The Truth about Companion Planting

So what is companion planting? Is it folklore passed down through the generations, or is there actual science behind it? In this article, we get down to the truth about companion planting.

Companion planting is the practice of growing two crops next to each other to optimize production, increase beneficial insects, and weed suppression.

Three Sisters Companion Planting

The Native Americans planted corn, beans, and squash together as companion plants. The three plants aided in the growth of each other. Legend has it the gods blessed the Native Americans with these three vegetables.

The corn serves as the structure for the beans. The beans climb the corn stalks as they pull nitrogen from the air. The nitrogen enriches the soil enhancing the growth of the three plants. The squash leaves shade the soil, keeping weeds at bay. The prickly squash leaves act as protection against animals.

The Science of Companion Planting

In the example of the three sisters, the plants use what the other plants provide. They are benefitting from each other. But do marigolds help tomatoes?

According to Cornell University, “the science of companion planting is often anecdotal. There appears to be no research-proven reproducible companion planting recommendations.”

Even though there aren’t reproducible studies on companion planting, gardeners notice benefits when using plants as the Native Americans did with the Three Sisters, to deter pests, provide weed suppression, attract beneficial insects, improve the soil, and obtain higher yields.

Let’s look at these benefits of companion planting close up.

Benefits of Companion Planting

Deterring Pests

Many people believe certain plants deter different pests. For example, chives may repel aphids. Some insects find their host plants through their sense of smell. When there are too many conflicting scents, the insects become confused, leaving the main crop untouched. Some studies indicate this repellent may work. However, the studies conducted were under laboratory conditions.

One scientifically proven deterrent of pests is marigolds. Marigolds do repel root nematodes. These microscopic parasites feed on plant roots. Marigolds produce a toxin that kills the nematodes. However, this benefit only occurs once the marigolds are tilled into the ground, releasing the chemicals.

A trap crop, or sacrificial crop, planted 8 to 12 feet away from your main crop, helps reduce the amount of damage the main crops sustain from pests. For example, planting a sacrificial blue hubbard squash crop lures squash bugs and squash vine borers away from the main crop.

Weed Suppression

Adding cover crops as companion plants between your main crops helps suppress weeds. The crops grow between plants, shading out the sun and prohibiting weeds from growing. Green manure also provides nutrients to the soil.

Rye produces an allelochemical. These chemicals affect other plants, some in good ways and others in destructive ways. The allelochemical present in rye suppresses weed germination. When used as a mulch, rye holds the weeds at bay but does not harm the crops.

Attracting Beneficial Insects

Attracting beneficial insects is by far the best way to use companion plantings. By growing plants, especially flowers, interspersed between your fruits and vegetables, you attract pollinators and those insects seeking out nectar. These insects pollinate your squash, tomatoes, and more to increase yield.

Other companion plantings for beneficial insects are those that attract predatory or parasitic insects. Creating a place for these insects to hunt and feed reduces the number of pests in your garden. Some of these predators are the lady beetle or ladybug, praying mantis, ground beetle, lacewing, damsel bug, assassin bug, parasitic wasps, and spiders.

When deciding which plants to add to your garden, take into consideration the blooming times of the flowering plants. Make sure to have something blooming year-round.

I included downloadable lists for attracting various beneficial insects.

Improvement of Nutrients

Planting, not just companions, but a diverse group of crops in the same area prevents the plants from competing against each other for nutrients. This shared existence means plants get the nutrients they need to thrive.

In a study published in the Journal of Integrative Agriculture, the scientists compared the growth of cucumbers when planted with only other cucumbers and cucumbers grown with wheat. The results showed the soil with all cucumbers had the lowest amount of potassium and phosphorus. Because of this lack of nutrients, the cucumbers had a low yield. Those cucumbers that grew alongside wheat had a much higher yield and more soil nutrients.

Plants that are heavy feeders of nitrogen grow best when next to plants that produce nitrogen, the givers. For example, potatoes are heavy feeders. They do very well next to legumes which take in nitrogen from the air and store it in their roots. When these plants die, they add nitrogen to the soil.

Obtain Higher Yields

As in the Three Sisters method, planting tall-growing vegetables with lower-growing, shade-tolerant vegetables takes up less space in the garden. Therefore, you grow more vegetables for the amount of land used.

Are Some Plants Incompatible with Others?

As mentioned earlier, some plants produce allelochemicals. When these chemicals affect other plants in harmful ways, it is detrimental to those plants. Black walnut emits an allelochemical called juglone. If this tree is in the vicinity of other plants, those plants’ growth becomes slowed by impeding cell division.

Wormwood is a plant that produces a toxin. This toxin can be fatal. When planted next to edible plants, the wormwood’s toxins seep into the roots and affect the nearby plants.

There isn’t much scientific research backing traditional companion planting. The lack of scientific evidence doesn’t mean it doesn’t work. Gardeners do see results from different plant combinations. Their observations and findings should not be overlooked.

Below I created charts for general guidelines in companion planting.