Last July I was out in the vegetable garden, when a neighbor gardener approached and asked, “What are you doing?” I responded, “Smashing Mexican bean beetle larvae.” My neighbor gave me a puzzled look, followed by a smirking smile, as if I was committing a gardening no-no. I just had to ask, “You’re not having a beetle problem on your green beans?” He smiled and said, “Of course not I plant marigolds with my beans, and they keep the bugs out of the bean patch.” WOW! Could this be the silver bullet in controlling this obnoxious pest?
Now that the gardening season is over and the cover crop is planted, I’ve got a chance to catch my breath from sowing, planting, mulching, weeding and harvesting. Did I mention weeding? Anyway, now that I have a little time to reflect back on the garden season as to what may or may have not worked well, I’ve been returning to that conversation with my neighbor about the wonders of the marigold. The seed catalogs are starting to show up in the mail, and the planning for next year’s garden is in the beginning stage, so before ordering a ton of marigold seeds, now would be a good time to do a little research on the magical repelling powers of the marigold plant. I’ve heard marigold stories for years, how marigolds will repel every garden pest known to mankind, including bugs, snails, rabbits, ground hogs, and deer! I’ve even heard that it has been used to target and kill selected weeds. This sunny annual has been employed as a companion plant for generations just to do that — repel pests from the garden. Do they really benefit the garden as a repelling machine or are they just pretty and their repelling powers just a gardening myth?
Marigolds belong to the aster family (Asteraceae), genus Tagetes. Their natural range extends from the southwestern United States into Argentina, with their greatest distribution being in south central Mexico. Approximately 50 species are known, but in general, the three most common are African marigolds (T. erecta), French marigolds (T. patula) and Signet marigolds (T. tenuifolia). However, regardless of their name, all marigolds are native to subtropical America and have been cultivated in Mexico for over 2,000 years.
Tagetes patula, commonly called French marigold, is a compact annual that typically grows 6-12” tall and features single, semi-double, double or crested flowers (1-2” diameter) in shades of yellow, orange, red and bicolor. Their pinnate leaves with toothed, lance-shaped leaflets are aromatic.
Tagetes erecta, commonly called African marigold, Aztec marigold, American marigold or big marigold, is native to Mexico and Central America. Big marigold may be the most descriptive of its names because plants are noted for their large flowerheads. They typically grow from 1-4’ tall and feature huge, mostly double-globular flowers (2-4” diameter) in various shades of yellow, orange, and white. Foliage and flowers are aromatic when brushed or crushed. Triploid F1 hybrids (T. erecta x T. patula) combine the large flowers of the African marigold with the more compact size of the French marigold into vigorous plants with 2-3” diameter flowers on stems reaching 10-18” tall. These triploids are largely unaffected by high heat and usually bloom all summer.
Tagetes tenuifolia. Signet marigolds are compact, mounding plants with smaller flowers and leaves than most other marigolds. Yellow, orange, golden, or bicolored flowers are held either well above the fine-textured, dark green foliage or tucked in with the foliage, depending on the cultivar. This plant doesn’t have that overwhelming marigold scent but has a light, citrusy smell.
For generations, many vegetable gardeners have planted marigolds in their vegetable patches to repel pests. However, there is a lack of scientific evidence to support the notion that marigolds actually repel pests. Research conducted at Rutgers University concluded that marigolds failed to repel cabbage, carrot and onion pests. In fact the USDA lists a total of 15 pests that attack marigolds; included on their list are aphids, Japanese beetles, snails, and spider mites, just to name a few. On the other hand, researchers at the University of Vermont have reported that marigolds have been effective in luring pests away from other ornamental plants.
Although science has yet to prove that marigolds actually repel pests from vegetable crops, there is scientific evidence that marigolds CAN be an important tool in controlling certain nematodes. Nematodes are tiny worms, usually microscopic in size. Nematodes that feed on plants — called plant-parasitic nematodes — have spear-like mouthparts used to puncture plant roots to obtain nutrients. As a result, plant-parasitic nematodes can seriously damage or even kill crops, turf, and ornamental plants.
Marigold roots release a toxic chemical (alpha-terthienyl), and the presence of this chemical inhibits the hatching of nematode eggs. Therefore, control of the nematode population is achieved by interrupting the nematode life cycle.
