Mighty Marigolds for Organic Gardening

The marigold that combats root knot nematodes best is the French Marigold (Tagetes patula).
The marigold that combats root knot nematodes best is the French Marigold.

A single flower crops up time and again in vegetable gardens, old and new.  Our grandparents may not have known why they were included, but they carried on this tradition “to keep bugs out”. But marigolds don’t control pests that bedevil foliage, so why did this practice become so ingrained in the home garden?  Agricultural studies have finally revealed the reasons for marigold planting in organic vegetable gardens and how they actually contribute to plant health.

Marigold plants, and especially roots, contain a chemical called alpha-terthienyl, which is toxic to harmful nematodes and has been shown to have antifungal and some antiviral properties. There is no better organic solution to counteract nematode pests, and if certain diseases are also dissuaded by marigolds, then there’s no reason not to plant them alongside tomatoes and other veggies.

Nematode Killing Power!

Roots knots or galls in tomato root systems is a sure sign of root knot nematodes.
Roots knots or galls in tomato root systems is a sure sign of root knot nematodes.

Those who’ve seen their tomatoes destroyed by root knot nematodes (Meloidogyne spp.) know this is among the few nearly incurable pest problems.  Once these microscopic soil-borne worms reach sufficient populations, little will survive long in that ground.  Nematodes invade the root tips of plants where they take up residence, laying eggs that contain up to 500 offspring, each to quickly infest the root system.  Nematodes are easily spread and may be introduced to gardens via nursery plants, topsoil, tillers, and manure.

Above-ground symptoms of nematode damage in tomatoes include stunting, leaf wilt, and leaf yellowing. Infested tomatoes are generally found in circular patches in the field or garden. Other pests and diseases can cause these symptoms, so the only way to accurately identify root knot nematode damage is to dig down and look for gall development on the roots. If the galls are there, your tomatoes are infected! This is where marigold magic comes in.

Most marigolds can do some good, but the marigold species that works best is Tagetes patula, the common dwarf French marigold you find in practically every annual bedding plant display.  (Note that varieties ‘Diamond Jubilee’, ‘Petite Harmony’

Most marigolds can help combat soil-borne pests but French marigolds work best. (photo by Jessie Keith)
Most marigolds can help combat soil-borne pests but French marigolds work best. (photo by Jessie Keith)

and ‘Bolero’ lack sufficient chemical content so avoid growing them alongside your tomatoes, if you want effective pest deterrence.)  Some alpha-terthienyl-filled varieties include  ‘Nema-Gone‘, a Burpee-exclusive variety proven to be effective in killing soil-borne nematodes, and the old-fashioned ‘Harlequin‘, which is an heirloom from 1871.

Alpha-terthienyl is present in a marigold’s living roots and branches, so you can’t chop up the plant and extract the chemical for use in a soil drench.  Living plants are the only way to solve nematode problems. When planted in infested soil, the worms will invade the roots of the marigold, but not for long. Once inside, the adults will not thrive and eggs will not hatch. The marigold roots act as “traps” to lure existing nematodes to enter and die, thus destroying a good number of them every year.

So how should you plant marigolds to maximize their nematode-killing power? Regular cultivation of marigolds with vegetables in nematode-prone regions, such as the South, is an ongoing means of reducing and preventing growth of root knot nematode populations. But, the only way to truly rectify infested ground is to set aside that zone for a summer marigold cover crop.  This, in addition to regular crop rotation, will reduce nematode populations significantly.

In the Epcot gardens in Florida, crops are flanked by dense rows of dwarf French marigolds to “trap” nematodes.

Plant the garden in tightly spaced masses of these heat-loving flowers, so the pests have no safe ground to reproduce.  Be sure to keep your marigold cover crop well weeded, because many weeds are also food for pesky root knot nematodes, supplement with Black Gold Rose & Flower Fertilizer, and give plants good care. Pick off spent flowers to prevent seed formation and water plants regularly.  While you may not be able to grow other crops during this “clean-out” year, the following year should be highly productive.

For best results, plant vegetable varieties listed with a “N”, an abbreviation that designates nematode resistance.  Nematode-resistant tomatoes include popular varieties like ‘Celebrity‘, ‘Better Boy‘, ‘Beefmaster‘, and ‘Sun Cherry‘. Flank their rows with dwarf marigolds to further reduce nematode  populations.  Then join all the old-time gardeners by planting marigolds every year after, alternating their positions with crop rotation to prevent populations from ever rising again.

About Maureen Gilmer

Maureen Gilmer is celebrating her 40th year in California horticulture and photojournalism as the most widely published professional in the state. She is the author of 21 books on gardening, design and the environment, is a widely published photographer, and syndicated with Tribune Content Agency. She is the weekly horticultural columnist for the Desert Sun newspaper in Palm Springs and contributes to Desert Magazine, specializing on arid zone plants and practices for a changing climate. She works and lives in the remote high desert for firsthand observations of native species. Her latest book is The Colorful Dry Garden published by Sasquatch Books. When not writing or photographing she is out exploring the desert on her Arabian horse. She lives in Morongo Valley with her husband Jim and two rescue pit bulls. When not writing or photographing she is usually out riding her quarter horse.

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