Besides fishing, there is probably no other hobby more steeped in traditions and folklore than gardening. "Place eggshells near your tomatoes to prevent blossom end rot." "Plant corn on a new moon and you'll get small ears." "Hang soap on your fence posts if you want to discourage deer." If you have been gardening for some time, you have undoubtedly come across one of these bits of garden wisdom or at least something similar in nature. Some of these traditions, passed down through the generations, are the result of decades worth of accumulated gardening experience. It's understandable that such knowledge would be passed on, especially when considering that garden produce once made up the majority of a family's sustenance. With so much riding on the outcome of your garden, you'd want to be sure that your techniques got passed down to your children before they left to set up their own households. But nowadays, with so much of this information accumulated and disseminated through books, magazines, the internet and social media, it can be difficult to sort out the facts from the superstition.
This seems especially true when it comes to companion plants and their use in controlling pests. When reading articles on this topic, I'm left wondering, "What is the original source of this information? Are they just repeating some superstition that has been passed on for decades?" And while garden books and blogs are replete with information on the supposed insect-deterring properties of certain plants and the authors seem authoritative on the subject, supporting evidence for these claims is seldom provided. If there is a citation for the information, it usually leads back to-- you guessed it-- another gardening book.
With this in mind, I've set upon a quest to verify the some of the most popular insect-deterrence claims. I looked into dozens of the most popular insect-deterring plants and unfortunately, the evidence for many of them is pretty scant. Nevertheless, I recognize that there may not be interest in (or funding for) researching a plant's supposed insect-deterring qualities, even though the claims may be based in fact. For this reason, I've chosen not to use a myth-busting format here, but rather to lift up and promote those plants that do, in fact, have scientifically proven efficacy against certain pests. In this article we're going to discuss ten plants with the most compelling evidence for efficacy, focusing our attention on those that are most relevant for North American gardeners. So, let's get started. In no particular order, here are ten of the most proven insect-deterring plants for the home garden.
1. Basil
For most gardeners and chefs, the sweet and spicy scent of basil is a delight to the senses, however over the years, gardeners have learned that some pests seem to be repulsed by basil's strong aroma. Scientific studies have borne out this observation. In fact, researchers in one study found that Gypsy Moth larvae would rather starve to death than eat leaves that had been treated with basil essential oils. When they separated the oil into its individual elements, the compounds reduced feeding by anywhere from 85 to 94 percent. And apparently some especially astute momma bugs recognize that a neighborhood with basil is no place to raise a family. Researchers have found that growing tomatoes in close proximity to a basil plant reduced the egg-laying behavior of female leafminers by 50% or more (Kostic et al. 2008.) Basil's pest-deterrent properties, either on the egg, larval, or adult stages has been confirmed in mosquitos, tobacco cutworms, and black cutworms. Seed savers may also take special notice that basil extracts have proven effective against the adult and egg stages of the ever-bothersome Mediterranean flour moth and Indian meal moth.
2. Calendula
Also known as pot marigold, Calendula has long been recognized as an important plant for deterring a variety of common garden pests. One study found that interplanting calendula with cabbage significantly reduced the prevalence of diamondback moth larvae and pupae by an average of 88% across three growing seasons. Similar results were observed in another study when interplanted cabbage was compared against monocultured cabbage for the presence of aphids. In the monocultured crop, aphids were quick to infest the plants with populations soaring to over 400 aphids per plant, on average, by mid-season. By contrast, the cabbages that were interplanted with calendula peaked at 50 aphids per plant. Twenty percent of the interplanted cabbages were still aphid-free at the end of the season.
As far as I can tell, research has yet to identify the mechanism responsible for calendula's insect-deterring properties. Both studies cited above found a similar number of parasitoid insects among the monocultured and interplanted treatments, suggesting that an increase in parasitism is not at play. Other hypotheses might include a reduced attraction to interplanted cabbages by egg-laying females or a reduced hatchability of eggs somehow induced by the calendula. Indeed, research has found that both leaf and flower extracts of calendula can significantly reduce the hatchability of eggs in other insects. In one study, calendula extracts decreased hatchability of tobacco cutworm eggs by up to 98 percent.
3. Nasturtium
Nasturtium has long been recognized as a versatile companion plant in the garden. Over the years, many gardeners have used nasturtium as a trap crop for aphids and a deterrent for a number of other garden pests. One of the mechanisms responsible for this effect is the plant's ability to recruit and support healthy populations of important parasitoids, which in turn feed on destructive garden pests, keeping their populations at bay. Researchers in one study found that the flowers of nasturtium selectively benefited the parasitoid Copidosoma koehleri without providing any benefit (in terms of food or reproductive advantage) to its host, the potato tuber moth. Selective strategies such as this have the potential to make biological control of pests even more effective. Likewise, another study found nasturtium to be an effective support plant for the parasitoid Trissolcus basalis which attacks stinkbug, a common pest for beans, tomatoes, corn and other garden vegetables.
