French connection
One way to keep pests at bay is to grow French marigolds – but check they pass the smell test,
ATRADITIONAL and commonly known companion planting suggestion is to have French marigolds in your greenhouse to prevent whitefly, a pest of so many greenhouse plants. These were once hard to eradicate, although now they are controlled by introducing the commercially available biological control Encarsia formosa. However, these wee wasp predators are not inexpensive and have to be re-introduced every year.
Thus, it’s always wise to closely inspect every new plant, not just for whitefly but also for other common culprits such as scale, red spider mite and aphids. You can even knock the plant out to see if vine weevil grubs are eating the roots. Ideally, it’s good to make a ‘quarantine cold frame’, somewhere plants can safely sit awhile so that hidden infestations or infections can show up before they have been introduced into your greenhouse.
This is where French marigolds come in. Allegedly, their strong pungent smell permeates throughout the greenhouse, masking the plants so whitefly cannot detect them. Now I have no doubt this works (unless you carry them in on a plant). Why? Because I never had whitefly until the year my marigolds failed and I didn’t bother with replacements, and then whitefly appeared as if by magic. I managed to eliminate these (by eliminating all susceptible plants) and have grown French marigolds ever since.
However, my experience is only personal and not scientific proof, so I was curious when I heard of trials in the USA. These oddly showed little in the way of effects, despite there being decades of anecdotal evidence. Remarking to an audience on how I could not understand this result, I was politely interrupted by someone working for a major seed company, who said: “I think I know why. Americans never liked their pungent smell, so they have spent decades breeding it out.” Yep, that would do it…
“Their strong smell permeates the greenhouse”