One drawback with using marigolds for nematode control is that the benefit is not realized until the following year. To be effective the marigolds must be planted before the vegetable crop — at least 2 months before — and must be planted at the same location where the vegetable crop is to be planted; otherwise, no benefits will be gained from the marigold root exudates. For example, California research showed that tomatoes grown after marigolds had significantly lower numbers of root galls due to root-knot nematodes (Meloidogyne incognito). In addition, the tomato yields were higher (root length, shoot weight, number and weight of fruits were all higher in plants grown after Tagetes). UniversityofCaliforniaRiverside.edu.
Care should be taken when purchasing marigolds for controlling nematodes. That’s because “not all marigold varieties control all types of nematodes.” Univ.ofFlaExt.edu/ng045. For example, the California research mentioned above revealed that particular varieties are more effective at controlling root-knot nematodes. In that experiment, the “Single Gold” variety of Tagetes patula outperformed other varieties. You’ll get the best results if you determine which types of nematodes are in your local soils, and you can do this by sending soil samples to a nematode assay laboratory. Univ.ofFlaExt.edu/ng045 (look at the chart in this article identifying marigold species and varieties by their resistance to and effectiveness against particular types of root-knot nematodes).
In addition to helping control nematodes, marigold flowers attract beneficial insects that not only pollinate, but also help control bad bugs. Beneficial insects attracted to marigolds include: hover flies, lady bugs and parasitic wasps.
To date there is very little scientific evidence that the aroma of the marigold plants actually repel pests, however it is a generally accepted scientific fact that marigolds help to control nematodes and attract beneficial insects that aid in controlling unwanted pests.
There is growing concern about pesticides’ non-target effects on humans and other organisms, and many pests have evolved resistance to some of the most commonly-used pesticides. Together, these factors have led to increasing interest in non-chemical, ecologically sound pest management. The marigold is not only pretty but offers the gardener another arrow for the quiver in the bug war. Who wouldn’t want to plant a beautiful plant that was edged out by the rose for our national flower. I know I will!
Thanks for stopping by The Garden Shed. We members of The Garden Shed Team wish you and your family a safe and happy holiday season.
“Effects of Selected Marigold Varieties on Root-knot Nematodes and Tomato and Melon Yields”, Department of Namatology, University of California, http://faculty.ucr.edu/~atploeg/PDF PAPERS/PLANT DISEASE/PLANTDISEASE.pdf
“Marigolds (Tagetes spp.) for Nematode Management,” University of Florida Institute of Food and Agricultural Sciences, Univ.ofFlaExt.edu/ng045.
“Murdering Thrips with Marigolds, Fungi & Mites,” University of Vermont Entomology Research Laboratory, https://www.uvm.edu/~entlab/Greenhouse IPM/Workshops/2015/AFEProjectDesc&SummaryDec2014.pdf
African marigolds (Tagetes erecta) are beautiful flowering plants that can brighten up any garden with their bright golden blooms. However, these popular plants can sometimes fall victim to boring insects that tunnel into stems and leaves, damaging the plant If you’ve noticed signs of borers on your marigolds, there are a few effective methods you can use to get rid of them naturally and protect your flowers
Identifying Borer Infestations
The first step is learning how to identify whether borers have infested your marigolds. Here are some of the most common signs:
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Wilting, dying, or collapsing stems and leaves even when the soil is moist. This happens when borers have tunneled inside and damaged the plant’s vascular system.
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Small holes on stems and leaves. These are entry and exit holes that borers create.
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Sawdust-like frass near holes. This debris is left behind by boring insects as they feed.
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Swollen or disfigured stems. Larvae tunneling inside causes abnormal stem growth.
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Presence of worms or caterpillars. Check around damaged areas and you may find borers inside stems or feeding on leaves. Common marigold borers include species like the marigold weevil, marigold tortoise beetle, slug caterpillars, and others.
If you spot any of these signs, it’s likely borers have infested your marigolds. The sooner you take action, the better chance you have of saving your plants.
Removing Borers Manually
One of the most effective ways to get rid of existing borer infestations is to manually find and remove the pests. Here are some tips:
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Check stems and leaves thoroughly for holes, frass, or swelling as these indicate active borers.
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Use a sharp knife to slit open stems lengthwise. Extract any borers, caterpillars, or larvae inside.
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Completely remove and destroy heavily infested or dying stems and leaves. This helps prevent spread.
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Hand pick any beetles, worms, or caterpillars you find on flowers or leaves.
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Drop pests into soapy water to kill them or place in sealed bags before discarding if composting.
Being diligent about manual removal ensures you get rid of as many borers as possible. Make sure to check your plants at least once a week for early detection.
