Although ants are occasionally beneficial in the garden, serving as natural pollinators, aerating the soil by digging tunnels, and carrying nutrients to the roots of plants, their overabundance can be very problematic. Some species of ants are relatively easy to control, but others are deceptively dangerous. Packing a painful bite and incredibly strong for their size, many red ants (e.g. fire ants, carpenter ants) are considered a nuisance.
Large ant populations can harm the garden by attracting pests. One of the most famous mutualistic relationships between two insects is that of the ant and the aphid. Aphids, which have a reputation for sucking the life out of plants, produce honeydew. This sweet substance is favored by ants, which may protect and even farm aphids to secure a consistent source of nourishment for themselves.
The nesting habits of ants can damage natural materials in the garden and may even endanger pets and young children. When provoked, they can leave their nests and travel in unpredictable swarms, possibly entering your home afterward. In the absence of anteaters and organic pesticides, there are fortunately some types of plants that can help keep their numbers at bay. Besides producing oils that deter many insects, their blooms may also attract pollinators that feed on your ants.
1) Mint (Mentha spp.)
Known for their aromatic properties and their versatility as kitchen staples, plants of the Mentha genus are fantastic additions to just about any garden. Numbering anywhere from 13 – 24 taxonomically accepted species, these perennial herbs have highly valuable essential oils with culinary, medical, and therapeutic uses. Some of the most popular varieties include peppermint (Mentha x piperita), spearmint (M. spicata), corn mint (M. arvensis), and apple mint (M. suaveolens).
Mint oils are considered natural and environmentally safe insecticides. They are capable of repelling not just ants, but also wasps, hornets, cockroaches, and mosquitoes. Their strong scent profiles and potent chemical compositions make for an effective deterrent. These oils are usually found all throughout the plant, with the highest concentrations in the textured leaves. Mechanically damaging or simply rubbing your fingers against the leaves should release some of the oils.
Most mint species thrive best under partial shade and in moist conditions. To use them as ant repellents, situate them close to your home’s entryways and around highly sensitive plants. You can occasionally harvest their leaves or sprigs and sprinkle these around more key points of your garden. Freshly picked leaves may continue to emit their scent for a few days.
2) Rosemary (Salvia rosmarinus)
Cultivated as a medicinal and culinary herb, the humble rosemary is an incredibly sturdy and highly textural plant. It is best known for the aromatic properties of its tough, needle-shaped leaves. These are borne on tender to woody branches and shoots which become quite thick and tall with age. The hardy components of this evergreen shrub make it remarkably tolerant of cool temperatures and dry conditions.
Rosemary’s scientific name, which is Latin for “sea dew”, alludes to its capacity to thrive in exposed conditions, such as those often observed along coastal areas. On average, it has a relatively slow rate of germination and growth but can live for decades once established. Its essential oils are packed in its leaves, which are known for being bitter yet aromatic. These contain cocktails of phytochemicals with antimicrobial and pesticidal properties.
Ants and many other animals, including common plant pests and mammalian grazers, usually keep away from the tough and bitter leaves of rosemary. Mature shrubs can be lined up along the perimeter of your home or garden. There, they’ll help protect your property from many wild animals while producing a steady supply of aromatic leaves that you can use in your kitchen.
3) Marigold (Tagetes spp.)
Grown as ornamentals for cultural and aesthetic purposes throughout their native range, marigolds are fragrant plants with some of the most eye-catching shades of yellow, orange, and gold in nature. These striking annuals can quickly germinate and gain height, becoming up to 2.2 meters (7 feet) tall once their buds come into bloom. Their generously leafy stems are borne on fibrous roots which exude a wealth of antibacterial compounds.
Some of these compounds, referred to as thiophenes, are known for their toxicity to nematodes or roundworms. These also offer marigolds a competitive edge as they can compromise the root growth of nearby plants. There has yet to be scientific proof that these compounds specifically repel ants, though the plants themselves are rarely attacked by these capable insects.
