Pests, which may come in the form of fuzzy aphids, microscopic spider mites, subterranean nematodes, leaf-eating caterpillars, and voracious grazers, are the bane of every crop farmer and garden enthusiast. When these are found in small and slow-growing populations, they are tamed by a well-balanced ecology. Nowadays, however, more than a handful of pests have proven to be extremely troublesome and destructive in both agricultural and urban landscapes.
A number of common trends in plant cultivation have led to the rise of pest infestation rates. These include but are not limited to the importation of non-native plants, the growth of crops in intensive monoculture systems, the destruction of pristine habitats, and the transportation of infested crops. As a result, chemical, biological, and physical means of pest control are now widely used all across the globe.
Unfortunately, many artificial methods of controlling pests can do more harm than good to our natural ecosystems. Many chemicals harm not only pests but also the beneficial insects, pollinators, and predators that would normally help manage infestations in the first place. A better way to naturally reduce pests is by growing pest-repellent plants. These have special phytochemicals for warding off a wide range of insects, herbivores, and pathogens.
1) Basil (Ocimum basilicum)
Basil is one of the world’s most popular annual herbs. This leafy plant is often used as a culinary garnish, main ingredient, and source of oils throughout its native range. It is now cultivated in many dedicated farms and aquaponic systems. Typically distinguished by its ovate, rich green leaves, tender shoots, and thick taproot, it gives off a distinct scent.
Crushing or rubbing basil leaves stimulates the release of its essential oil, which contains a potent mixture of aromatic phytochemicals. These include linalool, estragole, eugenol, myrcene, and 1,8-cineole. Collectively or on their own, these compounds can function as natural insect repellents. This is why stands of basil can be used to repel weevils, thrips, various nematodes, bacteria, and fungi.
2) Rosemary (Salvia rosmarinus)
Rosemary can be grown as an ornamental shrub in regions with mild climate conditions and relatively dry substrates. This wonderful species is known for its leathery, needle-shaped, and deep-green foliage. These are borne on increasingly woody shoots with either upright or trailing forms. In temperate zones, attractive inflorescences may appear on the tips of mature stems from spring through summer.
Widely cultivated for culinary and medicinal uses, rosemary can bring a wealth of benefits to both home gardens and farms as a border or intercropped plant. Its bitter leaves give off a strong scent and are unpalatable to many grazers. Its essential oil contains potent chemicals like camphor, alpha-pinene, rosmarinic acid, and carnosol. While these attract beneficial insects, they deter pests like cabbage moths, Japanese beetles, slugs, and carrot flies.
3) Lavender (Lavandula spp.)
Bound to be on every list of pest-repellent plants, members of the Lavandula genus are some of the most useful herbs for temperate landscapes. These lovely species have uses ranging from ornamental to medicinal. Their greyish-green leaves, spikes of eye-catching blooms, and their unmistakable fragrance are immensely valuable.
Entire fields in the Mediterranean region are dedicated to growing English lavender (L. angustifolia), Spanish lavender (L. stoechas), and French lavender (L. dentata). While their essential oils are especially prized in the cosmetic and herbal industries, they are also promising as biological repellents. Their major components include varied concentrations of linalool, linalyl acetate, and caryophyllene. These repel flies, fleas, moths, mosquitoes, and more!
4) Chrysanthemums (Chrysanthemum spp.)
No-nonsense additions to a perennial flower garden, chrysanthemums are wonderful plants with hundreds of varieties and prized cultivars. They are best known for their vibrant flower heads, which consist of dozens of petals and disc florets. Though they are chiefly cultivated for ornamental purposes, they are increasingly grown as a source of pyrethrins, which are concentrated in their blooms.
Now labeled as the “world’s deadliest flower”, the humble chrysanthemum may just be every farmer’s solution to pest infestations. Pyrethrins are deadly to pestilent insects as physical contact with these compounds can cause paralysis. Luckily, unless they are present in remarkably high doses, they don’t pose harm to humans or livestock. For this reason, dried pyrethrum can even be rubbed on cows to repel ticks and flies.
5) Marigolds (Tagetes spp.)
A symbol of divinity and the connectedness of life and death, marigolds are some of the most culturally important plants in Mexico. These produce vibrant blooms in shades of yellow and red-orange. While they evoke warmth and affection, they may also play many roles in the modern garden. Set apart by their multi-lobed leaves, overpowering fragrance, and their capacity for rapid growth, these members of the Asteraceae family are detested by many pests.
Marigolds are often cited as some of the best general pest-repellent plants because they can turn away both aerial and subterranean insects. Their root systems release a chemical called alpha-terthienyl, which can inhibit the development of root-knot nematode eggs in the soil. The scent of the leaves, on the other hand, can chase off those undesirable whiteflies. Situate their stands around susceptible plants, like lettuce and cabbage.