Using Row Covers as Preventive Barriers
Row covers made from fabric or netting act as protective barriers that keep many boring insects from ever reaching your marigolds in the first place. To use them:
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Cover marigold beds with a horticultural fleece or lightweight row cover early in the season before pests become active. Secure the edges well with stakes, rocks or garden soil.
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Check plants under the fabric daily for signs of pests. Lift up to remove weeds, water, or add beneficial insects as needed.
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Reapply covers after tending to the plants to maintain the protective barrier. Keep covered until flowering when pollination is required.
The row cover allows sunlight, air, and water to reach the marigolds while keeping borers and other pests out. It’s an easy organic prevention method.
Applying Neem Oil as a Pesticide Spray
Neem oil is a natural pesticide extracted from the seeds of the neem tree. It’s very effective against boring insects and safe for use on edible plants like marigolds. Follow these tips when applying neem oil:
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Mix neem oil concentrate with water according to label directions. Add a small amount of insecticidal soap which helps it adhere to the plants.
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Pour the neem solution into a spray bottle or garden sprayer. Spray on stems, leaves, and flowers, especially under leaves and on new growth where pests hide.
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Reapply after rain or every 7-14 days during peak borer activity. Neem oil kills eggs, larvae and adults it contacts.
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As a preventative, make neem oil sprays a regular part of your marigold care routine through the growing season.
The azadirachtin compounds in neem disrupt borers’ hormones and deter feeding. Just be sure to spray in the early morning or late evening to avoid hurting pollinators.
Using Beneficial Nematodes Against Borers
Beneficial nematodes (Steinernema spp) are microscopic worms that parasitize and kill boring larvae while safely leaving plants unharmed. Here’s how to use them:
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Purchase live nematodes from garden stores or online. Heterorhabditis bacteriophora is effective against many marigold borers.
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Mix nematodes with water according to package instructions. Apply this solution to moist soil around plants using a watering can.
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Reapply every 2-6 weeks as needed through seasons when borers are active. The nematodes persist in soil seeking out and infecting larvae.
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Provide adequate moisture and avoid applying chemical pesticides which can harm nematodes. They are compatible with neem oil sprays.
Releasing predatory nematodes into your garden beds gives ongoing protection against destructive boring pests on marigolds and other flowers.
Getting Rid of Borers in Soil with Beneficial Fungi
Some species of beneficial fungi like Beauveria bassiana act as endoparasites against boring larvae in the soil. Follow these tips to use them:
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Purchase beneficial fungi inoculant concentrated or powder online or from garden stores. Check the label to confirm it contains pest-killing strains.
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Mix into water according to package directions. Use a watering can to pour the solution over soil surrounding plants.
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Reapply every 7-14 days during peak borer activity. Water soil to aid fungi growth and spread.
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Fungi remain dormant waiting for larvae to infect. They pose no harm to plants or pollinators in your garden.
The fungi penetrate borer larvae bodies and use destructive enzymes to kill them within a few days. An ongoing fungi presence in soil provides biological pest control.
Trapping Adult Borers with Pheromone Lures
Many borer species are attracted to sex or aggregation pheromone lures. Traps baited with these chemical lures draw in and capture adult borers. Here are some tips for using pheromone traps:
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Obtain traps designed for boring pests like the clear-winged moth borer. Choose lures specific to marigold borers in your area.
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Place traps near plants during early season before borers begin laying eggs. Use at least 1 trap per 25 square feet.
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Check and empty traps weekly,
Piedmont Master Gardeners • Sharing knowledge, Empowering communities PMG News
- By Cleve Campbell
- /
- December 2017 – Vol. 3 No. 12
- /
Last July I was out in the vegetable garden, when a neighbor gardener approached and asked, “What are you doing?” I responded, “Smashing Mexican bean beetle larvae.” My neighbor gave me a puzzled look, followed by a smirking smile, as if I was committing a gardening no-no. I just had to ask, “You’re not having a beetle problem on your green beans?” He smiled and said, “Of course not I plant marigolds with my beans, and they keep the bugs out of the bean patch.” WOW! Could this be the silver bullet in controlling this obnoxious pest?
