What Should Not Be Planted Next To Onions

Getting your companion planting right is a game-changer for a healthy onion patch. If you’re wondering what should not be planted next to onions, you’re already on the path to a better harvest. Onions are fantastic garden allies, but they have a few neighbors that can really hold them back. Knowing which plants to keep apart helps your onions grow bigger and stronger, and it saves you from a season of disappointment.

This guide will walk you through the plants that compete with or harm onions. We’ll cover the why behind each bad pairing and suggest much better alternatives for your garden space. A little planning now leads to a much more productive and easier to manage garden later.

This list isn’t about plants that simply don’t help each other. These are combinations that can actively harm your onion’s growth or flavor. Keeping these apart is a key step for success.

Beans and Peas (All Types)

This is one of the most important rules. Onions and other alliums (like garlic and leeks) do not get along with legumes.

Why it’s bad: Onions release substances from their roots that can inhibit the growth of bean and pea plants. They can stunt their growth and reduce your pod yield significantly.
The effect on onions: Beans and peas fix nitrogen in the soil. While this is great for heavy feeders, onions prefer a more balanced soil and can produce excess leafy growth and weak bulbs if nitrogen is too high.

It’s best to plant these in completely different garden beds. If you must, allow as much distance as possible between them.

Asparagus

This is a classic example of allelopathy, where one plant’s natural chemicals affect another.

Why it’s bad: Onions and asparagus are directly antagonistic. The roots of each can interfere with the others growth. They compete vigorously in the soil.
The result: You’ll likely end up with spindly asparagus spears and small, underdeveloped onion bulbs. They simply shouldn’t share space.

Sage and Other Strong Herbs

While some herbs are wonderful companions, a few can cause problems.

Why it’s bad: Strongly aromatic herbs like sage can sometimes affect the delicate flavor of developing onion bulbs. The competition for resources in an herb garden can also be tough.
Practical issue: Sage is a perennial that likes drier conditions. Onions need consistent moisture, especially during bulb formation. Their watering needs are incompatible.

Stick with lighter herbs like chamomile or summer savory as neighbors instead.

Gladiolus

This pairing is a problem because of shared pests and diseases, not direct competition.

Why it’s bad: Both onions and gladiolus are susceptible to thrips. Planting them together creates a concentrated target, making it much easier for these pests to spread and cause severe damage.
Prevention is key: By separating these plants, you break the pest cycle and protect both your flowers and your food crop.

Other Alliums (In Crowded Conditions)

This might seem counterintuitive, but planting onions right next to garlic, leeks, or shallots can be risky.

Why it can be bad: They are all susceptible to the same diseases (like onion white rot) and pests (like onion maggot). Grouping them all together makes it easy for a problem to wipe out your entire allium crop.
The exception: If you have healthy soil and no history of these issues, planting them in the same bed is fine. Just ensure they have adequate space between plants to avoid competition. Proper crop rotation is essential here.

Why These Plant Partnerships Fail

Understanding the reasons helps you make better choices beyond just this list. There’s a few main causes of bad companionship.

Root Competition and Allelopathy

Plants compete for water and nutrients. Some, like onions and asparagus, go further by releasing natural chemicals that hinder nearby plants. This is allelopathy, and it’s a primary reason for keeping certain plants apart.

Clashing Nutrient Needs

Onions are not heavy feeders. They need a moderate amount of nutrients to form good bulbs. Planting them next to a heavy feeder like cabbage can leave the onions starved if you don’t fertilize carefully. Conversely, planting them next to nitrogen-fixers like peas can provide to much of a good thing.

Watering Incompatibility

As with sage, differing water needs create stress. One plant will always be at a disadvantage if they have to share the same soil moisture conditions.

Pest and Disease Sharing

This is a huge one. Grouping plants that attract the same pests (like onions and gladiolus with thrips) invites an epidemic. Diversifying your plantings acts as a natural pest control.

What You Should Plant Next to Onions

Now for the good news! Onions have many excellent companions that help with pest control, space use, and growth.

Carrots: A legendary pairing. Onions deter carrot rust flies, and carrots may help deter onion flies. They occupy different soil depths, so they don’t compete heavily.
Lettuce & Spinach: These are quick, shallow-rooted crops. You can plant them between slower-growing onion rows to maximize space. They provide light shade that can help keep onion roots cool.
Beets: Another root crop that gets along well. They have similiar moderate nutrient needs and don’t seem to interfere with each others growth.
Chamomile & Summer Savory: These herbs are believed to improve the growth and flavor of onions. They also attract beneficial insects.
Tomatoes, Peppers, & Eggplant: Onions can help deter pests like aphids and spider mites from these nightshades. It’s a mutually beneficial arrangement.
Strawberries: Some gardeners find onions help keep pests away from strawberry plants. The strong scent seems to confuse many berry-loving insects.

Practical Steps for Planning Your Onion Beds

Here’s a simple process to put this knowledge into action.

Draw a Simple Map: Sketch your garden space, even if it’s just a rough drawing on paper. Outline your beds.
Place Your Onions: Mark where you want your main onion crop to go. Remember they need full sun and well-drained soil.
Apply the “No-Go” List: Check your map. Ensure none of the plants listed above (beans, peas, asparagus, etc.) are in the immediate neighboring rows or squares.
Add Positive Companions: Intentionally place good neighbors like carrots, lettuce, or beets next to or between your onion rows.
Consider Succession Planting: After you harvest early summer onions, you can plant a fall crop of beans in that same space. This is fine! The problem is only when they grow simultaneously.

A final tip is to always practice crop rotation. Don’t plant onions or any allium in the exact same spot year after year. Wait at least three years before replanting alliums in a bed. This prevents soil-borne diseases from building up and depleting the same nutrients consistently. It’s one of the smartest things you can do for your garden’s long-term health.

FAQ: Onion Companion Planting

Can I plant onions next to garlic?
You can, but with caution. They are fine companions if your soil is healthy. However, because they share pests and diseases, it’s often safer to separate them to avoid losing your entire allium crop if a problem arises.

What about planting onions near potatoes?
This is generally considered a neutral pairing. They aren’t harmful to each other, but they aren’t especially beneficial either. Its okay if your garden layout requires it, but there are better companions for both.

Why shouldn’t you plant beans with onions?
Onions secrete substances that can stunt the growth of bean plants. Conversely, beans add nitrogen to the soil, which can cause onions to put energy into leaves instead of forming nice, tight bulbs.

What is a good companion plant for onions to deter pests?
Carrots are the classic choice for mutual pest protection. Herbs like chamomile and savory also help. The strong scent of onions themselves is a great pest deterrent for many other crops, like tomatoes and strawberries.

How far apart should I keep onions from bad companions?
A good rule is to use separate garden beds if possible. If you’re working in a single large bed or row, try to keep at least 3-4 feet between incompatible plants. This minimizes root interaction and pest spread.

Getting your plant relationships right takes a bit of thought, but the rewards are worth it. You’ll have healthier plants, fewer pest issues, and a more bountiful harvest from the same amount of space. By avoiding the plants that hinder onions and choosing helpful friends instead, you set your garden up for its best season yet.