6) Garlic (Allium sativum)
A must-have in any home chef’s pantry, garlic is one of the most versatile and flavorful root crops. Its bulbs are packed with a complex mixture of phytochemicals, usually dominated by sulfur-rich compounds. These undergo chemical changes when the bulbs are damaged or cut, releasing a distinctly strong odor that can work against dozens of problematic grazers and pests.
Intact garlic bulbs are not the most effective at repelling pests, so they should ideally be harvested and processed to create a potent spray. Simply crush the cloves until they are reduced to a pulpy substance, and then add the pulp to a water-based solution with a few drops of soap. You may also incorporate other repellent compounds, such as those from the essential oils of other plants or from crushed peppers. Spray outdoors to control aphids, caterpillars, mosquitoes, and slugs.
7) Catnip (Nepeta cataria)
Popularized by its seemingly magical effect on cats, catnip is an attractive perennial herb. This member of the Lamiaceae or mint family can be cultivated as flowering edges around delicate crops. Its shoots grow to a maximum height of about 40 inches (1 meter). Its square stems bear triangular leaves with noticeably toothed margins. These are packed with a potent essential oil.
The chief component of catnip essential oil is nepetalactone, a compound that has shown promise as a repellent by irritating the nervous systems of insects. It is particularly effective against mosquitoes, termites, cockroaches, squash bugs, and aphids. Moreover, the oil contains compounds (e.g. iridoids) that attract beneficial wasps, some of which produce larvae that feed on plant-eating pests.
8) Lemongrass (Cymbopogon citratus)
Named for the lemon-like fragrance and flavor of its essential oil, lemongrass is commonly grown as a culinary ingredient throughout its native range. As it imparts the taste of lemon without its signature tartness, it is also commonly added to herbal teas. This source of traditional supplements is, unsurprisingly, quite promising as a pest repellent in farms and gardens.
Rich in phytochemicals like citronellol, linalool, geraniol, myrcene, and citral, lemongrass essential oil can be used to repel stable flies. These flies are incredibly troublesome in many tropical areas as they are able to bite livestock and transmit dangerous pathogens. The plant itself can also be cultivated as a standalone herb for deterring cockroaches, mosquitoes, and rats. Just make sure to occasionally trim its leaves to release a stronger scent cloud.
9) Peppermint (Mentha x piperita)
Peppermint is a natural hybrid between two widespread mint species – spearmint (M. spicata) and watermint (M. aquatica). It is often found in environments where both of its parent plants thrive. Partial to moist conditions, it is a fantastic herb for the borders of water features and rain gardens. Keep in mind that it is prone to self-propagating and forming competitive colonies.
Like many other members of the Lamiaceae family, peppermint has a knack for attracting pollinators while repelling many kinds of pests. Its deep green and markedly toothed leaves are packed with strongly-scented and bitter phytochemicals. These include menthol, menthone, limonene, 1,8-cineole, pulegone, caryophyllene, and pinene, all of which are known for their capacity to deter insects and mammalian grazers.
10) Common wormwood (Artemisia absinthium)
This ornamental herb is infamous for being the source of thujone, a potentially dangerous compound that can cause fatal symptoms in high doses. An ingredient in absinthe, which can no longer be sold legally in some countries, this chemical likely evolved to protect the plant from pathogens, pests, and grazers. Along with a few other phytochemicals, such as absinthin and chamazulene, it is the main component of wormwood’s essential oil.
To take advantage of this herb’s pest-repellent nature, grow it as a hedge or border plant close to susceptible vegetables and fruit trees. Under full sun, it is able to produce shoots measuring up to 3 feet (91 cm) tall. In dense stands, its leaves should keep lice, beetles, mosquitoes, and flies away.
11) Dill (Anethum graveolens)
Widely cultivated for its flavorful leaves, dill is related to parsley and fennel. This flowering plant is set apart by its summer blooms, which are clustered in umbel-shaped inflorescences. The spritely yellow flowers attract many small pollinators and beneficial insects. This species is occasionally grown in butterfly gardens as it serves as the host plant of the stunning black swallowtail’s (Papilio polyxenes) larvae.
Although it attracts butterflies, its aromatic shoots repel harmful pests like mites and aphids. For this reason, its young specimens are often grown close to tomatoes as a companion plant. Tomato growers must prune the shoots before they produce inflorescences as these emit chemicals that may compromise the growth of nearby crops.
12) Common sage (Salvia officinalis)
Once believed to have properties for warding off evil spirits and curses, common sage has been intentionally cultivated for hundreds of years. This attractive perennial herb, perfect for adding muted color and texture to partly shaded gardens, comes in many ornamental cultivars. Its charming foliage is soft to the touch due to the presence of fine hairs. Breaking the leaves releases an overpowering fragrance.