Now that the gardening season is over and the cover crop is planted, I’ve got a chance to catch my breath from sowing, planting, mulching, weeding and harvesting. Did I mention weeding? Anyway, now that I have a little time to reflect back on the garden season as to what may or may have not worked well, I’ve been returning to that conversation with my neighbor about the wonders of the marigold. The seed catalogs are starting to show up in the mail, and the planning for next year’s garden is in the beginning stage, so before ordering a ton of marigold seeds, now would be a good time to do a little research on the magical repelling powers of the marigold plant. I’ve heard marigold stories for years, how marigolds will repel every garden pest known to mankind, including bugs, snails, rabbits, ground hogs, and deer! I’ve even heard that it has been used to target and kill selected weeds. This sunny annual has been employed as a companion plant for generations just to do that — repel pests from the garden. Do they really benefit the garden as a repelling machine or are they just pretty and their repelling powers just a gardening myth?
The Plant
Marigolds belong to the aster family (Asteraceae), genus Tagetes. Their natural range extends from the southwestern United States into Argentina, with their greatest distribution being in south central Mexico. Approximately 50 species are known, but in general, the three most common are African marigolds (T. erecta), French marigolds (T. patula) and Signet marigolds (T. tenuifolia). However, regardless of their name, all marigolds are native to subtropical America and have been cultivated in Mexico for over 2,000 years.
Tagetes patula, commonly called French marigold, is a compact annual that typically grows 6-12” tall and features single, semi-double, double or crested flowers (1-2” diameter) in shades of yellow, orange, red and bicolor. Their pinnate leaves with toothed, lance-shaped leaflets are aromatic.
Tagetes erecta, commonly called African marigold, Aztec marigold, American marigold or big marigold, is native to Mexico and Central America. Big marigold may be the most descriptive of its names because plants are noted for their large flowerheads. They typically grow from 1-4’ tall and feature huge, mostly double-globular flowers (2-4” diameter) in various shades of yellow, orange, and white. Foliage and flowers are aromatic when brushed or crushed. Triploid F1 hybrids (T. erecta x T. patula) combine the large flowers of the African marigold with the more compact size of the French marigold into vigorous plants with 2-3” diameter flowers on stems reaching 10-18” tall. These triploids are largely unaffected by high heat and usually bloom all summer.
Tagetes tenuifolia. Signet marigolds are compact, mounding plants with smaller flowers and leaves than most other marigolds. Yellow, orange, golden, or bicolored flowers are held either well above the fine-textured, dark green foliage or tucked in with the foliage, depending on the cultivar. This plant doesn’t have that overwhelming marigold scent but has a light, citrusy smell.
Fact or Fiction
For generations, many vegetable gardeners have planted marigolds in their vegetable patches to repel pests. However, there is a lack of scientific evidence to support the notion that marigolds actually repel pests. Research conducted at Rutgers University concluded that marigolds failed to repel cabbage, carrot and onion pests. In fact the USDA lists a total of 15 pests that attack marigolds; included on their list are aphids, Japanese beetles, snails, and spider mites, just to name a few. On the other hand, researchers at the University of Vermont have reported that marigolds have been effective in luring pests away from other ornamental plants.
Although science has yet to prove that marigolds actually repel pests from vegetable crops, there is scientific evidence that marigolds CAN be an important tool in controlling certain nematodes. Nematodes are tiny worms, usually microscopic in size. Nematodes that feed on plants — called plant-parasitic nematodes — have spear-like mouthparts used to puncture plant roots to obtain nutrients. As a result, plant-parasitic nematodes can seriously damage or even kill crops, turf, and ornamental plants.
How marigolds help fight nematodes
Marigold roots release a toxic chemical (alpha-terthienyl), and the presence of this chemical inhibits the hatching of nematode eggs. Therefore, control of the nematode population is achieved by interrupting the nematode life cycle.
One drawback with using marigolds for nematode control is that the benefit is not realized until the following year. To be effective the marigolds must be planted before the vegetable crop — at least 2 months before — and must be planted at the same location where the vegetable crop is to be planted; otherwise, no benefits will be gained from the marigold root exudates. For example, California research showed that tomatoes grown after marigolds had significantly lower numbers of root galls due to root-knot nematodes (Meloidogyne incognito). In addition, the tomato yields were higher (root length, shoot weight, number and weight of fruits were all higher in plants grown after Tagetes). UniversityofCaliforniaRiverside.edu.
Varieties Matter
Care should be taken when purchasing marigolds for controlling nematodes. That’s because “not all marigold varieties control all types of nematodes.” Univ.ofFlaExt.edu/ng045. For example, the California research mentioned above revealed that particular varieties are more effective at controlling root-knot nematodes. In that experiment, the “Single Gold” variety of Tagetes patula outperformed other varieties. You’ll get the best results if you determine which types of nematodes are in your local soils, and you can do this by sending soil samples to a nematode assay laboratory. Univ.ofFlaExt.edu/ng045 (look at the chart in this article identifying marigold species and varieties by their resistance to and effectiveness against particular types of root-knot nematodes).
