A good companion can be supportive, offer protection from a cold, harsh world and nurture you when you need it most. So can a good plant. In fact, plants do that for each other.
Companion planting is the practice of growing different plants together for their benefit. The practice offers a multitude of advantages, such as shielding delicate plants from too much sun, providing a natural trellis for other plants to climb and creating a living mulch that can fertilize the soil, says Troy Cobb, greenhouse and garden shop manager at Fort Collins Nursery.
Because of this, gardeners may want to consider how different plants can support one another when they are designing their gardens. Not only does companion planting help gardeners boost the productivity of their plants, but it also provides a way to keep gardens diverse and therefore healthy. The practice even provides natural pest controls without the use of herbicides or pesticides.
“There are so many benefits for you and the plants,” Cobb says.
CSU Extension horticulturist Cassey Anderson likes companion planting, but she says not all the believed benefits are scientifically backed. For instance, the old adage that growing tomatoes with basil creates better-tasting fruit seems a bit off to her.
There’s another adage that “the three sisters”—corn, beans and squash—should be planted together because 1) the beans will climb the corn, 2) the squash will shade the ground for the corn and beans and 3) the beans will add nitrogen to the soil for the corn and squash.
“It is probably not doing everything it is touted to do,” Anderson says. Specifically, when it comes to the nitrogen, studies indicate that benefits to the soil primarily happen when the plants decompose, she says.
Despite her doubts, Anderson does tout companion planting. She even calls the three sisters method “a really cool idea and a great way to plant.” Ultimately, she advocates for diversity, which is a key component of companion planting.
However, rotating families of vegetables in gardens and raised beds can be of even greater help to soil health, Anderson says.
“I change between plant families each season,” she says. “That way, you can prevent the same pest issues or disease issues from year to year. Go for diversity in your garden rather than just having a monocrop.”
Anderson and Cobb share their top suggestions for companion planting that are often as pleasing to the eye as they are practical.
For soil health
Cobb recommends trying a cover crop between plants. Nurseries sell bags of mixed seeds that often include legumes, grains and grasses that shade the soil and help it retain moisture. Cover crops can also protect the soil against erosion, especially in heavy rain.
As the cover crop grows, it should be cut with a mower or shears (depending on the size of your garden) regularly and left to decompose back into the soil. Rye, common in cover crop mixes, can add nitrogen to the soil as it decomposes, and it can also inhibit weed growth, Cobb says.
Anderson advocates for planting vegetables with deep roots next to those with shallow roots. Differing root depths mean the plants don’t have to compete as much for water, and the space for root growth is less crowded. On the contrary, two deeply rooted plants, like tomatoes and melons, “just won’t play good together,” Anderson says.
Plants can also become “living rototillers,” Cobb says. Try planting forage radishes, or daikon, to open up spaces in the soil. These radishes can grow large and deep. You shouldn’t harvest all of them: If they’re left to decompose, they are a great way to loosen up the soil, especially the compact clay soil we have in Northern Colorado.
For pest and disease control
Not only can companion planting repel the wrong kinds of bugs—think aphids, spider mites and whiteflies—but it can also attract the right ones. Flowers, for instance, attract pollinators such as bees and butterflies. They also attract spiders and other bugs that will eat up the bad ones.
“And you get the beauty, too,” Anderson says of the flowers.
Trap planting, or the practice of planting certain vegetables or flowers to draw away harmful insects, can protect a prized crop. For instance, Anderson says to plant Brussels sprouts the next row over from broccoli because they will draw aphids away from the broccoli. You may lose the Brussels sprouts, but your broccoli will be happy.
Cobb says certain flowers make great trap crops. Nasturtiums can attract aphids as well, and they grow easily from seed, he adds. He suggests putting them next to any plant that is especially susceptible to aphids, like tomatoes or maybe that extra crop of Brussels sprouts. He also recommends planting them next to squash plants because they will deter squash bugs.
Marigolds are another handy flower to have in the garden. Cobb says marigolds’ scent repels some insects.
“The smell they give off may be stronger to some insects than the smell of the plants they are actually attracted to,” he says.
Companion planting can even deter some of the bigger critters that might try to invade your garden, Cobb says. Racoons are reported to avoid the fuzzy leaves of squash plants, which in turn may protect your corn plants from the masked invaders. Marigolds may also keep squirrels from digging around in your garden.
Combinations to avoid
Not all plants get along. For example, broccoli and cauliflower aren’t great next to tomatoes because they will compete for the same nutrients, Cobb says. Mint is an aggressive grower and can choke out other plants. Rhubarb may look manageable at first but can grow in a hurry and crowd other plants.
“Research what your plants are going to do ahead of time and decide what you want to do,” Cobb says. “Companion planting can probably help you do some of it.”
For support and shade
Certain plants, like corn, can provide a natural trellis for vining plants, Cobb says. Anderson suggests planting beans on a trellis and lettuce underneath. The beans will provide the needed shade for delicate lettuces and give you a longer season of crops.
