Stories, articles and books abound on the topic of companion planting. In today’s “Ask a Master Gardener” presentation, Nancy Louwman takes a critical look at this subject to see whether current companion planting advocacy is based on anecdotal or scientific evidence.

Do carrots really love tomatoes?

Claims have been made that carrots grown in close proximity to tomatoes provide a benefit by breaking up the soil, allowing water and air to flow to the tomato roots. It is said that tomatoes protect carrots by exuding the alkaloid solanine.

Carrots do break up the soil and improve soil aeration.  Solanine however, found in plants of the nightshade (Solanaceae) family,  particularly potatoes, is not the alkaloid found in tomatoes. The substance in tomatoes is in fact a more benign alkaloid known as tomatine.  Little research has shown that tomatine exuded from tomatoes protects any neighbouring plants.

Does basil improve tomato yields (as stated by many websites and books extolling the virtues of companion planting) when used as a companion plant?

According to companion plants enthusiasts, planting basil and tomatoes together improves tomato yields and even flavour. However, when Nancy examined this claim, she found no evidence of increased yields or improved taste as a result of inter-planting these two crops. While there may be other reasons for growing these together, from a yield and flavour perspective perhaps basil is best served with tomatoes after harvest, in a sauce!

Do marigolds repel insects and root nematodes?

It is a belief among many home gardeners that planting marigolds (Tagetes sp) in the vegetable patch controls insect pests or root nematodes. While there are approximately 50 species of marigolds, we commonly find only three in our local garden centres -T. patula (French marigold), T. tenuifolia (Signet marigold) and T. erecta (African/Mexican marigolds). Research has shown that in fact, intercropping marigolds and tomatoes DOES slow the development of whitefly populations. Both however must be planted at the same time. There is little to no effect if marigolds are planted once a whitefly population exists.

Root knot nematodes are a pathogen of many crops, including carrots, where they invade and disfigure the root tissue. Various plants, including marigolds, produce a nematicide, a substance that is toxic to nematodes. However, Nancy’s investigation indicates that some species of marigold can reduce nematodes when they act as a trap crop. (Signet marigolds are one species that are ineffective in this regard). The nematode enters the root system of the marigold but is unable to develop further in its life cycle or is killed by the plant.

In our short growing season it is impossible to grow a crop of marigolds first, to be followed by a root vegetable crop such as carrots. In any case, doing so does not fit the definition of companion planting.

Do citronella mosquito plants (as our companions) repel mosquitos?

Some plants, for example citronella grass (Cymbopogon nardus), are sold to be used as mosquito repelling plantings, on the grounds that their scent is unappealing to these insects. However, studies have shown that the amount of citronella in this plant is relatively small, with some other species (e.g. lemon balm) having a higher concentration of citronella. Regardless, the scent is released when the leaves are crushed, so a plant simply sitting in a pot is unlikely to repel mosquitos.

Will growing legumes supply nitrogen to surrounding plants?

The legume plant family (Fabaceae) includes peas, beans and clover, among others. Legumes have a symbiotic relationship with bacteria living in nodules on their roots, which convert nitrogen in the atmosphere into a form that is available to the plant. These plants get only about 5% of their nitrogen from the soil, with the rest coming from the air around them.

Nancy found that the nitrogen supply that moves from the legume to soil and other plants in the vicinity is negligible, at least initially while the plant is growing. There may be some benefit to using legumes as a cover crop – that is, leaving it to die and decompose in the garden bed, where the nitrogen it took up when growing can be returned to the soil.

Companion planting as we know it today includes the following practices:

• Intercropping to confuse predators by visual or olfactory cues (including scent masking)
• Trap cropping
• Providing physical protection or support (e.g. the three sisters model)
• Attracting beneficial insects including pollinators and natural enemies
• Avoiding plant combinations where harmful effects may occur because of resource competition or allelopathy (e.g. sunflowers and potatoes)

In conclusion, Nancy suggests following the advice of Robert Kourik:

The trick to a lifetime of good effective gardening is to stay flexible and to be willing to change with the feedback of unprejudiced observation and the guidance of modern research

Robert Kourik, Garden Myths, the Good the Bad and the Unbelievable