If you’re not already growing marigolds in your vegetable garden, you need to be! Marigolds have long been used to organically manage common vegetable pests, and science is beginning to prove what many gardeners have always believed–that marigolds are distasteful to a number of bad bugs and crawling nuisances.
French and African marigolds produce a toxin that kills soil-borne root-knot nematodes, and experienced gardeners speculate that the plant’s pungent flowers repel a number of other garden pests. Plant marigolds in your garden to draw beneficial scents that help keep pest infestations to a minimum.
Continue reading to learn more about which varieties of marigolds are the most effective at deterring pests, and how to use marigolds to manage existing pest infestations and safeguard against future invasions.
The three different types of marigolds
Marigolds come in three types – signet marigolds are the smallest plants, producing minuscule flowers that rarely grow more than a foot high. French marigolds are somewhat bushier plants that may grow up to two feet tall. African marigolds boast the tallest stems and largest flowers, some plants stretching three or four feet tall!
All varieties of marigolds produce strong-smelling flowers ranging in color from pale yellow to the deepest mahogany. Marigold flowers can be double or semi-double, and some blooms are bicolored. While African marigolds tend to have more ruffled flowers than French marigolds, it’s the French varieties that are the most studied and seem to provide the most benefit in the garden.
Best marigold varieties for pest control
- Orange Flame
Hands-down the most impressive marigold in the garden, Orange Flame is an heirloom French cultivar that is as functional as it is beautiful. The burnt-orange petals frame a vibrant orange center, and their strong fragrance seems to keep pests at bay. Baker Creek Heirloom Seeds has Orange Flame seeds available.
- Crackerjack
An heirloom African marigold mix that effectively deters and kills root-knot nematodes. Plants grow two to three feet tall and produce a fragrant and balanced blend of fully ruffled orange and gold blooms perfect for cut flowers or border plants. Purchase seeds from Botanical Interests.
- Disco Mix
A blend of single-petaled French marigolds ranging in color from bright yellow to deep red, Disco is among the earliest marigolds to bloom. Pollinators love the early, simple flowers for easy access to sweet nectar. A compact variety that withstands wet weather, Disco is a must-grow in any garden! Buy seeds from Park Seed.
Using marigolds for pest control
Marigolds have always been upheld as an organic pest management tool in the garden. Folk knowledge claims that marigolds are effective at fighting everything from pesky aphids to soil-borne nematodes.
Scientific data on the subject is somewhat limited, but it’s no secret that marigolds are a relatively pest-free crop. Several universities have studied the effectiveness of marigolds in pest management and concluded that some marigold cultivars do have significant pest-repelling properties – but the power of marigolds to repel pests comes not from their flowers, but from a toxin that the plants excrete from their roots.
Root-knot nematodes
Scientists at The North Carolina Department of Agriculture and Consumer Services have proven that both African marigolds and French marigolds are effective at fighting root-knot nematodes. The study concluded that “most cultivars of African marigolds (Tagetes erecta) and French marigolds (T. patula) are effective in reducing the most common root-knot nematode populations. The attractive flowering plants contain chemicals that kill nematodes.¹”
The Department recommends that gardeners and farmers intercrop marigolds with susceptible plants in the garden in a strip-crop arrangement, or rotate blocks of marigolds throughout the garden from season to season. Marigolds can be planted in between rows of like crops or in the paths between beds to slow the spread of nematodes from one crop to another, but the real effect of marigolds as a biocontrol won’t be felt until the following season.
It’s believed that nematodes are drawn to marigolds, and once in a close enough proximity to the marigold root system, nematodes are killed and their breeding cycle interrupted, decreasing the nematode population over time. An in-depth study by the University of Hawaii at Manoa found that marigolds produce several bioactive compounds, the most potent of which – α-therthienyl – has devastating effects on root-knot nematodes:
This sulfur-containing compound is abundant in marigold tissues, including roots. […] Thus nematodes may be killed either by entering the root system of a marigold plant or contacting soil containing the marigold’s bioactive compounds.²
The University makes it clear that different marigold varieties target different species of nematodes –and not all marigolds secrete the toxic compound that kills root-knot nematodes. If possible, perform a soil test before you buy marigold seed or starts, so you know which species of nematodes are in your soil.
Contact your local university extension office or county agricultural department for more information on how to take a soil test and where to send it to get your results. Take the time to do the proper research and make sure you plant the appropriate marigold variety for the best results.
