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Tuesday, February 14, 2012

The Truth About Marigolds

French marigolds (Tagetes petula)

I’ve been hearing tales for years about how one should always plant marigolds in the garden to keep away inspect pests. Never quite believing the stories, I decided to search for the truth. Like many stories and myths, there is a grain of truth inside this one. Fortunately lots of research has been conducted on the subject. I looked at research from the Universities of Florida, North Carolina, Louisiana, Arizona and California. Here’s what I learned.

First, companion planting, meaning scattering marigold plants among vegetables, has no proven beneficial effects. Second, what marigolds truly are useful for, is combating root nematodes. Root nematodes are microscopic, not seen by the human eye and whether you have them or not can only be confirmed by a soil nematode analysis from a University Extension Office.

All soil has nematodes and some of them perform good soil-building functions while. others will attack the roots of vegetables. The symptoms are excessive wilting, with stunting or weakness in a plant. (Once that is observed, provided the cause is root nematodes, there is no treatment other than to remove the plant and destroy it).

What is true and proven about planting marigolds is this. First determine that you have nematodes with a soil analysis. Next, devote the entire bed or that part of the garden to growing marigolds as a cover crop for one season. Nematodes don’t thrive when soil temperatures are below 64 degrees F., so you could potentially grow a spring crop of peas, radishes, lettuce, etc. As soon as those are done, plant that entire bed in a variety of marigolds that are proven to combat root-knot nematodes. It’s recommended you plant a marigold every 7 inches, in all directions and keep the area weeded. Weeds attract destructive nematodes, so keep them pulled.

You can follow the total-bed marigold planting with vegetable crops the next season. However, to be effective, you will have to alternate planting marigolds then vegetables, season to season. Fortunately, most gardeners in the Ozarks aren’t severely bothered by nematode infestations.
Calendula flowers on the left, French marigolds on right, they're not the same plant.

If you do plan to do this crop rotation with marigolds this year, here are the varieties that work best and some to avoid. (Keep in mind, we are talking about the little French marigolds (Tagetes patula) we plant in the flower bed and not the “pot marigold” (Calendula officianalis) some people confuse with marigolds).
Bolero is a good one to use for nematode control.

Red Sophie is another good one.

Bonita mixed, another excellent choice for nematode control.

Best varieties for combating root-nematodes: Bolero, Bonita mixed, Goldie, Gypsy Sunshine, Petite Harmony, Petite Gold, Scarlet Sophie, Single Gold, Petite Blanc, Sophia and Tangerine. Avoid “signet” marigolds (Tagetes signata or tenufolia) IF you have nematodes, because nematodes feed and reproduce on those and will make the problem worse. Included in the signet series are: Lemon Gem, Red Gem and Tangerine Gem varieties.
For controlling nematodes, avoid any in the Signet series.

February is the month to prune grape vines, clean beds and dispose of old garden debris. Turning the soil now is a useful, to help expose grasshopper and cucumber beetle larvae where the sun and cold weather will destroy them. Happy gardening!

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