A cockroach infestation in one’s yard or garden, especially if it is located in an urban setting, is anyone’s worst nightmare. These seemingly malevolent and destructive insects are some of the most troublesome pests to deal with. Their tolerance for pollutants, along with their resilient nature, allows them to thrive off of decaying plant matter, trash, and even human waste!
That said, cockroaches aren’t actually known for feeding on intact vegetables and for damaging live plants. They are an important part of the food chain in the wild as many larger reptiles, birds, and even mammals may obtain high-quality proteins from feeding on them. The problem lies in the consequences of their entry into our homes, schools, factories, and more. Those that come into contact with sewers, canals, and other structures where pathogens are likely to breed become vectors of dangerous diseases.
The best way to eliminate cockroaches involves removing potential sources of food and converting habitats that may sustain their rapid reproduction rate. This is easier said than done, particularly in highly populated areas. Pesticides and traps would certainly help, but they can become destructive and dangerous in their own ways. Fortunately, some strongly-scented plants and their oils might help!
1) Mint (Mentha spp.)
The Mentha genus, a wondrous group of plants in the Lamiaceae family, contains more than a dozen valuable species. In the world of aromatic herbs, these are set apart by their strong and sharp scent and taste. Some of the most popularly grown types, which are used for both industrial and ornamental purposes, include spearmint (M. spicata), watermint (M. aquatica), corn mint (M. arvensis), and peppermint (Mentha x piperita).
One thing all mint species have in common is the high concentration of menthol in their essential oils. Though the fragrance and flavor of this compound are appealing to humans, menthol is undeniably offensive and toxic to many pests. Studies that exposed several types of cockroaches to pure extracts of mint plants have shown that the oils are capable of killing these bugs.
To maximize the cockroach-repellent effect of mint plants, you can either plant them outdoors (in key parts of the garden and along entryways), grow them indoors, or directly make use of their essential oils. As cockroaches tend to favor moist areas, which usually promote the growth of most mint species, you can place these plants around their potential nesting habitats.
2) Rosemary (Salvia rosmarinus)
Among the dozens of valuable herbs from the Mediterranean region, rosemary is one of the most useful species when it comes to repelling cockroaches. These troublesome critters are unlikely to feed and occupy habitats close to dense stands of rosemary shrubs for a couple of reasons. The scent of rosemary stems and leaves, especially those that are oozing out oils, is repulsive to them. Moreover, the blooms attract beneficial insects that may help control small populations.
Rich in potent phytochemicals like camphor, rosmarinic acid, and carnosol, rosemary oil is bitter and highly fragrant. When tested for its repellency and toxicity against the brown-banded cockroach (Supella longipalpa), a common vector of pathogens, the oil was shown to be remarkably effective at a low concentration. Mere contact with droplets of a rosemary oil solution was found to cause 100% mortality.
Of course, this is great news for anyone who likes cooking with rosemary and appreciates its ornamental contribution to gardens! By cultivating rosemary shrubs, you can naturally reduce the chances of cockroach infestations. Regularly collect cuttings and spread these around entryways and moist areas to dispel their scent.
3) Chrysanthemums (Chrysanthemum spp.)
Pyrethroids, which are synthetically produced and incorporated into chemical-based pesticides, are some of the most effective insect-repellent ingredients. Amazingly, the formulations of these are based on compounds (i.e. pyrethrins) that are found in the blooms of just about every type of chrysanthemum. When dried and pulverized, chrysanthemum flower heads can be utilized as an organic pest repellent in large-scale farms.
Cockroaches are just one of the many insects that can be controlled with pyrethrin-rich pesticides. Flies, fleas, ants, moths, and mosquitoes are also affected by chrysanthemum powder. Though this has a low-toxicity profile for humans and other mammals, it can both kill and repel widespread pests. To avoid allergic reactions, just make sure you yourself don’t breathe in or touch the powder with your bare hands.
To use chrysanthemums as a cockroach repellent around your home or garden, you’ll need to collect their blooms. The plant itself is not typically effective as a repellent unless its flowers are present. Place either fresh or dried cuttings of blooms around areas that are most likely to be frequented by roaches.
4) Catnip (Nepeta cataria)
A member of the Lamiaceae or mint family, catnip is unarguably one of the most effective organic repellents for a wide range of insects. Though cats seem to be obsessed with this plant as it stimulates their nervous system in a peculiar way, its oil can literally make insects go crazy. Nepetalactone, the primary component of catnip essential oil, effectively repels cockroaches, mosquitoes, and even termites.