Growers have found that marigolds are most effective at reducing nematode pressure with a “chop and drop” approach. Marigolds are grown as a summer cover crop, and before the plants die back in fall they can be tilled into the soil, keeping the nematode-killing toxins in the soil for future plantings.
Crawling and flying insects
The strong scent of marigold flowers is thought to repel a variety of pests, including aphids, cabbage worms, potato beetles, cucumber beetles, squash bugs–just to name a few.
Many growers have success interplanting marigolds with other crops, including tomatoes and cabbages. For more information on companion planting marigolds in the vegetable garden, check out this article. [Link to “How and Where to Plant Marigolds in the Vegetable Garden”].
Homemade pest spray with marigold extract
It’s been scientifically proven that marigolds contain a toxin that kills and repels certain types of soil-borne nematodes, especially the species that attack tomatoes. Some organic pest sprays even contain marigold extracts to kill and repel pests.
If you’re feeling crafty and determined to level up your organic pest-control efforts, you can make your own insecticide from marigold flowers. All you need to do is pick about a cup (250 mL) of fresh marigold flowers and blend the heads with two cups (500 mL) of water. Transfer the mixture to an airtight container and allow it to sit for two days.
Strain the mixture through a cheesecloth to catch the solid particles, and the remaining liquid can be diluted with water to make a homemade spray. Add a few drops of liquid Castille soap to increase the spray’s sticking power, and you’re ready to tackle any bad bug that gets in your way!
Attract beneficial insects with marigolds
Repelling pests is only half the battle – insects might decide to bypass your marigolds, but in favor of something a little more appetizing. Fortunately, marigolds follow through on their pest-repelling work by inviting beneficial insects into the garden. Predatory insects and parasitic wasps are drawn to the marigold’s sweet-smelling fragrance and brightly colored flowers.
Predatory and parasitic insects
Ladybugs eat aphids and mites at all stages of development, but parasitic insects do their dirty work a little differently. Parasitic wasps feed on nectar-rich marigolds and lay eggs inside soft insects like aphids and caterpillars. When the eggs hatch, the larvae eat the host insect from the inside out.
Green lacewing larvae didn’t earn the name, “aphid lions” for nothing. While adult lacewings are docile creatures that feed on flower nectar and pollen, their offspring will happily eat hundreds of aphids a week for several weeks! Adult lacewings are naturally drawn to marigolds, but you can establish a population quickly by ordering lacewing eggs from online suppliers.
Hoverflies are also attracted to marigold flowers, and their life cycles work in much the same way as lacewings and ladybugs. The adults are drawn to marigolds for the sweet nectar, and after they breed they lay their eggs near aphid colonies. When hoverfly larvae hatch, they get right to work munching on aphids and other soft-bodied insects!
Marigolds are effective at controlling pest pressure for short periods of time, but by bringing in populations of beneficial insects marigolds ensure a system of biological pest control that far outlives them. Once the beneficial insects settle in your garden, they won’t go anywhere so long as their food source is near.
Native pollinators
While marigolds are celebrated for their ability to draw predatory insects, the pollinators that their flowers bring to the garden are just as important to the health of the garden. Bees, butterflies, beetles, and flies are drawn to the fragrant, brightly-colored flowers and are very likely to stick around and pollinate any other plants too.
Conclusion
Marigolds aren’t just beautiful plants with vibrant flowers–these half-hardy annual plants have long been used to biologically control pests in the garden. While the jury is still out on whether or not pests are repelled by the strong-smelling flowers, science proves that marigolds actually do secrete a toxin that kills root-knot nematodes.
Either way, there’s no harm in planting more marigolds in your vegetable garden. The bright flowers attract pollinators and predatory insects that help keep pest populations from ever getting out of hand. Why not try growing marigolds as an experimental crop this season, and just see if and how the flowers positively affect your harvests this year?
Resources
¹ Root-Knot Nematodes: Biocontrol with Marigolds. North Carolina Department of Agriculture and Consumer Services, January 2006, https://carteret.ces.ncsu.edu/wp-content/uploads/2013/07/Root-Knot-Nematodes-Biocontrol.pdf?fwd=no.
² Wang, Koon-Hui, et al. Protecting Crops from Nematode Pests: Using Marigold as an Alternative to Chemical Nematicides. University of Hawaii, July 2007, https://www.ctahr.hawaii.edu/oc/freepubs/pdf/pd-35.pdf.
