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
Mealybugs are common garden pests that can infest many plants, including popular flowers like African marigolds These small, sap-sucking insects secrete a sticky substance called honeydew that attracts other pests and can promote fungal growth Left unchecked, a mealybug infestation can weaken and even kill marigold plants. The good news is there are effective organic methods to get rid of mealybugs on African marigolds. With persistent monitoring and treatment, you can protect your flowers and avoid serious damage.
Identifying Mealybugs on Marigolds
Mealybugs are tiny, soft-bodied insects about 1/8 to 1/4 inch long. They are usually light gray or white in color. Mealybugs get their name from the white, powdery wax coating that covers their bodies. This waxy layer helps camouflage them on plants.
Carefully inspect African marigolds to spot early signs of a mealybug infestation
- White cottony masses on stems, leaves, buds, or flowers
- Honeydew secretions that make plant parts appear shiny or sticky
- Sooty mold growing on honeydew deposits
- Leaves turning yellow, wilting, or dropping
Check undersides of leaves and hidden crevices where mealybugs like to hide and feed. A magnifying glass can help you spot them more easily. Catching infestations when they first start gives you the best chance to control these pests.
Cultural and Physical Control Methods
For light or localized mealybug infestations non-chemical control methods may be effective for African marigolds. Start with these techniques
Remove heavily infested parts – Prune off leaves, stems, or flowers with large numbers of mealybugs. Dispose of the clippings immediately to prevent spread.
Use pressurized water – Blast mealybugs off plants with a forceful spray of water. Focus on undersides of leaves and crevices. Repeat weekly as needed.
Apply alcohol – Use a cotton swab dipped in 70% isopropyl alcohol to rub off mealybugs. This method works best for light infestations.
Attract natural predators – Ladybugs, lacewings, and parasitic wasps feed on mealybugs. Plant nectar flowers like daisies or yarrow to draw in these beneficial insects.
Manage ants – Ants often “farm” mealybugs for their honeydew secretions. Control ants to help eliminate this protective relationship.
Adjust growing conditions – Avoid overfertilizing and maintain moderate humidity levels for marigolds. Mealybugs thrive on succulent, rapidly-growing plants.
For any spreading or severe mealybug infestations, however, pesticide sprays are the most effective treatment option.
Insecticidal Soaps and Oils
Insecticidal soaps and horticultural oils kill mealybugs on contact while posing minimal risks to people, wildlife, and the environment. With regular applications, these organic pesticides can eliminate heavy infestations:
Insecticidal soaps – Potassium salts of fatty acids disrupt soft bodied insects’ cell membranes. Spray mealybug-affected parts of marigold plants until fully coated. Allow soap to penetrate for several hours before rinsing off.
Neem oil – Derived from the neem tree, this oil smothers insects and interferes with their reproduction and feeding. Mix neem oil concentrate with water according to label directions and spray mealybug infestations.
Horticultural oils – These lightweight petroleum oils suffocate insects. Coat all infested plant parts for best results. Oils are less effective on waxy insects like mealybugs, so repeat applications are needed.
Apply soaps or oils every 5-7 days over 2-3 weeks to kill juvenile mealybugs as they hatch. Be sure to spray undersides of leaves and get good coverage in hidden crevices where mealybugs hide. Test on a small area first, as marigold foliage can be sensitive to oils. Avoid spraying during the heat of day or when flowers are in bloom.
Systemic Insecticides
Systemic insecticides are absorbed into plants and transported to all tissues. They provide extended control as they poison insects for weeks after application. While stronger than other options, systemic insecticides may also kill beneficial insects. Use them selectively when other methods fail:
Imidacloprid – Apply this neonicotinoid chemical as a soil drench or spray on marigolds. It causes neurological damage in feeding insects. Use caution, as imidacloprid can impact pollinators and aquatic life.
Acephate – This water-soluble organophosphate insecticide is absorbed into plant tissue and ingested by feeding mealybugs. It breaks down quickly but can be toxic to bees and beneficials during application.
Horticultural oil mixtures – Some products mix oils with systemic insecticides like imidacloprid for enhanced effectiveness on waxy insects. However, these combinations are stronger and require caution around pollinators.
Carefully follow label application directions when using systemic insecticides. Avoid spraying them on blooming marigolds. Insect growth regulator products containing azadirachtin can also deter mealybugs when applied early before populations explode.
Ongoing Prevention and Monitoring
Getting rid of a mealybug infestation takes diligence and repeated efforts. Continue inspecting and treating plants until all signs of the pesky pests are gone. Prune back any permanent damage to encourage new marigold growth.
Going forward, check for mealybugs regularly and take action at the first signs. Maintain optimal conditions for marigolds by:
- Giving plants adequate sunlight
- Watering at the base to keep foliage dry
- Promoting air circulation
- Fertilizing lightly to avoid excessive new growth
- Removing weeds which may harbor mealybugs
Watch for natural predators like ladybugs and lacewings. Their presence indicates your other strategies are working to restore balance. With preventative care and prompt organic treatments, you can enjoy vibrant, healthy African marigolds mealybug-free!
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 Get Rid of Mealybugs – Part 1 of Logee’s “Pest Prevention” Series
FAQ
How do you get rid of mealy bugs on African Violets?
What is the best home remedy for mealybugs?
How do I permanently get rid of mealybugs on my plants?
What is the best killer for mealybugs?