Two marigold species, T. erecta (African marigold) and T. patula (French marigold), may be quite effective at controlling pests when they are present in considerable densities and placed in strategic locations around the home or garden. The flowers can also be harvested, crushed, and placed in soapy water (with potash-based soap) to create an organic pesticide solution.
4) Catnip (Nepeta cataria)
Able to throw the most respectable of felines into a sudden frenzy, catnip is a potent herb containing many volatile compounds. Though cats tend to gravitate towards its fragrant leaves and shoots, ants and many other insects do their best to keep away. Its essential oil is principally composed of nepetalactone, a terpenoid with known pest-repellent qualities.
Catnip essential oil is supposedly quite effective at keeping squash bugs, aphids, weevils, and mosquitoes away from the garden. The isolated oils can be as strong as industrial-grade pest repellents when sprayed into a given space rather than placed directly on the skin. Another compound in the oil, iridodial, attracts instead of repels the lacewing fly. If your garden plants tend to be attacked by aphids or mites, this fly can serve as their natural predator.
A member of the mint (Lamiaceae) family, N. cataria has square stems with leaf pairs that occur opposite one another along the nodes. Deer-resistant and drought-tolerant, it is able to tolerate a wide range of conditions. If you live in USDA hardiness zones 3 – 10, consider growing a few of these around your more precious plants to keep ants away.
5) Sage (Salvia officinalis)
An effective ant deterrent in its whole or crushed form, sage is known for its aromatic and medicinal properties. Its mildly sweet and fragrant essential oils may also repel other insects. Pulverizing the fresh leaves releases a stronger scent and is highly recommended if you intend to dissuade ants from entering the home. The crushed material can then be sprinkled or placed in small mounds along key entry points and gaps where ants tend to make their homes.
The main components of this species’ essential oil include thujone, borneol, and cineole. The leaves contain many acids, vitamins, and glycosides as well. This cocktail of components has been shown to be toxic to a fair number of insects, which you will seldom see creeping close to dense stands of the plant. It has thus been suggested as an alternative to chemical pesticides, for which some types of pests may develop a resistance.
Apart from keeping pests away from the garden, sage certainly won’t hurt as an ornamental addition in spots that could use some color and soft textures. Its leathery leaves can also be harvested and used to add amazing flavor to many dishes. Keep a pot of sage right next to a sunny window to brighten up indoor spaces and deter troublesome insects.
6) Anise (Pimpinella anisum)
The sweet aroma and flavor of anise is the secret component of many Middle Eastern snacks and deserts. Its complex taste has been likened to that of licorice, tarragon, and fennel. Due to its popularity as an herb, it is now widely cultivated in many parts of the globe. Its fruits, which are called anise seeds, are grocery staples in many regions. The aroma of these fruits is produced by their essential oils, which may comprise up to 7% of their nutritional profile.
Anethole, the chemical constituent responsible for the flavor and scent of aniseed, is quite effective at keeping grain pests away while attracting beneficial insects. This combination of effects would, of course, be quite advantageous to have in the garden, where pest-eating insects would act in sync with natural repellents to control pest populations.
Apart from keeping those pesky ants away, anise would undoubtedly add interest to just about any outdoor space because of its fantastic features. A perennial, low-maintenance, and drought-resistant plant, it can produce weightless umbels of delicate flowers without fail each year. As the shoots can grow to 3 feet (1 meter) tall, they can be used in border plantings and as a backdrop for more low-growing herbs.
7) Pennyroyal (Mentha pulegium)
The toxic effects of this seemingly harmless mint species have been exploited for centuries. Once used as an herb and an alternative form of contraception (or as an abortifacient), pennyroyal contains substances that may be dangerous to humans when consumed in large doses. Pulegone, the very compound that gives it its peppermint scent and taste, can induce dizziness, vomiting, and gastrointestinal pains.
