Onions and the Allium Effect: A Companion Guide

Onion plants

Onions are one of those vegetables you can’t do without. They’ll make you cry in the kitchen the same way they’ll inflict pain on numerous pests in the garden, with their smell and pungent fumes. Onions act as a natural defense mechanism, driving away some of the most annoying and destructive bugs. Just think of them as the neighborhood watch for your garden.

Onions can be pretty easy to start and cultivate, but will take some patience as they are typically long season crops. No matter, all the while they’re in the ground they will be repelling nasty pests. They’re also versatile and able to grow to any desired size. This makes it possible to fit them into tight places, namely among the companion plants they will be protecting.

We’ll now examine the many ways onions provide expert security to the crops in your garden.

Benefits of Onions as a Companion Plant

As far as companion plants go, onions are the reliable employee that takes on loads of overtime hours. Because onions take so long to mature, vegetables, fruits, flowers and herbs that are planted close by will benefit greatly, all throughout the season, from their ability to ward off pests. They are a true natural pesticide, no chemicals or sprays required.

There are few plants that can match the allium effect. A couple plants in this list add benefits of their own, but for the most part it’s the underrated aspects of the onion that makes it the MVC (Most Valuable Companion).


Confidently interplant carrots in between rows of onions. The two are friends with mutual benefits; carrots reduce the presence of onion flies and onions ward off carrot flies. They won’t compete for resources either. The long taproots of carrots reach down deep for water and nutrients, while onions only occupy the very top layer of the soil. This traditional tandem should always be grown together in some capacity.


Tomatoes receive a boost in flavor when grown with onions. And don’t expect any issues with aphids or spider mites on your tomato plants when onions are planted nearby.



Strawberry plants attract an endless array of pesky bugs and crafty critters that seek their hydrating, sugary sweet berries. Who wouldn’t want that first taste of fresh spring fruit? Poor strawberry plants are so troubled by nearly every pest there is and it can be extremely frustrating trying to protect them.

Onions to the rescue once again! If you were going to plant onions anyway, why not start some in the vicinity of your strawberry patch? The presence of their odorous emissions will confuse, divert and repel most common pests. Luckily, onions won’t transmit their strong odor or flavor to your delectable berries.


The two root crops can be grown in rows side by side. Onions ward off flea beetles, which are notorious for shredding beet greens. The sweet roots may even be protected from larger wildlife, like deer and rabbits because of the strong onion odor.


Aphids, Japanese beetles and spider mites can become big problems for a pepper crop. Mix some onions in around your pepper plants to keep these pests at bay.

Brassica Vegetables

Planting some onions amongst broccoli, cabbage, cauliflower, Brussels sprouts, kale, and kohlrabi is a must. The signature allium aroma keeps away multiple pests that can destroy brassica crops. Flea beetles, cabbage maggots and cabbage loopers won’t venture anywhere near an onion patch.


Leaf lettuce heads mature in a flash and can be interplanted next to rows of onions. Once the lettuce is ready it can be harvested just in time for the onions to start sizing up. Typical lettuce nuisances like flea beetles and aphids are driven away by the presence of onions.

Companion crop Lettuce


The tiny, rapid growing root forms a nice trio with onions and lettuce. Onions deter flea beetles that usually bother radish greens. Multiple crops of radishes and lettuce can be turned over, taking full advantage of the natural pest repellent, planted around the slow and steady growth of a row of onions.


Marigold roots keep harmful nematodes from invading your garden soil. Onions, being a root crop, are especially benefited by this signature service that marigolds provide.


Onions and roses, sounds like a bad band name. And the combination won’t exactly make a romantic bouquet that you would present to a date.

All kidding aside, nothing is more discouraging than rose plants that wither and die due to pest invasion. The two don’t necessarily have to occupy the same growing space, but by keeping onions somewhere in the vicinity, unwanted pests could be deterred.

Pigweed (Amaranth)

Pigweed is known as a dynamic accumulator that pulls up nutrients from the depths of the soil. The newly available minerals are gladly taken up by hungry onion bulbs. Just be extremely cautious when cultivating this plant, it has a reputation for being obnoxiously invasive.


Onions pair well with seemingly every type of herb commonly grown in the garden. Onions’ synergistic effects improve growth, enhance flavor and drive pests away from herbs like dill, chamomile, summer savory, mint and parsley.

Companion Vegetables Garden

Enemies of Onions: What Not to Grow

There is a small handful of plants that may be negatively affected when grown in close proximity to onions. 

Peas, beans and onions should all have their own separate sections of the garden. Stunted growth for onions and the legumes will be the result of this ill-advised combination.

Onions will also stunt the growth and of early season asparagus spears. It is detrimental to the flavor of asparagus to grow onions close by. It’s best to keep onions and asparagus on opposite ends of the garden.

Onions: The Bouncer at the Border of Your Garden

The humble, honest allium seems to be a fast friend to a wide range of garden favorites, save a few exceptions. So if you wish to protect your most prized crops, it’s crucial that you incorporate onions as a first line of defense.

Frequently Asked Questions (FAQ)

How much space do onions need?

Young onion transplants should be planted 2-3” apart. Onion sets can be spaced a little further, 4-6” as they size up more quickly. Onions can be used to fill the space between rows of crops or other vacant spots in the garden and harvested at the desired size.

What are some tips for growing onions?

Onions like to be grown in full sun. The soil should be well-drained and can be enriched ahead of planting with compost or other organic material. The bulbs don’t grow very deep, so make sure the top layers of soil remain moist. Apply a layer of mulch to help this cause.

Should protruding onion tops be covered with dirt?

It’s normal for the shoulders of onions to emerge over the course of the growing season. Top soil naturally washes away over time. There’s no need to cover it back up, pulling the dirt around it may damage the underground bulb. Apply mulch early in the season to prevent this from happening.

How many days until onions are ready to pull?

Check your specific variety. Onions mature anywhere from 100 days all the way up to 180 days.

How can you tell when onions are ready to harvest?

Usually by late summer the green tops will begin to yellow out and eventually flop over. The leaves dry out indicating the bulb has stopped growing. Onions should be harvested at this time.

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