15 Companion Plants You Should Grow With Onions—and 6 You Shouldn't

These onion companion plants improve flavor, repel pests, and more.

Onions are an easy-to-grow root vegetable that require very little maintenance—especially when they're grown next to companion plants. Companion planting is the method of growing plants together that will benefit both species.

Onion companion plants include flowers, herbs, and other vegetables that can do everything from improve the flavor of your crop to repel unwanted bugs and improve soil quality. On the other hand, some plants don't grow well with onions as they can compete for nutrients and hinder growth. To help your onion crop thrive, we spoke to experts who shared the best onion companion plants—and what you should never plant alongside this root vegetable.

Meet the Expert

• Linda Langelo, horticulture specialist at Colorado State University, where she specializes in various gardening topics, including herbaceous plants, vegetables and vegetable insects, landscape design, and rock gardening.

• Carrie Spoonemore, co-creator of Park Seed's From Seed to Spoon app, an app that makes garden planning easier.

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Lettuce

Some lettuce (Lactuca sativa) varieties are ideal companions for onions because they can be grown as a cover crop. "Cover crops keep more moisture in the soil," says Linda Langelo, horticulture specialist at Colorado State University. "These fast-maturing lettuce varieties efficiently use the space and benefit crop yield. The cover crop also keeps weed pressure down."

• Zones: 2 to 11

• Size: 6 to 12 inches tall x 6 to 12 inches wide

• Care requirements: Full sun to partial shade; well-draining soil

Carrots

Carrots (Daucus carota subsp. sativus) and onions have different root depths, so they don't compete for nutrients. "Planting them together can help deter carrot fly and onion fly pests, as their strong aromas confuse them," says Carrie Spoonemore, co-creator of Park Seed's From Seed to Spoon app.

• Zones: 2 to 11

• Size: 3 to 36 inches tall x 2 to 3 inches wide

• Care requirements: Full sun to partial shade; moist, well-draining soil

Dill

Dill (Anethum graveolens) is beneficial for enhancing the flavor and growth of onions. Additionally, dill attracts beneficial insects like wasps and hoverflies, which prey on pests that may damage onion crops, says Spoonemore.

• Zones: 2 to 11

• Size: 2 to 5 feet tall x 2 to 3 feet wide

• Care requirements: Full sun to partial shade; well-draining soil

Brassicas

Any crop in the cabbage family, such as broccoli, cabbages, cauliflower, kale, collards, mustard, and kohlrabi is beneficial to onions. These plants will keep unwanted pests away from onions and help enhance flavor, says Langelo.

• Zones: 5 to 8

• Size: Depends on the type of plant you're growing

• Care requirements: Full sun; well-draining soil

Radish

Radish (Raphanus sativus) can loosen compacted soil, allowing onions to swell to their mature size, says Langelo. This crop also matures quickly and can be interplanted with onions to maximize space and yield, says Spoonemore.

• Zones: 2 to 11

• Size: 6 to 36 inches tall x 12 to 24 inches wide, depending on variety

• Care requirements: Full sun to partial shade; well-draining soil

Beets

Beets (Beta vulgaris) share similar growing conditions with onions, so they grow well together. Additionally, beets help improve soil structure and act as a ground cover, shading the soil and reducing moisture loss, says Spoonemore.

• Zones: 2 to 11

• Size: 12 to 18 inches tall x 8 to 12 inches wide

• Care requirements: Full sun to partial shade; moist, rich, well-draining soil

Chamomile

Chamomile (Matricaria recutita) repels pests like aphids, flies, and mosquitoes that can damage onions. "Its delicate flowers attract beneficial insects like hoverflies, which prey on aphids," says Spoonemore.

• Zones: 2 to 8

• Size: 12 to 30 inches tall x 8 to 12 inches wide

• Care requirements: Full sun to partial shade; well-draining soil

Marigold

Marigolds (Tagetes) are a go-to companion plant due to their insect-repelling properties. Marigolds emit a strong scent that repels pests such as nematodes, onion flies, and aphids," says Spoonemore. "Planting them around onions can help deter these pests and protect the onion crop."

