Help! If planting marigolds (Tagetes spp.) in Australia to deter grasshoppers actually helps these pests to breed and succeed, why do well known authors and garden clubs pass on misinformation? Are marigolds useful for anything other than ornament? How do I control the grasshoppers plaguing my garden? asks Vivienne in Queensland.
Question: “Hi Jerry, I have always planted French marigold (Tagetes patula) around my garden in the belief these companion plants produced pyrethrum to repel and control certain insects, especially grasshoppers. However, I find scores of these insects feeding and breeding on my marigolds. Have grasshoppers become immune? Is it just a certain grasshopper species? Many thanks”, Vivienne.
Reply: Hi Vivienne, You are discovering that companion planting is often misapplied. Certain members of the genus Tagetes can be used effectively in natural pest control. I explained on television some ten or so years ago when the CSIRO published its investigation into why some farmers reported success with using Tagetes while others had little or no benefit from them.
The genus Tagetes is mostly known for its ornamental members with aromatic leaves. As a child in London, I grew lemon marigold (Tagetes tenuiflora) and French marigold (T. patula) as summer flowering annuals. I won my first prize at the autumn show of the Lewisham Gardener’s Guild and Horticultural Society with my giant African marigolds (T. erecta).
Here in subtropical Brisbane, I grow Lemmon’s marigold (T. lemmonii), a beautiful autumn flowering shrub and Mexican tarragon (T. lucida), also autumn flowering, a knee-high perennial culinary herb.
Stinking roger, (T. minuta), is a controversial member of the genus. It is weedy. If horses eat it, they suffer bad indigestion. It pops up as a volunteer in my garden. Resin is harvested from stinking roger and it is used by the pharmaceutical industry. I have blogged on this site about using it to make a traditional South American dish ocopa with huacatay sauce.
Colgate-Palmolive once asked farmers in mid-northern NSW to farm this annual. They promised a handsome price if enough was grown – they needed around 12 tonnes of resin a year – to guarantee they did not need to import any into Australia. Owing to its reputation as a weed, the offer did not interest enough farmers to meet the target, so resin continues to be imported for use in toothpaste.
The CSIRO reported that the best plants to use to control root knot nematodes in the soil are mustard (Brassica juncea) and stinking roger.
Root knot nematodes are a major pest in a warm climate. Worldwide, they ruin 5% of harvests and they can kill seedlings. These minute, worm-like creatures invade and live inside the roots of various crops, interfering with the flow of nutrients and carbohydrate within plants, causing tumour-like growths to form. These growths reduce plant vigour and yield, and harvesting time is affected.
In the past, steam pasteurisation of glasshouse soil has been used as an organic control (expensive, complicated) or highly toxic, persistent chemicals like fenamiphos (Nemacur) have been used risking human, animal and environmental health.
There are many species of Tagetes, and the annuals bred as ornamentals are the least effective for control while stinking roger is the most potent. Whilst the CSIRO conducted field research, I trialled Tagetes in my vegetable garden to see which worked best at nematode control. I stripped all eight of my ten square metre vegetable beds and sowed them with various kinds of Tagetes.
The winning technique is to:
* Dig knee-high Tagetes minuta (or mustard) into the soil;
* Water the soil, keeping it damp for two to three weeks so they rapidly decompose;
* Replant once they have decayed, which takes up to six weeks in Brisbane;
Moist decomposition is the crucial factor. Variability in soil moisture content is what determined the success or failure reported by growers. During moist decomposition, these plants release isocyanate gas which fumigates the soil, killing root knot nematodes and their cysts. No chemical solution is as effective or safe.
I controlled the root knot nematodes in the bed sown with stinking roger. I had limited control elsewhere. To complete my nematode disinfestation project, I applied crop rotation, ensuring crops which this pest attacks like tomato (and other Solanaceae) are not planted in the same bed two seasons in a row.
The project took a year to see results, but crops have not had any significant nematode damage in the decade since. I keep a store of fresh stinking roger seed so I am able to grow a crop should the need arise.
Plant companions may be good, bad or useless
You are not alone in being misinformed about companion planting; much of what is taught has been thoroughly plagiarised and repeated uncritically by gardening magazines, television programmes and garden clubs.
Marigolds are eaten by a range of grasshoppers and caterpillars, so to control them you are better off using my grandad’s organic grasshopper control based on molasses.
Using French or African marigolds as a companion to cabbages is for deterring aphids, not grasshoppers. It has nothing to do with their chemistry, it is to do with their silhouette. Winged female aphids flying in search for cabbage recognise their leaf shape, and the finely divided leaves in the genus Tagetes changes the silhouette of a cabbage planting and they keep moving.
Ethical authors of companion planting ideas are few, but some have gone to great lengths to explain that many of these ideas are mere folklore.
My grandmother’s cabbage companion planting recommendation has been confirmed by the University of Queensland, it’s one of the minority of suggestions that have been scientifically tested and confirmed.
What works in the northern hemisphere doesn’t necessarily work in the south. Some apply in a specific climate zone or in conjunction with local biodiversity.
Lastly, please do not stop growing marigolds in your garden. In the subtropics they grow all year round, they are very cheery and you don’t need a scientist to confirm they lift the spirits 
Jerry Coleby-Williams
Director, Seed Savers’ Network Inc.
Patron, National Toxics Network Inc.
28th March 2020
