Bad Companion Plants For Onions

Getting your onion crop right starts with knowing what not to plant next to them. Understanding the bad companion plants for onions is key to avoiding stunted growth and poor yields in your garden.

Onions are fantastic garden allies. Their strong scent helps deter many pests. But they don’t get along with everyone. Some plants will compete too fiercely or even harm the onion’s development. Let’s look at which plants to keep seperate.

This list covers the main plants you should avoid placing near your onion beds. Keeping these apart will give your onions the space and resources they need to thrive.

Beans and Peas (All Types)

This is one of the most important pairings to avoid. Onions and legumes are poor neighbors.

Beans (pole, bush, runner) and all peas fix nitrogen in the soil. Onions are light feeders and don’t require high nitrogen levels.
Excess nitrogen from legumes can cause onions to put energy into lush green tops instead of developing large, firm bulbs.
Some gardeners also find that the strong onion scent can inhibit the growth of the bean plants themselves.

Asparagus

This perennial vegetable and onions are a bad match. Their root systems compete directly.

Onions can exude substances that hinder asparagus root development.
Asparagus beds are permanent, and disturbing them yearly to plant onions is not practical. The competition for nutrients can weaken asparagus spears over time.

Other Alliums (Sometimes)

This one requires a bit of nuance. Generally, onions can be planted with other alliums like garlic and shallots.

The main risk is spreading disease. If one plant gets a fungal issue like onion rust, it can quickly jump to its close relatives.
They also attract the same pests (like onion maggot). Grouping them together can create a target-rich environment. For this reason, some gardeners consider them less-than-ideal companions.

Sage and Other Strong Herbs

While onions themselves are aromatic, very strong herbs can sometimes cause problems.

Sage, in particular, is often listed as a poor companion. Its vigorous growth and potent essential oils may inhibit onion bulb formation.
It’s best to keep onions away from dense, woody perennial herbs. They simply have different growing requirements.

Gladiolus

This is a classic companion planting rule. Avoid planting ornamental gladiolus near your onions.

Gladiolus are susceptible to many of the same diseases and pests that affect onions, particularly thrips.
Planting them together makes it easier for these problems to spread between your flowers and your vegetables.

Why These Plants Don’t Mix

Knowing the “why” helps you make better decisions. The conflicts usually fall into a few categories.

Nutrient Competition and Imbalance

Plants have different dietary needs. Onions are light to moderate feeders. Planting them next to heavy feeders like cabbages or corn can starve them. Conversely, as with beans, too much of a specific nutrient (nitrogen) leads to all leaf, no bulb.

Allelopathy: Chemical Warfare

Some plants release biochemicals into the soil to suppress competitors. While onions do this to some pests, other plants might do it to them. The suspected issue with asparagus is a form of this allelopathic interaction.

Pest and Disease Sharing

This is a huge one. Plants in the same family often share enemies. Onions, leeks, garlic, and gladiolus can all be attacked by the same fungi and insects. Grouping them is a risk.

Root System Competition

Shallow, fibrous roots versus deep taproots can create conflict. Onions have relatively shallow roots. A deep-rooted neighbor might drink all the water, or a dense, spreading root system might strangle them.

Great Companions for Onions Instead

Now for the good news! Many plants love growing near onions. Here’s who to plant them with.

Carrots: A legendary pairing. Onions deter carrot fly, and carrots may help deter onion fly.
Beets and Lettuce: These are light feeders with compatible root systems. They make efficient use of space without competing heavily.
Tomatoes, Peppers, and Eggplant: Onions can help deter aphids and other pests that bother these nightshades.
Strawberries: The onion’s smell is said to confuse pests that would otherwise eat the berries.
Chamomile and Summer Savory: These herbs are believed to improve the onion’s flavor and growth.

Planning Your Garden Layout

Putting this knowledge into practice is simple. Follow these steps for a successful plan.

Map It Out: Sketch your garden beds on paper before you plant anything. Mark where your onion sets or seeds will go.
Check the List: Refer to the “bad companion” list above. Ensure none of those plants are in the adjacent squares or rows.
Add Friends: Populate the spaces around your onions with the recommended good companions listed.
Rotate Annually: Never plant onions in the same spot two years in a row. This prevents disease buildup. A 3-4 year rotation is ideal.

Common Mistakes to Avoid

Even experienced gardeners can slip up. Watch for these errors.

Ignoring Family Ties: Forgetting that gladiolus is in the iris family and can share onion diseases.
Overcrowding: Even with good companions, onions need space for air circulation to prevent fungal issues. Don’t plant to closly.
Poor Timing: Planting onion sets too late in spring can result in small bulbs, regardless of companions. Get them in early.
Watering Wrong: Overhead watering wets the foliage and can promote disease. Use soaker hoses or water at the base of plants.

FAQ: Onion Companion Planting

Can I plant onions next to garlic?

Yes, you can, but with the caveat about shared diseases and pests. It’s generally fine but not a risk-free pairing. Keep an eye out for problems.

What should you not plant near onions and garlic?

The list is similar for both. Definitely avoid beans, peas, asparagus, and sage. They both dislike the same poor companions.

Are onions and potatoes bad companions?

They are not recommended. Potatoes are heavy feeders and need hilling, which can disturb the onion’s shallow root growth. Its best to give them separate space.

Where is the best place to plant onions in a garden?

Choose a spot with full sun and loose, well-draining soil. Raised beds are excellent. Ensure the area hasn’t grown alliums in the previous 1-2 years.

Do onions improve soil for other plants?

Not directly. Their main benefit is pest deterrence. After harvest, they leave behind a fairly neutral soil, which is fine for most crops that follow in rotation.

By steering clear of bad companion plants for onions, you set the stage for a healthier, more productive garden. The strong smell of onions is a gift to many plants, but a few simply don’t appreciate it. A little planning goes a long way. Your reward will be a bounty of crisp, flavorful bulbs to enjoy all season long.