I’ve been curious to learn more about companion plants lately, and I wanted to dive in a bit deeper on those which help prevent pests and diseases. So, I did some research. Here’s what I found.
To repel pests such as aphids and caterpillars, plant companion plants such as nasturtium, dill, lavender, and rosemary. To boost the disease resistance of many plants, plant garlic, onions, borage, and horseradish. Along with companion planting, intercropping is one of the best practices to limit pests and diseases.
But first, how exactly does companion planting help prevent pests and diseases?
How Companion Plants Prevent Pests and Diseases
If I could use one word to explain most of our experiences with pests and diseases, it would be defeating.
However, this is mainly because we’ve been overly dependent on monocropping. For example, if only one cabbage gets infected with a pest or disease, nothing is stopping that pest or disease from spreading swiftly to the other, adjacent cabbages.
On the other hand, by intercropping, the natural diversity of plants limits and sometimes completely prevents many pests and diseases. This is due to intercropping serving as a natural genetic wall or barrier, either containing the infection to a single plant or bringing certain traits which repel it completely.
This effect is then compounded when combined with companion planting.
Plants that repel pests and diseases usually do so by emitting a natural substance either through their roots, flowers, or leaves. As a result, many of these plants make popular and useful companion plants.
Let’s now take a look at 10+ companion plants that can help your garden prevent pests and diseases.
5+ Pest Repelling Companion Plants
Nasturtium is occasionally thought to be a weed, but it’s gaining popularity as one of the best companion plants around. It’s great at attracting pollinators such as hummingbirds due to its super sweet nectar and longer flowers, but it’s also useful to repel pests.
Well, repelling might not be the right word, as its sweeter nectar actually attracts aphids away from other plants (source).
Aphids love fruit trees, sweeter herbs, and most other garden plants, so using nasturtium to attract them away from your other plants can save you a lot of trouble from having to deal with them yourself.
Nasturtium itself can handle aphids and often won’t get overrun by them. However, if you find your garden is getting overrun, consider planting dill, calendula, and cosmos.
2. Calendula and Cosmos
Calendula and cosmos are both flowering plants that attract beneficial wasps and hoverflies. I know, I know—how can ANY wasps or flies be beneficial? Who would invite these “pests” into their garden?
However, some wasps, such as paper wasps, feed on caterpillars, which we know are all too often damaging to plants. They take the caterpillars back to their nest, where they feed them to their larvae (source).
But, like most balanced ecosystems, the wasps don’t consume all of the caterpillars, leaving just enough of them to prevent major plant damage and develop into butterflies—pollinating the garden.
So, it’s effectively a win-win. You avoid decimating caterpillars, your garden gets butterflies, and the wasps get a snack. Oh, and you get some pretty nice flowers too.
Hoverflies are also beneficial as they prey on aphids, reducing their population to reasonable numbers as well. As a bonus, planting nasturtium with your calendula and cosmos will keep aphids manageable. Additionally, many flies, including hoverflies, assist in pollination, although not as well as bees (source).
Small note: calendula is also called pot marigold, but it’s not the same as a traditional marigold. However, they are in the same family.
To see more companion flowers, check out my other post: The Top 10 Companion Flowers for Gardens, Vegetables, & More.
Dill is another plant that prevents aphids. Only this time, it’s done by attracting ladybugs—a common aphid predator.
Unlike wasps and flies, ladybugs naturally have a more positive reception by us in our gardens, which makes my job writing about them a bit easier.
Ladybugs also eat spider mites, which, like aphids, suck the sap from plant tissues. This leads to issues such as yellow or brown spots on leaves. Aphids and the like are commonly found underneath the plant’s leaves.
By planting companion plants that attract ladybugs, such as dill, and releasing them into your garden, you’ll have one more method of aphid and pest control. If you’re interested in releasing ladybugs, you can find ladybugs for sale at most nurseries.
4. Lavender, Sage, and Rosemary
While lavender, sage, and rosemary attract pollinators and some beneficial insect predators, their main method of pest control is from their strong scent.
Their potent scents repel many pests including snails, slugs, deer, and other browsing animals (source).
For best results, interplant lavender, sage, and rosemary in your gardens. They’ll help repel pests while significantly increasing pollination. For example, the purple sage in my parent’s backyard is always attracting bees, butterflies, and other pollinators.
5. Garlic, Chives, and Onions
Garlic, chives, and onions are all part of the same family and, no surprise—they’re more potent-smelling than the above aromatic plants.
The pest repelling aspect of these plants likely comes from their ability to accumulate sulfur, which is also useful in preventing some diseases (more on this later).
You can interplant any of these onion-family plants with just about any other plant, except for legumes.
You can also combine garlic, chives, or onions with some or all of the other pest repelling plants on this list to make an aphid knockout team.
5 Disease Limiting Companion Plants
While garlic has a significant pest repelling ability, its sulfur is actually a naturally occurring fungicide. Since most diseases are of a fungal origin, as opposed to bacterial, garlic is a super useful companion plant.
However, when fungal or bacterial diseases do take hold, garlic cloves can be mixed into organic sprays along with neem oil and applied as a treatment.
In fact, if you look at store-bought organic sprays, many of them contain garlic.
Chives are another companion plant that not only have pest repelling qualities but disease limiting ones as well.
Interplant chives near apple trees to prevent scab, and near rose bushes to help prevent black spot.
When planting, a good spot for chives is just outside of the other plant’s canopy. An exception to this is if they’re pruned enough to let in a sufficient amount of sunlight for the chives below. As chives have shallow roots, they often won’t compete with the other plants’ roots.
As you can see, the onion family is an ace at naturally preventing many pests and diseases. And with onions themselves, you’ll also find similar benefits.
Specifically, intercrop onions with strawberries to boost the strawberry plant’s disease resistance. While the runners on strawberries spread aggressively, onions grow tall and deep enough to avoid competition from them. The onions then benefit from the strawberry’s ground cover, including increased shade and water retention.
Together, these two plants make a sufficient ground cover for a larger garden or food forest.
Another companion plant for strawberries, borage is incredibly useful to limit and prevent diseases (source). Borage is also known as starflower and it’s an annual herb native to the Mediterranean. After planting, borage is easy to grow as it self-seeds. Its flowers are also edible.
Along with strawberries, you can interplant borage with tomatoes for a similar disease-resistant effect.
Horseradish is a perennial root vegetable part of the brassica family and is closely related to mustard and wasabi. It’s thought to be native to southeastern Europe and western Asia and brings some disease-reducing properties to the garden.
Specifically, intercrop horseradish along with potatoes to boost their disease resistance.
There are more companion plant friends than there are foes, and one of the best ways to find benefits is to simply observe your garden. Give these plants a shot and see how they do!
Where to Buy Companion Plants
Generally, you can find these companion plants at your local nursery. However, many gardeners have found that online nurseries such as Fast Growing Trees are a bit more convenient (with delivery and all).
I’ve also found this to be the case after ordering several fruit trees (apple, fig, and cherry) from Fast Growing Trees. All of the plants were delivered swiftly, neatly, and healthy.