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Onions are a gem to grow in your garden. They make good friends with a lot of other crops and enhance the flavor of others. Plus, they help keep pests away. If you’re looking to do a little companion planting this year, you can’t go wrong with onions.
Onions are easy to grow and they’re a cash crop. Combine that with the fact onions are super-useful in the kitchen, and you can’t go wrong with planting as many as will fit in your garden.
So, if you want to make the best of your onion crop by embracing companion planting, let’s look at what to plant with them, and a few to avoid.
Best Companion Plants for Onions
Onions need full sun and don’t like being shaded by other plants. The soil pH should be between 6 and 7, and well-draining. Wet soil will rot onions. They grow best in temperatures ranging from 55ºF to 75ºF.
Here are some of the best plants to share space with onions.
1. Carrots
Onions benefit carrots (Daucus carota subsp. sativus) because they help to repel the carrot fly. If you have a patch of carrots growing, surround them with onions. The onions act as a border, repelling the carrot fly, which hates the smell of onions.
You could also have alternate rows of carrots and onions.
2. Beets
Onions are a great deterrent to many pests, so it makes sense to surround them with pest-prone plants like beets (Beta vulgaris), which need protection. Some aphids and sugar beet beetles don’t like onions thanks to the sulfur-like scent they emit.
If you have trouble with rabbits, hares, and deer eating your beet leaves, plant plenty of onions around them. I like to plant the beets toward the middle of the planting area, and then surround them with rows of all types of onions.
Just make sure you can get to the beets when you need to.
3. Swiss Chard
Swiss chard (Beta vulgaris subsp. vulgaris) can provide shade to the soil to help keep it moist. Because it doesn’t grow too big, chard won’t prevent sunlight from reaching the onions. A little shade is good, but too much will hurt the onions.
Because onions are short rooted, they appreciate moist (but not wet) soil. The companion planting of swiss chard helps with this.
Plant swiss chard among the onion rows, but keep the base about 12 inches from the nearest onions. You could plant the swiss chard behind the onions if the garden faces the afternoon sun.
4. Leeks
It’s often advisable to not plant similar plants from the same family together due to the pests and diseases they may attract. Leeks (Allium ampeloprasum) and onions are a pair made in heaven though. They both have similar soil, temperature, and nutrient needs.
Leeks are a valuable weapon for onions as well. When the onion fly comes looking for the onions, they get confused by the leek scent. Plant alternate rows of leeks and onions, or rows of onions and rows of leeks next to them.
5. Summer Savory
Some companion plantings don’t offer protection, but rather an improvement in quality and quantity. Summer savory (Satureja hortensis) makes onions slightly sweeter and improves their size as well.
Because summer savory can grow up to two feet tall, be wary of the shade issue. Plant onions in front facing the sun. Water both well, but don’t saturate the soil.
6. Brassicas
The whole family of Brassica genus benefits from being companion planted with onions. These include:
- Cabbage
- Cauliflower
- Broccoli
- Kale
- Kohlrabi
- Mustard
Just avoid planting turnips near onions. They can affect the flavor of the turnips.
Brassica is always inundated by pests but can be protected by onions. The smell of onions repels many of the pests attracted to brassicas. This can save you a large amount of spraying and treating.
7. Lettuce
Lettuce (Lactuca sativa) is a swiftly growing crop compared to onions. Sow the lettuce seeds while the onions are still small. You’ll be able to harvest the lettuce, and then give the onion bulbs time and room to grow.
Onions will also repel many of the pests that plague lettuce like slugs, snails, and aphids.
8. Strawberries
This isn’t a common combination because you may think the onions will affect the sweet flavor of strawberries (Fragaria × ananassa). This isn’t the case, but onions will prevent many pests associated with strawberries.
Plant rows of strawberries on the sides of the onion patch, keeping about 12 inches spacing from each other.
9. Chamomile
Not only is chamomile (Matricaria recutita and Anthemis nobilis) a great plant to grow for herbal remedies and teas, but it also improves the taste of onions when grown close together.
Chamomile seems to grow really well when planted in the onion garden. Plus, when the weather is humid, chamomile’s antifungal properties can help onions avoid fungal diseases.
10. Peppers (Hot and Sweet)
Onions protect pepper plants by repelling many of the pests that love them. Aphids and cutworms love the young pepper plants but don’t like onions. Plant pepper within 12 to 18 inches of rows of onions.
11. Melons
Pests that are attracted to melons are repelled by onions. These include melon aphids and spider mites. Most melons suit being planted with onions as long as the melon vine doesn’t trail directly through the onion rows. They will likely smother the onions from light and moisture and may damage them with their fruit. Try these melons:
- Honeydew
- Watermelon
- Cantaloupe
12. Radish
I’m pretty sure that radishes (Raphanus raphanistrum subsp. sativus) are friends with everyone in the garden. They grow so quickly that they can be planted in among just about everything, onions included.
Plant radishes in between rows of onions and even in between onions within the row.
There’s no huge benefit to either onions or radishes with this companion planting combination, but the timing suits both harvests. Radishes grow quickly and will be long gone before you even think about harvesting the onions.
You may even get two plantings of radishes, but be careful not to disturb the onion bulbs.
13. Parsnips
Parsnips (Pastinacasativa)benefit from onions much as carrots do. They share space well because of the scent of the onions. Parsnips suffer from the same pests as carrots do, but the onions repel them.
I have a few rows of onions with carrots on one side and parsnips on the other. It works well.
14. Dill
Dill (Anethum graveolens) grows well with onions without causing any issues. The presence of dill helps with the flavor of onions and is a useful herb in the kitchen, just like onions.
15. Borage
Borage (Borago officinalis) attracts predatory wasps to the garden that attack many of the pests that destroy crops including onions.
This underappreciated plant will also help protect those other plants in the garden you’re using as companion plants. Borage repels both cabbage worms and tomato worms.
16. Marigolds
Marigolds (Tagetes spp.) are well known to repel aphids and are suitable to be planted in the rows with the onions. You can also plant marigolds around the onion rows as a barrier, or on the edges of the garden.
17. Mint
Mint gives off such a strong smell for insects, onion flies in particular, that it confuses them.
Remember, mint will take over the garden wherever it’s planted, so I suggest you plant it in pots that you can remove at the end of the growing season.
If you plant tomatoes with onions, mint will assist them as well, so a good idea is to place the mint containers between both crops. Be on the lookout that the mint roots don’t grow through the bottom of the containers and into the ground.
4 Plants to Avoid Putting With Onions
Not everything plays well with onions. Here are four plants to avoid.
1. Legumes
Onions and beans definitely don’t mix well. Onions give off a chemical that kills good bacteria on the roots of beans. This will stunt their growth, or even kill the plant.
With beans, include other legumes that don’t work well with onions like:
- Peas
- Lentils
- Alfalfa
- Clover
- Chickpeas
- Soybeans
- Lupins
Legumes fix the soil with nitrogen, so consider planting onions after you have legumes in an area of the garden. Onions are big nitrogen feeders.
2. Sage
Sage isn’t bad for onions, but the conditions it requires are different. If you try to get things right for the sage, the onions will suffer, and the same for the sage if you try to get the growing conditions set for the onions.
The best thing to do is keep both plants well separated.
3. Asparagus
Onions and asparagus compete for the same nutrients, so unless you are always in the garden monitoring them, one will suffer if not both.
Asparagus also takes a long time to get established and grow year after year. You will disturb them if you harvest the onions close to the asparagus.
4. Other Alliums (Except Leek)
Garlic and shallots are to be kept well away from onions. Onion maggots will move from onions to garlic and shallots easily and you’ll lose all three crops.
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