Companion planting is a technique where different plant species are grown in close proximity to provide mutual benefits, often involving pest control or improved growth. Marigolds and tomatoes form one of the most successful pairings in the garden ecosystem, offering a natural and effective defense for the tomato plant. Understanding the specific mechanisms behind this relationship is the first step toward successfully integrating these two plants.
Why Marigolds and Tomatoes are Ideal Companions
The primary benefit of this pairing operates beneath the soil surface, targeting microscopic pests that can devastate tomato roots. Marigold roots release compounds called thiophenes, which are natural nematicides that poison and suppress root-knot nematodes (Meloidogyne spp.). These nematodes cause galls on tomato roots, severely limiting the plant’s ability to absorb water and nutrients. The release of alpha-terthienyl, a specific thiophene, can reduce the population of these harmful soil-dwelling worms, leading to healthier tomato plants.
Above ground, marigolds act as a general pest deterrent. Their strong, pungent aroma can help mask the scent of tomato plants, making them harder for pests like whiteflies, aphids, and tomato hornworms to locate. Marigolds also attract beneficial predatory insects, such as hoverflies and ladybugs, which prey on common tomato pests. This dual-action protection, both below and above ground, makes the marigold an effective ally for any tomato crop.
Selecting the Best Marigold Types
Not all marigolds offer the same degree of protection, making variety selection important. French marigolds (Tagetes patula) are recognized as the most effective species for controlling root-knot nematodes. Their roots produce the highest concentrations of nematicidal thiophenes, and specific varieties like ‘Nemagold’ or ‘Petite’ are bred for nematode suppression.
African marigolds (Tagetes erecta), while larger, are primarily better for general pest deterrence above ground. Although they possess nematicidal properties, they require a longer period as a cover crop to achieve the same level of nematode suppression as French varieties. For comprehensive defense, gardeners often choose a mix, prioritizing the smaller French marigolds for direct interplanting.
Practical Guide to Planting Together
The most effective planting strategy is to establish marigolds as a dense border or intercrop them directly with the tomatoes. For immediate defense against above-ground pests, marigolds should be planted concurrently with the tomato seedlings. This ensures the marigolds are established enough to begin releasing their scent and compounds as soon as the tomatoes are transplanted.
For optimal nematode suppression, French marigolds should be planted within a 12-inch radius of the tomato base. This proximity ensures the roots of both plants intermingle, maximizing the nematodes’ exposure to the protective thiophenes. A good density is to plant at least one marigold per tomato plant, or create a continuous ring of smaller marigolds spaced 6 to 8 inches apart.
If dealing with a severe nematode problem, the most rigorous method involves planting marigolds as a dense cover crop for two to four months before planting the tomatoes. This pre-planting strategy, often called biofumigation, reduces nematode populations before the tomato roots are introduced. For typical home gardening, planting marigolds in a dense ring or row alongside the tomatoes provides a simpler, continuous protective effect throughout the growing season.
Maximizing Effectiveness: Maintenance Tips
To ensure marigolds provide continuous pest defense, the flowers must be deadheaded regularly. Removing spent blooms encourages the plant to produce new flowers and maintain vegetative growth throughout the season. This sustained growth ensures the continuous release of protective volatile compounds above ground and nematicidal thiophenes from the roots below.
At the end of the growing season, leave the marigold roots in the soil. Allowing the roots to decompose in place is a form of green manure that releases remaining nematicidal compounds into the soil. This process helps reduce the nematode population, providing a beneficial residual effect for the next season’s planting.