Is seen as an invasive weed in some parts of the world
To be fair to the nursery, this wasn’t entirely an original suggestion, and over the years, other organisations have made similar claims. Even the ancient pre-Inca peoples are believed to have used T. minuta to cleanse the soil for their potato crops, although clearing a bed of ground elder does seem a bit extreme.
The supposed mechanism is essentially that chemicals exuded by the roots have all manner of wonderful weed and pest-controlling properties, but the only one for which I’ve seen
anything like serious experimental data is an ability to suppress certain kinds of soil-inhabiting nematodes (eelworms).
Common sense must tell us that if these claims are valid and widely applicable, ground elder and comparable weeds would have vanished from horticulture’s list of problems long ago. Call me a cynic, but a lifetime as a professional scientist has taught me that nothing is impossible, although that same experience has also told me to rely on the facts. So, if the nursery making this claim will provide the results of full, proper, scientifically-based, replicated trials, supported by a full statistical analysis, I’ll be the first to tell you about it. Hearsay and anecdote are fine, but real evidence is something quite different.
And just in passing, it’s worth adding that Tagetes minuta itself is considered a serious and invasive weed in many parts of the world, so should always be grown with caution.