Nasturtium has also been found to be especially effective at deterring both the striped and spotted cucumber beetles, which are common pests to cucumbers, watermelon, and other cucurbits. One study tracked cucumber beetle populations across a growing season and found that companion plantings that included nasturtium reduced cucumber beetle populations by up to half.
4. Thyme
Thyme oil is perhaps one of the most well-documented natural insecticidal extracts known to exist, so it is easy to understand why thymol, the major component of thyme oil, and its derivatives are often the basis for many commercial bioinsecticides. Thymol has been shown to cause greater than 90% mortality in larvae of the tobacco cutworm. Thymol and its derivatives have additionally shown insecticidal, larvicidal, or feeding inhibition in armyworms, Japanese beetles, cabbage loopers, aphids, thrips, and whiteflies. With such a wide range of pest-deterring abilities, thyme deserves a place in every garden. In fact, it probably deserves more than one place in every garden.
5. Peppermint
In a test of 41 different essential oils, peppermint oil was found to be one of the most effective treatments for reducing attraction of Japanese Beetles to field traps. Traps that were treated with peppermint oil had roughly 70% fewer beetles in the trap compared with controls. Presumably this effect would, at least to some extent, be exhibited by the peppermint plant itself. Just be cautious as peppermint is known for being invasive. It might be worth a try to sink a pot of peppermint into the ground near the base of your pole beans, as these plants can be especially attractive to Japanese beetles.
In one of the more interesting mechanisms of deterrence, studies have identified one particular compound found in Mentha piperita that seems especially effective in repelling and killing a wide variety of aphid species. As it turns out, that compound, known as (E)-β-farnesene (Eβf) is identical to one produced by aphids in response to an attack by predators. This so-called alarm pheromone causes other aphids in the vicinity to stop feeding and move away.
Finally, besides its usefulness in the garden, mint extracts have long been known to repel and kill a wide variety of mosquito species. This makes mint an excellent choice for the patio garden where it can help facilitate a relaxing, slap- and itch-free evening.
| I'm Getting Mixed Signals Here |
| One of the mechanisms by which companion planting works is to confuse a pest's navigation system by interfering with the intricate biochemical interactions that occur between the pest and its host plant. Certain companion plants, such as marigolds (Tagetes spp.), release allelochemicals like limonene and alpha-terthienyl, which are known to disrupt insect olfactory receptors. These compounds interfere with the insects' ability to detect host plants, effectively confusing them and reducing pest damage. Additionally, studies have shown that companion plants like basil (Ocimum basilicum) emit volatile organic compounds (VOCs) such as estragole and linalool, which interfere with insect navigation and feeding behavior. By harnessing these chemical cues, gardeners gain not only a beautifully scented garden, but also a sophisticated defense mechanism to help protect their garden against destructive pests. |
6. Summer Savory
Although not among the most popular herbs for the kitchen garden, summer savory (Satureja hortensis) deserves a closer look for both its culinary quality and amazing insect deterring properties. In fact, while scouring the scientific literature for insect-deterring plants to include this article, time and time again I found summer savory tested against common pests, often alongside other plants, and time and time again it was found to be the most effective. For example, researchers in one study found that the essential oil of summer savory elicited a 100% mortality rate in adult broadbean weevils in just 36 hours after treatment. Similar effects have been observed for the cowpea beetle, tomato leafminer, cotton whitefly, maize weevil, and at least a dozen more common pests.
7. Rosemary
Are you catching a trend here? There seem to be a lot of aromatic herbs on this list, and the next insect-deterring plant is no exception. Rosemary is a fragrant, woody-stemmed plant that is native to the Mediterranean. Research has shown that interplanting rosemary with sweet pepper significantly reduces the prevalence of three common pepper pests: thrips, green peach aphids, and whiteflies. Furthermore, while the presence of rosemary was a deterrent to these costly pests, no adverse effects were observed on the populations of their natural enemies, indicating a selective effect that may further improve integrated pest management strategies.
Possibly most interesting to gardeners is the effectiveness of rosemary at deterring the diamondback cabbage moth. This garden nuisance is a constant challenge to gardeners, laying its eggs on cruciferous vegetables like broccoli, cabbage, and kale. Once emerged, the larva chew holes in the leaves and sometimes florets of these plants and are quite an unappetizing sight to the person responsible for cleaning and preparing the produce. The diamondback moth was one of the first pests to show resistance to pyrethrin and has since developed variable resistance to Bacillus thuringiensis (Bt), so its control can be difficult.