Unsurprisingly, pennyroyal has been used as a natural pesticide and repellent. Its aroma is able to fend off common pests, such as fleas. The crushed leaves are thus also likely to keep all sorts of ants away. The leaves can be gathered and placed in parts of the garden where ants are likely to make their homes. Be careful when handling them with bare hands, however, as their toxins may be absorbed through the skin.
If you intend to plant stands of pennyroyal in your garden, do make sure that pets or children are fully supervised around them. To minimize accidents, pennyroyal should ideally be situated in areas that are harder to access. As it favors damp and heavy substrates and can tolerate partial to full sun, it is suited for areas that are close to water features.
8) Chrysanthemum (Chrysanthemum spp.)
With an incredible number of varieties and colorful cultivars, chrysanthemums have become one of the most popular groups of flowering perennials. These showy plants now occupy significant portions of botanical gardens all over the world. Many are recipients of the RHS Award of Garden Merit and have exhibition varieties that can truly bring out the beauty of their genus’ unique features.
The appeal of chrysanthemums doesn’t simply end with their ornamental contributions to gardens. Amazingly, these stunning plants have culinary, insecticidal, and air cleansing uses too. They can naturally repel ants and are a fantastic alternative to frequently loading the garden with pesticidal sprays. The active insecticidal compound, called pyrethrin, is contained in their dried flower heads. These can be pulverized and distributed around the garden.
By controlling the spread of ants, chrysanthemums can also significantly reduce rates of aphid and mealybug infestations. One particular species, C. cinerariifolium, has been cited as an extremely potent source of pyrethrin and is the “deadliest flower in the insect world”. This goes to show that chrysanthemums are extremely important sources of far more than just pretty flowers.
9) Garden thyme (Thymus vulgaris)
Fragrant and delicate, the common thyme is a must-have in every herb garden, where it can serve as both a culinary ingredient and an effective ground cover plant. This valuable perennial eventually grows into a subshrub with evergreen features that can last through mild winters. As it reaches a maximum height of about 12 inches (30 cm), it can also serve as a petite potted plant to enliven a kitchen windowsill.
The essential oil of T. vulgaris contains antimicrobial and antifungal components that are likely to deter insects, including ants, from coming close. A study has shown that it can significantly reduce the survival and hatching rate of weevils in stored-bean seeds, highlighting its potential as an environmentally safe alternative to chemical insecticides.
Spikes of thyme can be harvested and distributed around the garden to dissuade sensitive insects from infesting your property. These are also likely to keep flea and tick populations at bay. For best yield rates, the shoots should be exposed to full sun and should be rooted in a highly porous substrate. Note that this species is more likely to tolerate droughts compared to periods of excess moisture.
10) Garlic (Allium sativum)
One of the world’s most important seasonings and traditional medicines, garlic is best known for the incredibly strong scent and flavor of its bulbs. The sharp and tangy taste is caused by the presence of phytochemicals which are automatically released once the plant’s cells are damaged or scratched. These compounds are potent sources of sulfur, making them highly reactive and odorous. They are likely produced as a natural defense mechanism to dissuade the likes of grazers, insects, and even bacteria.
Rooted in the garden, unharvested garlic may not be the most effective ant repellent as its scent-producing tissues are left compact and underground. For garlic to truly keep ants away, its cloves should be cut and distributed around areas where ants are most likely to congregate or pass through. Once the bulbs have dried up, they will no longer emit an odor or be effective against ants. They should be replaced with freshly cut cloves.
An all-natural pesticide and ant repellent spray can also be made by mixing a few finely sliced pieces of garlic with soapy water and chilis. This should keep not only ants but also termites, spider mites, slugs, beetles, moths, mosquitoes, and fungi away. The solution can be sprayed directly onto foliage or grass but should be used in moderation to prevent the killing off of beneficial microbes.
I remember my grandmother growing all these plants around her trailer to repel ants.