• Zones: 2 to 11

• Size: 6 to 36 inches tall x 6 to 24 inches wide, depending on variety

• Care requirements: Full sun; well-draining soil

Basil

Basil (Ocimum basilicum) is a culinary companion to onions, as it can enhance their flavor when planted close by. "It also repels insect pests like aphids, mosquitoes, and flies," says Spoonemore.

• Zones: 4 to 10

• Size: 8 to 12 inches tall x 8 to 12 inches wide, depending on variety

• Care requirements: Full sun; well-draining soil

Rosemary

Rosemary (Salvia rosmarinus) acts as a natural insect repellent, deterring pests like carrot flies, onion flies, and cabbage moths that can damage onions. "Its woody stems provide structural support and shade for onions," says Spoonemore.

• Zones: 8 to 10

• Size: 4 to 5 feet tall x 3 to 4 feet wide

• Care requirements: Full sun; well-drained soil

Thyme

Another herb that's a great companion plant for onions is thyme (Thymus vulgaris), a low-growing herb that acts as a living mulch, suppressing weeds and retaining soil moisture around onions. It also repels pests like onion flies and cabbage moths.

• Zones: 5 to 9

• Size: 6 to 12 inches tall x 12 to 18 inches wide

• Care requirements: Full sun; well-draining soil

Rue

The strong scent of rue (Ruta graveolens) deters onion root fly. "The larvae of the onion root fly cause the onion leaves to curl and rot the bulbs," says Langelo. "The strong aroma of rue overpowers the onion’s fragrance, thus protecting it from pests."

• Zones: 4 to 10

• Size: 2 to 3 feet tall x 2 to 3 feet wide

• Care requirements: Full sun to partial shade; well-draining soil

Hot Peppers

Hot peppers (Capsicum annuum) attract parasitic wasps that can feed on beetles and flies, which are attracted to and can be harmful to onions, says Langelo.

• Zone: 4 to 11

• Size: 12 to 44 inches tall x 6 to 12 inches wide

• Care requirements: Full sun; moist, well-drained soil

Celery

Celery (Apium graveolens) is a great companion for onions as its aroma will deter pests that are attracted to onions. "Celery roots go 2 to 3 inches deep by about 6 inches wide and are not in competition with onions," says Langelo.

• Zone: 3 to 6

• Size: 18 to 36 inches tall x 12 to 18 inches wide

• Care requirements: Full sun; moist, well-drained soil

Summer Savory

Summer savory (Satureja hortensis) is a great companion because it can enhance the sweet flavor of onions, says Langelo. Additionally, it can increase the growth of onions.

• Zone: 2 to 11

• Size: 8 to 18 inches tall x 24 to 36 inches wide

• Care requirements: Full sun; alkaline soil

Swiss Chard

Swiss chard (Beta vulgaris L. var. cicla) and onions share different levels of root zones and do not compete for nutrients, making them great companion plants. "The Swiss chard can shade the soil, helping to keep moisture in the ground, which is beneficial to the onions," says Langelo.

• Zone: 2 to 11

• Size: 12 to 24 inches tall x 8 to 30 inches wide

• Care requirements: Full sun to partial shade; well-draining soil

What to Never Plant With Onions

While many plants can help your onion crop thrive, there are others you should never grow next to this root vegetable.

• Asparagus: The growth of both onions and asparagus will be inhibited when they're grown next to one another because they compete for the same nutrients, says Langelo.

• Turnips: The growth of turnips is stunted when they are planted next to onions as both crops will compete for space when growing together, says Langelo.

• Peas: Peas have similar nutrient and water requirements as onions, so they may compete for resources in the garden, says Spoonemore, adding that their vining growth habit may inhibit onion growth.

• Beans: Beans, particularly pole beans, can compete with onions for nutrients and water in the soil and can lead to an imbalance in soil nitrogen levels, says Spoonemore.

• Sage: This herb produces allelopathic compounds that can inhibit the growth of nearby plants, including onions, says Spoonemore.

• Garlic: Although garlic is a close relative of onions, planting them together may increase the risk of cross-contamination with diseases and pests, says Spoonemore.
  

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