With all of these challenges mounting, any approach that can keep these pests at bay is welcomed. That's why I was so excited to discover from a 2022 study, that rosemary essential oils showed a high degree of effectiveness at killing larva and deterring oviposition (the laying of eggs) in adult diamondback moths. A 72% reduction of egg-laying was observed in this study. Similarly, another study found that including dried, crushed rosemary leaves in samples of dried beans reduced the egg-laying behavior of bean beetles by more than 87%.
8. Sage
Rosemary and sage don't just go together in Simon and Garfunkle songs. It turns out their essential oils are both effective at killing diamondback moth larva and inhibiting egg-laying behaviors in adults. What's more, neither of them had any detrimental effects on the parasitoid, Cotesia vestalis, a parasitic wasp that lays its eggs in diamondback moth larva. With the possibility of further synergistic effects by planting them together, sage definitely deserves consideration if you're already planning on growing rosemary near your Cole crops.
If you end up with a few extra seedlings and happen to have some space near your patio, sage and rosemary might also be worth a try there, especially for their ability to repel the bothersome blowfly. Blowflies are attracted to organic matter (like that found in compost piles), meat, and manure from pet droppings. In one study, the essential oils of sage and rosemary were both found to completely deter female blowflies from depositing their eggs for up to 24 hours after application. I'm wondering if this might be another reason why sage has for centuries been one of the most popular herbs for seasoning meat.
9. Onions and Garlic
Intercropping onions with cabbage is a commonly practiced technique for keeping pests at bay and this practice is one that has lots of evidence. Researchers in one study found that intercropping cabbage with either onions or garlic significantly reduced the prevalence of pests, reduced damage to leaves, and increased the overall yield of the cabbage. Interestingly, the researchers found that this arrangement was most effective when the onions were planted between cabbage rows rather than within. These benefits have been confirmed in numerous other studies (Baidoo et al. 2016) . Intercropping has also been effective at reducing pests in potato, strawberry, and mustard, to name a few.
Furthermore, scientists have identified a specific compound within garlic that seems to have especially potent insect-deterring properties. Allium sativum leaf agglutinin (ASAL) has been shown to have anti-feedant and insecticidal effects against a variety of sap-sucking insects. Some scientists have even sought to genetically engineer crop plants to produce this compound in order to protect against common pests.
If you live in the South, you may appreciate that garlic provides a great deal of protection against some common garden pests including spiny and cotton bowlworms, red cotton bugs, and tobacco cutworms. Researchers in one study found that eggs of these species displayed slowed or arrested development when exposed to volatile compounds produced by garlic.
10. Marigold
No article on pest-deterring plants would be complete without mentioning marigolds. Long recognized as the workhorse of companion plants, marigolds are beloved for their ability to deter a wide variety of garden pests. Today, Marigolds (Tagetes spp.) are one of the most widely studied genera, with over 80 years of research supporting their effectiveness as a suppressor of plant parasitic nematodes (PPN). Owing to their vast economic toll and wide range of host plants-- which includes most garden vegetables-- PPN are regarded as one of the most destructive agricultural pests. And while a few chemical solutions have been developed, prevention and natural approaches remain the most effective and widely used strategies for home gardeners. In addition to management strategies like crop rotation, proper sanitation, and soil solarization, adding marigolds to the garden can be an effective strategy for controlling these bothersome pests.
Several mechanisms have been proposed to explain the mechanisms by which marigolds control nematodes, including acting as a trap crop, producing chemicals that inhibit the development of PPN, and attracting antagonistic organisms. These mechanisms may act separately or in combination. Research seems to suggest that planting the marigolds as a cover crop and subsequently incorporating the plant matter is the most effective way to take advantage their nemacidal properties (summarized here). Interplanting may also be used but is less effective. In terms of their usefulness, marigolds seem to be very good at increasing yield. One study found that planting marigolds as a cover crop preceding tomatoes increased tomato yields by 50 percent. The same study found that planting marigolds before melons increased yields by up to 95 percent. Significant increases in yield have also been observed for potato (8-14%), carrots (65-173%), and cowpeas (106-123%), to name a few.
Conclusion: The Best Pest-Deterring Plants for the Home Garden
Well, there you have it-- ten plants with proven effectiveness against a variety of common insect pests. If you're looking for some natural ways to protect your garden against bothersome pests, I hope this article will at least give you a place to start. And if you've been battling a particularly bothersome pest, I hope you found the next tool to add to your insect-fighting arsenal. Of course, as with everything in gardening, experimentation is key. You won't know exactly what works for you until you try, but by harnessing the wisdom that has been gathered and tested by generations of our gardening predecessors, you'll be off to a great start.
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