For the first time since beginning this column, I’m going to write only about the most common flowers that many of you already have in your gardens. It’s the reason I suggest for growing them that is uncommon this time, and I expect it will make them all the more popular.

The plants for this month are the French marigold (Tagetes patula) and the Madagascar or rosy periwinkle (Catharanthus roseus, formerly Vinca rosea). The purpose in planting them which I present to you today is not for the color they add to the garden or their attractiveness to butterflies, but for their ability to control one of the worst garden pests in Florida: nematodes.

Nematodes are microscopic round worms which suck the nutrients out of a plant’s roots, or even enter the roots and infest the plants tissues, making it difficult for the plant to transport nutrients and water to the leaves. Such weakened plants are susceptible to fungal, viral and bacterial diseases, which eventually kill them. If you have had difficulty growing vegetables, or your annual flowers look healthy one week and dead the next, nematodes are probably the reason.

In my teens and twenties, I always planted French marigolds with my tomatoes to control nematodes, but the effect seemed to be inconsistent, so I gave it up. Recent studies explain why my marigolds didn’t always work – I was planting the wrong varieties. Although they are the same species, some French marigolds are actually susceptible to nematodes, while others poison them. According to these studies, the best varieties readily available in the U.S. are Scarlet Sophie and Gypsy Sunshine, which can be purchased from Thompson & Morgan Seeds. Burpee also lists a variety called Nema-gone, at $6.95 for 2000 seeds. The recommendation is to plant an entire bed thickly with marigolds for a season to kill all the nematodes, then plant vegetables or ornamentals the following season, and continue alternating in that manner.

Madagascar periwinkle is now available with flowers in many colors and patterns. It thrives in the Florida climate, and seeds itself readily. With only one exception, the sources I consulted do not single out any particular varieties for nematode control, so I’m going to say that they all work. Unlike marigolds, Madagascar periwinkle is recommended to plant in with your potted plants, to keep out the nematodes during the current growing season.

I’m trying both in my potted plants and in the ground around susceptible plants. The abundant flowers will be an added bonus to my landscaping, and I’ll let you know how it goes with the nematodes!

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