Do Marigolds Help or Hinder?

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Using marigolds as companion plants might be a mistake. Photo: hermionesgarden.blogspot.com.

Many years ago, when I still believed in companion planting (I no longer do: read Companion Planting: Myth or Reality?), I was in the habit of planting marigolds—both French marigolds (Tagetes patula) and African marigolds (T. erecta) and sometimes other species like T. minuta or T. lucida—near my vegetable plants. They were supposed to repel nematodes and whiteflies as well as to attract pollinators. 

French marigold (Tagetes patula) is the smaller of the two common marigolds. Photo: http://www.noelwilson.ca

I stopped after a decade or so, because I wasn’t getting any positive results with whiteflies (the marigolds seemed to have no influence on them, either positive or negative, at least not in my community garden of the time); plus they seemed to attract aphids and (especially) leaf hoppers that then spread to my other plants. Also, it turns out that there weren’t any harmful nematodes in my area, so I had no need to protect against them.

As well, I simply wasn’t finding bees, hoverflies and other pollinating insects particularly attracted to marigold flowers. (I now use purple tansy [Phacelia tanacetifolia] to attract pollinators: it’s much more effective!)

African marigold (Tagetes erecta) have larger flowers, often like big puffballs. Photo: ayushvedah.com

Worse yet, it seemed to me that the plants growing next to marigolds just weren’t performing as well as they should. I just figured the marigolds were outcompeting my vegetables, so stopped growing them.

It turns out it I might have been right.

Some studies now show that exudates produced by marigold roots, notably alpha-terthienyl, terpenes and thiophenes, are harmful to many other plants. In other words, marigolds would appear to be allelopathic. They apparently evolved these compounds to hinder the growth of their neighbors so they could get more than their share of sunlight, minerals, moisture, etc. This effect is apparently most evident when marigolds are planted in the same spot for two years or more, so apparently the compounds accumulate in the soil.

Tagetes lucida has smaller flowers, but a nice scent. Pollinators seem to prefer it to the more common garden marigolds. Photo: http://www.specialplants.net

In one study, marigolds were planted around bean plants to see if they kept Mexican bean beetles (Epilachna varivestis) away, while a control plot contained only beans, not marigolds. It turns out the plants in the control plot did sustain a bit more beetle damage, but even so, produced more beans than those in the plot with marigolds as companion plants, where the bean plants remained stunted.

That is just one example and really proves very little. It will take decades of study to determine whether marigolds really are seriously allelopathic and, if so, to which plants and under what circumstances. 

In the meantime, though, if you’re finding plants growing near your marigolds not performing as they should, you might want to drop marigolds from your planting list.

11 thoughts on “Do Marigolds Help or Hinder?

  1. nancy marie allen

    I’ve found that Japanese beetles love the “Vanilla” variety African marigold with its near-white rounded flowers. It was easy for me to drown the beetles when I gently dislodged them from the blooms into a plastic container filled with soapy water AND they avoided my roses this year. So, at least there’s one good reason to plant marigolds in the garden!

  2. duh.
    However, I do not doubt that companion planting in the right situations is effective. Marigolds just might be effective in the right situations. I just do not know what those situations would be, since I never noticed any benefits from them either.
    Mulberry trees were a companion plant to some of the orchards of the Santa Clara Valley. They fed birds that would have otherwise eaten ripening apricots, prunes or cherries. Cultivars of mulberry were selected to ripen at about the same time or just prior to the fruit that they were intended to protect.
    Italian cypress were companion plants to some of the vineyards. They did not occupy much space, or shade large areas, but provided nesting sites for martens who chases off other birds who would have eaten more of the grapes.

  3. I find that marigolds deters chipmunks, to a degree, but I find your hypothesis interesting because in one of my small raised beds I had marigolds last year. This year, I planted the bed with shishito peppers, but a few marigolds popped up by themselves. The peppers close to the flowers are much smaller than the others, and the marigolds are, of course, looking fabulously huge and full.

    • Very interesting comment! Oddly, chipmunks are chewing on the flowers of some of my marigolds. That doesn’t really bother me, as it was an experimental variety that I decided I didn’t much like anyway, but its interesting that they would stay away from yours, yet eat mine!

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