones such as lettuce or onions. Their roots sit in different layers and compete less.
Keep plants of the same species as far apart as you can, mixed with other crops, so pests and disease don’t walk from plant to plant.
Avoid companions or living mulches that grow fast enough to smother the crop.

In a Thai bed you can apply this by sowing lettuce or coriander between young chilli plants. The leafy crop is picked before the chillies spread and shade it. Another is growing leafy greens in the light shade on the east side of a yardlong bean trellis. Both are about sharing space and time; neither claims the plants repel pests for each other.

Pair plants for reasons you can measure: roots, light, harvest timing and beneficial insects. Not because a chart says they “like” each other.

Citronella grass and mint as pest repellents

Many Thai gardeners ring their beds with citronella grass or mint to keep insects away. WSU concludes that the scent compounds work in the lab, but field studies haven’t confirmed it, because the smell disperses quickly outdoors. Grow them for the kitchen by all means, but don’t count on them as a pest fence. Mint also spreads fast by runners and can crowd vegetables in an open bed, so a pot is safer.

Common mistakes

Treating a companion chart as law, moving plants around when the real problem is too little sun or compacted soil.
Planting a trap crop and walking away. Pests breed on it and come back to the crop.
Planting marigolds with tomatoes at the same time to beat root-knot nematodes. They need a two-month head start.
Packing in so many flowers there’s no room for vegetables. Flowers help beneficial insects, but they cost space. Start with the edges.
Spraying insecticide in a bed meant to host beneficial insects. The natural enemies die too. The University of Saskatchewan notes that parasitic wasps only help if no insecticides are used in the garden.

Sources

General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.