You will come across gardening advice suggesting that marigolds are an excellent companion planting for your vegetables. They … something something something … and bad bugs begone. Plus, they attract pollinators. And they’re pretty, to boot.
What’s not to like?
Admittedly, “companion planting” is probably the most folklore-based topic in gardening. With a handful of exceptions, companion planting advice is offered with no actual testing or hard evidence whatsoever. And there’s never any quantification of the size of the impact that any particular companion planting has.
The evidence-free nature of most companion planting advice is par for the course. If you’re of a mind to do companion planting, you pick you companions and you get what you get. Not a lot of science involved in that.
But at some point, you have to draw the line. Pro-marigold propaganda fails to mention that marigolds are allelopathic. That is, they produce chemicals that may suppress or kill other nearby plants.
You can find the occasional gardening site that will give you adequate warning that marigolds are allelopathic (such as this one). In fact, they are so strongly allelopathic that if you do even a cursory search for lists of allelopathic plants, you’ll find that marigolds appear on most lists (E.g., reference, reference, reference).
The impact of marigolds will depend on both the strain of marigolds chosen, and the vegetables growing nearby. For example, there is some research to suggest that marigolds increase tomato yields slightly (reference). But that marigolds reduce bean yields (via this blog post).
In this year’s garden, I noticed that the okra plants and green beans that were growing near french marigolds seemed to produce next-to-no fruit. The negative effect of marigolds on beans was noted above. But as far as marigolds suppressing the development of okra pods, you’re on your own to figure that out. If it’s true. In fact, you’ll come across sites touting marigolds’ ability to suppress hatching of root nematode eggs as a good reason to plant marigolds with your okra (reference, reference).
All I have to say is, my folklore contradicts that folklore. Okra pods leave a scar or stem when picked, so I can compare the yield of individual plants. For a stand of a dozen okra plants, the two plants adjacent to marigolds had an average of 5 pod scars, the remaining plants had an average of 9. That’s hardly proof, but it’s that difference that caught my attention. It’s not merely the lack of mature pods, it’s also that the pods that have matured seem to take forever to do so, compared to the remaining okra. Based on my tiny sample of plants, marigolds depress okra yield.
So far, this year has been a poor one for my vegetable garden. Other than peas, I haven’t gotten much of a yield of anything. Even my tomatoes seem to have taken a mid-summer vacation. Some of which is my fault. Some of which is just random.
That happens sometimes.
But it’s a tough gardening world where you can’t even trust your flowers.
For all paired images in this post, an image from Gencraft AI is on the left, an image from Freepik AI is on the right. Weirdly enough, I think I could pick out, by eye, which AI did which picture. The Gencraft general style appears darker and more crowded, in general, compared to the Freepik style. At least, to me, today, it does.