Compared to DEET, a synthetic insecticide that has been incorporated into many commercial bug repellents, nepetalactone is reportedly up to 100x better at repelling cockroaches! This goes to show just how well-evolved plants are at protecting their stands from pests in the wild. Biotoxicity assays (experiments that look into the lethal effects of substances on live animals) have shown that catnip oil can disrupt the nervous systems of cockroaches.
If you have a dog or small children in your home, using catnip cuttings and oil extracts is a fairly safe alternative to chemical-based products. If you have cats, however, it may not be the most suitable plant because its sustained presence and repeated ingestion can lead to toxic symptoms.
5) Lemongrass (Cymbopogon citratus)
A tropical grass grown for both ornamental and culinary purposes, lemongrass is one of the most notably fragrant members of the Poaceae family. Known for its smoothly margined and lengthy, blue-green leaves, it is remarkably easy to maintain and propagate in warm areas. Both fresh and dried leaves are able to impart a strong flavor to dishes because of their citrus-scented essential oil.
Limonene, a phytochemical with insecticidal properties, is a dominant component of lemongrass oil. Also contained in lemon and many other citrus fruits, this compound naturally repels cockroaches and many other insects. A toxicity test that looked into the effectiveness of lemongrass as a cockroach killer proved that direct contact with the oil was lethal. Sub-lethal doses, on the other hand, resulted in 100% repellency.
While intact tufts of planted lemongrass should produce a detectable fragrance, especially if some of the leaves are rubbed or cut, pure extracts of the oil should work best. You may also collect cuttings of the blades of grass and place them in areas that seem to be frequented by pests.
6) Osage orange (Maclura pomifera)
Found in savannas, mountains, and prairies throughout its native range, the Osage orange is a valuable tree. Its knotted and twisted wood is known for being able to withstand rot and for serving as good-quality timber. With a high heat value and durability, it is a fine choice for making wooden posts and door knobs. Apart from these merits, this phenomenal tree produces fruits that can repel cockroaches!
Despite the plant’s common name, it isn’t actually related to oranges of the citrus family at all. Instead, it is a member of the fig or Moraceae family. Unlike the typical oranges you’re likely to find in the supermarket, those of the Osage orange tree are bumpy and yellow-green. Usually called hedgeapples, the fruits are inedible and contain milky, chemical-rich latex with insect-repellent properties.
Though hedgeapples on their own may not be enough to prevent insect infestations, they can definitely be used to repel cockroaches, houseflies, and mosquitoes in small spaces. If you’re able to cultivate this tree in your own yard, you can collect its fruits and place its fragrant slices close to entry points and windows. Note that hedgeapple slices may not be as effective as natural repellents outdoors.
7) Garlic (Allium sativum)
A list of insect-repellent plants is seldom complete without garlic, which is one of the most fragrant and strongly-flavored root crops in mass cultivation. This member of the Amaryllidaceae family of bulbs produces oil-rich tissues. When mechanically damaged, these are undeniably odoriferous as they secrete oils that can quickly waft through small spaces.
Sulfur-rich compounds, many of which have pesticidal properties, dominate the chemical profile of garlic essential oil. When these break down, they produce the tangy, hot, and strong scent and taste that we look for in garlic-flavored dishes. Though these compounds are treasured by humans for the sensory experiences they induce, they function as defensive mechanisms to protect the plant against grazers and pests.
As the pungent smell of garlic can turn away a cockroach, consider crushing cloves and placing them in areas where you are most likely to see cockroaches and other bugs. In the absence of actual garlic, you may also use store-bought garlic powder. Sprinkle this around loose doors and windows. Of course, the plant itself can be used to repel insects in the garden, but it may not be as effective in its intact form.
8) Bay laurel (Laurus nobilis)
The bay laurel is a small, evergreen tree that produces remarkably aromatic leaves. Rich in a wide range of phytochemicals, the essential oil of the plant gives it its unique scent and flavor. When dried, the leaves continue to retain their intensity for about a year. This is why they are used as a common seasoning in Mediterranean soups and sauces.
The most dominant phytochemical in bay laurel essential oil is eucalyptol (i.e. 1.8-cineole). It also contains terpenes like alpha-pinene, beta-pinene, and linalool. All of these compounds function as natural mechanisms for defending the plant against potential grazers and leaf-eating insects. Their collective scent is strong enough to repel cockroaches, especially if they are present in pure laurel extracts or in pulverized leaves.
If you happen to have access to fresh bay laurel leaves, you can pulverize them on your own and spread the powder around your home or garden. The key to getting it to repel roaches is correct placement. Make sure to situate the crushed leaves or essential oil droplets in places where you previously spotted cockroaches. In the garden, you may consider sprinkling the crushed leaves liberally. Do note that they may repel more than just pests, however.