There is perhaps some truth to sage being a “holy herb” as it does have a knack for repelling pests and for aiding in the treatment of bites and stings. Extracts of its essential oil can repel leafworms, beetles, moths, and their larvae. Sage can promote the growth of vegetables when it is used in intercropping systems.
13) Tansy (Tanacetum vulgare)
A member of the Asteraceae family, tansy is a strongly-scented perennial herb. This versatile flowering plant has a reputation for being a natural protector in the garden. Though it is often morbidly associated with death as its flowers are almost guaranteed to be present in funerals, it has a multitude of less ominous uses. Its blooms are a source of natural dyes and its foliage can be incorporated into herbal infusions, pancakes, and puddings!
Tansy is able to repel pests in the garden because its volatile essential oil contains compounds like camphor, 1,8-cineole, myrtenol, and bornyl acetate. As a companion plant, it can drive away ants and pestilent beetles. Its oils can also be used to formulate an organic mosquito repellent.
14) Scented geraniums (Pelargonium spp.)
The remarkable fragrance of scented geraniums doesn’t actually come from their vibrant blooms; it is emanated by the potent oils in their patterned foliage. Though these plants are naturally consumed by the larvae of some Lepidopterans, they are generally effective against other troublesome pests. Zonal geraniums (P. x hortorum), for example, are known for having paralytic effects on the Japanese beetle, which plagues many agricultural farms around the world.
15) Garden thyme (Thymus vulgaris)
Frequently grown as a ground cover or decorative edge plant, garden thyme is a multi-purpose herb. This evergreen member of the mint family typically grows as a subshrub. With trailing stems that rarely grow to more than a foot (30 cm) tall, it is fairly straightforward to maintain and propagate. Apart from filling in gaps, reducing erosion rates, and stifling the growth of weeds, this wonderful perennial can also repel a wealth of pests!
Garden thyme now comes in many eye-catching hybrids and cultivars, some of which have received the RHS Award of Garden Merit. Its leaves contain distinctly spicy yet sweet-flavored oils dominated by compounds like thymol and carvacrol. These phytochemicals protect the plant from plant-eating insects and grazers. They can also be extracted and used as botanical ingredients in mosquito repellents.
16) Nasturtiums (Tropaeolum spp.)
Nasturtiums are generally grown as ornamental plants due to their showy, vibrant flowers and their circular leaves. Depending on the species, they may possess a vine-like habit or develop a more bush-like form. Garden favorites include monk’s cress (T. majus), canary nasturtium (T. peregrinum), and flame nasturtium (T. speciosum).
Often used as companion plants, nasturtiums contain oils with insecticidal properties. The right selection of species may aid in repelling squash bugs, cucumber beetles, whiteflies, and even aphids. They may also function as trap crops in the sense that they may cause damage to some of the pests that they attract, preventing them from attacking nearby crops.
17) Neem tree (Azadirachta indica)
The neem tree, also known as the Indian lilac, is an evergreen, tropical to sub-tropical plant. Able to grow to a full height of about 66 feet (20 meters), its dense crown can contribute shade, shelter for wildlife, and ornamental texture. While this fast-growing species has the tendency to persist as a weed in optimal conditions, its biological contributions cannot be discounted. In its leaves, bark, seeds, and stems are a diversity of insecticidal phytochemicals.
Azadirachtin, just one of the secondary metabolites found in neem seeds, can affect hundreds of species of insects by inhibiting their capacity for feeding and by acting as a growth disruptor. It can even interfere with the reproductive process of some pests, causing sterility. For these reasons, neem oil extract is often sold in plant nurseries as an organic pesticide and as an overall protectant against pathogens.
18) Pencil cactus (Euphorbia tirucalli)
Named for the pencil-like appearance of its slender stems, the pencil cactus grows in a wide range of semi-arid climates. It is a member of the Euphorbiaceae family, which is generally known for its overall hardiness in desert regions and its adaptations against herbivory. Like its close cousins, the pencil cactus repels potential grazers and pests by secreting a toxic, milky sap.
Studies that looked into the pesticidal properties of the pencil cactus have revealed its effectiveness against nematodes, mosquitoes, and aphids. As the caustic sap acts as an irritant, hardly any animals are able to consume large amounts of this succulent. Its stands can be cultivated as protective hedges around susceptible herbs.
19) Crown imperial (Fritillaria imperialis)
Named for the crown-like appearance of its emergent inflorescences, the crown imperial is widely cultivated as an ornamental plant. A member of the Liliaceae or lily family, it is set apart by its orange, downward-facing blooms. Though the vibrant foliage and flowers may visually beckon one to come closer, they have a surprisingly unpleasant scent that does the opposite!
