Companion planting is a time-tested gardening practice that involves growing different plant species near each other for mutual benefits, such as improved growth or natural pest control. This strategy creates a diverse ecosystem where plants interact chemically and physically. The pairing of marigolds with tomato plants is a frequently discussed and scientifically examined relationship. Gardeners often seek to understand if these bright flowers truly offer a protective advantage to their tomato harvest.
Marigolds and Nematode Control
The most significant and scientifically supported benefit of planting marigolds near tomatoes occurs beneath the soil surface, targeting root-knot nematodes (\(Meloidogyne\) species). These microscopic roundworms are parasitic, attacking tomato roots and causing galls that impede the plant’s ability to absorb water and nutrients. Severe infestations result in stunted growth, yellowing foliage, and greatly reduced yields.
Certain marigold varieties, particularly French marigolds (\(Tagetes\ patula\)), release chemical compounds called thiophenes from their roots into the surrounding soil. The most potent of these compounds is alpha-terthienyl, which is toxic to the nematodes. This natural nematicide suppresses the nematode population by either killing them or disrupting their reproduction cycle.
For maximum effectiveness, marigolds must be grown as a dense cover crop for a period of two to four months before the tomato plants are introduced to the same soil. This pre-planting strategy allows the thiophenes to build up in the soil, leading to documented reductions in nematode populations of up to 90% in subsequent tomato crops. Simply interplanting marigolds simultaneously is less effective for nematode control, as the protective compound needs time to accumulate.
Deterring Above-Ground Pests
Marigolds also offer a secondary layer of protection against several common pests that attack the above-ground portions of the tomato plant. These benefits rely on the strong scent and visual characteristics of the flowers and foliage. The highly pungent aroma of marigold leaves acts as a natural repellent or confusion mechanism for certain insects.
This strong scent can mask the attractive smell of tomato plants, making it harder for airborne pests like whiteflies to locate their preferred host. The visual presence and scent may also deter larger pests, including the destructive tomato hornworm. The flowers also attract beneficial insects, such as hoverflies and parasitic wasps, whose larvae prey on soft-bodied pests like aphids.
Successful Companion Planting Techniques
Maximizing the benefits of this companion planting strategy requires careful selection of the marigold variety and proper placement.
Variety Selection
French marigolds are the superior choice for nematode suppression because they produce the highest concentration of the root-exuded alpha-terthienyl. Specific cultivars like ‘Nema-Gone’ are bred specifically for this purpose. African marigolds (\(Tagetes\ erecta\)) are generally taller and offer a stronger, more pervasive scent, which is useful for deterring above-ground pests.
Placement and Considerations
For practical interplanting, place marigolds within 12 to 18 inches of the tomato plants to ensure their volatile compounds benefit the nearby crop. Smaller French marigolds can be planted closer than the larger African varieties, which might otherwise grow tall enough to shade out young tomato seedlings.
Marigolds are not a complete solution and can occasionally act as a trap crop for spider mites in dry conditions. Ensuring the soil stays consistently moist helps mitigate this risk. By implementing the correct variety selection and technique, marigolds are an effective, natural component of a healthy tomato garden.