Attracting Beneficial Insects
In addition to helping control nematodes, marigold flowers attract beneficial insects that not only pollinate, but also help control bad bugs. Beneficial insects attracted to marigolds include: hover flies, lady bugs and parasitic wasps.
Conclusion
To date there is very little scientific evidence that the aroma of the marigold plants actually repel pests, however it is a generally accepted scientific fact that marigolds help to control nematodes and attract beneficial insects that aid in controlling unwanted pests.
There is growing concern about pesticides’ non-target effects on humans and other organisms, and many pests have evolved resistance to some of the most commonly-used pesticides. Together, these factors have led to increasing interest in non-chemical, ecologically sound pest management. The marigold is not only pretty but offers the gardener another arrow for the quiver in the bug war. Who wouldn’t want to plant a beautiful plant that was edged out by the rose for our national flower. I know I will!
Thanks for stopping by The Garden Shed. We members of The Garden Shed Team wish you and your family a safe and happy holiday season.
Sources:
“Tagetes L., Marigold,” Plant Profile, USDA https://plants.usda.gov/core/profile?symbol=TAGET
“Tagetes erecta, ” Missouri Botanical Garden, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=277371
“Effects of Selected Marigold Varieties on Root-knot Nematodes and Tomato and Melon Yields”, Department of Namatology, University of California, http://faculty.ucr.edu/~atploeg/PDF PAPERS/PLANT DISEASE/PLANTDISEASE.pdf
“Marigolds (Tagetes spp.) for Nematode Management,” University of Florida Institute of Food and Agricultural Sciences, Univ.ofFlaExt.edu/ng045.
“Companion Planting and Insect Pest Control,” Rutgers University, http://pemaruccicenter.rutgers.edu/assets/PDF/publications/2013-InTech-Companion_planting_and_insect_pest_control.pdf
“Murdering Thrips with Marigolds, Fungi & Mites,” University of Vermont Entomology Research Laboratory, https://www.uvm.edu/~entlab/Greenhouse IPM/Workshops/2015/AFEProjectDesc&SummaryDec2014.pdf
“Root-knot Nematodes: Biocontrol with Marigolds,” North Carolina Cooperative Extension, https://carteret.ces.ncsu.edu/wp-content/uploads/2013/07/Root-Knot-Nematodes-Biocontrol.pdf?fwd=no
“The Repellent Marigold And Other Myths Of Companion Gardening,” The Chicago Triburne, http://articles.chicagotribune.com/1992-09-13/news/9203230490_1_companion-plantings-carrots-love-tomatoes-bedding-plants
“Dispelling Marigold Myths,” Alabama Cooperative Extension System, http://www.aces.edu/dept/extcomm/specialty/marigolds.html
“Parasitic Wasps: A Gardener’s Best Friend,” North Carolina Cooperative Extension, https://brunswick.ces.ncsu.edu/2014/04/parasitic-wasps-a-gardeners-best-friend/
“PROMOTING THE MARIGOLD AS NATIONAL FLORAL EMBLEM,” The Dirksen Congressional Center, http://www.dirksencenterprojects.org/promotingmarigold.pdf
“Controlling Insects on Flowers,” USDA, https://naldc.nal.usda.gov/download/CAT87210014/PDF
2003 Annual Report of Accomplishments and Results, The Connecticut Agricultural Experiment Station, https://portal.nifa.usda.gov/web/areera/Reports/2003/Ct/AES.NewHaven.CT.pdf
“Attracting Beneficial Insects, Penn State University,” https://extension.psu.edu/attracting-beneficial-insects
Winter, the dormant season, is when the bare bones of the landscape are revealed.
In the Ornamental Garden
The eastern redcedar — is it a “weed” or a valuable native tree?
In the Edible Garden
“What are you doing?”
December is a good time to reflect on your gardens successes and failures.
Happy holidays from The Garden Shed! You’ve completed all your Christmas shopping and done all your decorating, so now youre looking for a few gardening tasks. Right? Well, heres our list of December tasks and tips for the edible garden:…
Recipe of the Month
Wow your holiday guests with this tasty make-ahead appetizer!
How To Treat Tree Borers.
FAQ
What repels vine borers?
Do coffee grounds repel squash vine borers?
What do you spray for vine borers?
How to remove vine borers?