Marigolds (Tagetes species) are popular flowering plants known for their bright colors and pungent aroma. This strong scent has given them a long-standing reputation as a natural pest repellent among gardeners seeking non-chemical solutions. This common practice raises a question: does the marigold’s celebrated pest-repelling ability extend specifically to ants?
Evaluating the Claim: Do Marigolds Repel Ants?
Scientific evidence does not strongly support the idea that simply planting marigolds in a garden will reliably deter or repel ant populations. The belief that the strong odor of the foliage or flowers acts as a direct deterrent to ants is largely anecdotal rather than fact-based. In fact, gardeners have observed ants actively crawling on marigold plants, suggesting their passive presence is not a significant barrier to these insects.
Marigolds contain several bioactive compounds, such as thiophenes and limonene, which have insecticidal or repellent properties against certain pests. For a plant to act as an effective repellent in a garden setting, it must release these chemicals at a high enough concentration to disrupt the insect’s navigation or behavior. Current research indicates that the concentration of volatile compounds released by a living marigold plant is too low to significantly impact a mobile insect like an ant.
Marigold compounds show potent activity only when highly concentrated, such as in essential oil extracts. Applying a concentrated extract directly to an ant trail might disrupt pheromone signaling, but planting the flowers nearby does not achieve the same result. Relying on a planted marigold to control an ant problem is therefore not a reliable method for managing garden ant populations.
Marigolds and Nematode Control
The marigold’s reputation as a powerful pest repellent is often confused with its proven effectiveness against a different type of organism. Marigolds are highly effective biological control agents against plant-parasitic nematodes, which are microscopic roundworms that feed on plant roots. This specific action is the true basis for the plant’s pest-control fame.
The mechanism involves the roots of the marigold releasing a toxic sulfur-containing compound called alpha-terthienyl. This compound, which is one of the most potent natural toxins known, is nematicidal, meaning it kills or suppresses the nematodes in the surrounding soil.
To achieve this effect, marigolds must be planted densely and remain in the ground, often as a cover crop or in crop rotation, allowing the toxic exudates to build up in the soil. French marigold (Tagetes patula) and African marigold (Tagetes erecta) are the species most commonly used. This established ability to manage root-damaging pests makes the marigold a beneficial companion plant, even though its effect on above-ground insects like ants is negligible.
Proven Natural Methods for Ant Deterrence
Since planting marigolds is not an effective deterrent for ants, gardeners must turn to other reliable, natural methods for control. One of the most effective non-chemical strategies involves the use of diatomaceous earth (DE), a fine powder made from the fossilized remains of diatoms. Food-grade DE acts as a physical barrier and desiccating agent, causing ants to dehydrate when they walk across it.
Another practical approach is to disrupt the ants’ crucial pheromone trails, which they use for navigation and communication. Spraying a mixture of equal parts white vinegar and water, or diluted lemon juice, directly onto ant trails can successfully mask these chemical signals, causing the ants to lose their way and retreat.
Common kitchen spices also offer deterrent properties, as the strong scent of ground cinnamon can interfere with ant communication. Sprinkling a line of cinnamon near entry points or along trails can create a boundary the ants are reluctant to cross.
Reducing access to food sources is equally important. Promptly cleaning up spilled sweets, fruit drops, and pet food makes the garden less attractive to foraging ants.