The odor of crown imperial blooms is often compared to that of a skunk or fox. This is probably why it repels rodents and other small mammals. If you’ve had issues with wildlife coming into and damaging your garden, consider growing this species around the perimeter of your property. Don’t situate its stands too close to your windows, however, as late spring winds may cause its musky scent to waft into your home.
20) Eucalyptus (Eucalyptus spp.)
The essential oils of eucalyptus plants have long been favored by herbalists and scent enthusiasts for their calming and therapeutic effects. While their use may reap many benefits for us humans, they are typically detested by wild animals. With effects that are comparable to those of Cymbopogon species, the aroma of eucalyptus oils can repel mosquitoes, ants, spiders, lice, cockroaches, fleas, and some types of moths.
Rich in phytochemicals like 1,8-cineole, citronellol, p-cymene, etc., eucalyptus trees can be grown around your home to deter many insects. You may also make use of store-bought extracts to create a diluted, pest-repellent solution. Spray onto susceptible plants to prevent them from being attacked by common pests.
21) Fennel (Foeniculum vulgare)
A notable member of the Apiaceae or celery family, fennel is set apart by its swollen basal stem and its throes of fine, dissected leaves. It is often grown for its bulbs and its seeds, both of which have a scent and flavor that is likened to that of licorice or anise. The aroma is derived from its complex mixture of volatile compounds. These include estragole, limonene, and rosmarinic acid.
Like other herbs containing a similar combination of phytochemicals, fennel tends to function as a natural pest-repellent in vegetable gardens. To be effective, it needs to be grown in thick stands. Gypsy moth larvae (Lymantria dispar), fleas, aphids, and slugs are just some of the pests that are thrown off by fennel oils.
22) Petunias (Petunia spp.)
Most of the ornamental petunias grown as seasonal garden plants are hybrids of P. axillaris and P. integrifolia. These avid bloomers thrive in moist, well-draining substrates and under at least five hours of full sun exposure each day. Remarkably popular with home gardeners, they can brighten up verandas, hedges, and containers with their pink, purple, red, and white blooms.
Though they are seldom cultivated for purposes beyond visual interest, petunias are actually beneficial in vegetable gardens. They can help repel some of the most common crop pests, such as squash bugs, asparagus beetles, tomato hornworms, and more! Some of the sugar esters and glycolipids found in petunia leaves and flowers act as natural pesticides.
23) Common rue (Ruta graveolens)
Grown as an ornamental plant because of its highly textural, bluish leaves, the common rue is an impressively hardy and useful perennial. The national herb of Lithuania, this species can easily be cultivated as a fragrant, low-growing hedge around susceptible flower beds and crops. Its shoots may become increasingly woody as they lengthen to about 3 feet (91 cm) tall.
Common rue is an ideal companion plant for roses, raspberry shrubs, lavender bushes, and fig trees. When its leaves are mechanically damaged, they release strongly-scented oils that repel maggots, snails, slugs, Japanese beetles, fish moths, aphids, and flies. Rich in mutagenic and hepatotoxic compounds, the toxic oils can even cause blisters upon contact with the skin.
24) Citronella grass (Cymbopogon nardus)
When this close relative of lemongrass was discovered to have mosquito-repellent oils, it became remarkably popular as a garden plant for general pest-repellence. Citronella oil is rich in pesticidal compounds like citronellal, citronellol, limonene, and geraniol. These are strongly-scented phytochemicals with a non-toxic mode of repellence against several hematophagous insects and other common crop pests.
Intact and healthy stands of citronella grass are able to deter pests to an extent, but do note that cuttings or extracted oils may be more effective as they emit a stronger scent. You may grow this low-maintenance grass around key outdoor areas, like verandas and porches. Aim to restrict its roots to within large containers or pots as they can spread aggressively in optimal conditions.
25) Borage (Borago officinalis)
Like many other herbs that share its native range, borage is equipped with adaptations for protecting its delicate leaves and blooms from pests and grazers. When it is present in considerably large stands, its repellent effects may extend to adjacent plants. Moreover, its eye-catching cymes of deep-blue to purple blooms attract a wealth of beneficial insects. These aid in the growth of economically important crops as their larvae parasitize common pests.
As a companion plant, borage is usually most beneficial alongside strawberries, legumes, cabbages, broccoli, and spinach. Its leaves and seeds contain pyrrolizidine alkaloids. When consumed, these can cause toxic symptoms in herbivores. Interestingly, the larvae of some pollinators accumulate these compounds to make themselves unpalatable to potential predators.
