Yellow Onions repay a little planning around what grows beside them. Yellow Onions are the all-purpose storage onion of the kitchen garden, grown for cooking and storing through winter. They need full sun, soil with a pH of 6.0–7.0, 4–6 inches of space and 90–120 days to reach maturity, so their neighbours need to suit the same bed and the same season. This guide lists the companions for yellow onions from our crop data, how each one is thought to help, and which close relatives to keep apart.
There are 4 companions listed for yellow onions here: Marigolds, Basil, Onions and Lettuce. Companion planting is traditional garden practice rather than proven science, so benefits below are described in those terms — what gardeners have long said and done. Yellow Onions are in the Amaryllidaceae family, need moderate water, and are started indoors or sown direct; they are hardy and tolerate cold, so they can go out early in the season. Check any pairing first with the companion-planting finder.
Best companion plants for yellow onions
| Companion | Why it is grown with yellow onions |
|---|---|
| Marigolds | Grown as a border or dotted through the bed; its roots are traditionally said to discourage some soil pests, and its flowers attract beneficial insects. |
| Basil | Traditionally said to improve flavour; its strong scent may confuse pests. Draws pollinators when it flowers. |
| Onions (other varieties) | Other onion varieties share the bed happily at the same close spacing, and the shared scent is traditionally said to confuse pests. |
| Lettuce | A fast, shallow-rooted filler that uses the space between slower plants and shades the soil; harvest it before the main crop needs the room. |
Marigolds are the first companion to reach for with yellow onions, and it is grown to keep, so the bed needs to stay clean and dry as the bulbs mature and the tops fall over. Plant it as a border around the bed or dotted through the rows, and let a few plants flower all season. The flowers keep beneficial insects coming, and the plants are easy to pull out at the end of the year.
How companion planting works
Companion planting works through four overlapping mechanisms.
Pest confusion. Many pests find their host plant by scent. Strong-smelling companions such as Basil, Onions are traditionally said to mask that scent, so the pest passes by. It is a delaying effect, not a barrier.
Pollinator and beneficial-insect attraction. Several companions here flower — Marigolds, Basil — drawing pollinators and beneficial insects in. Let a few flower.
Nitrogen fixing. Nitrogen fixing does not apply here — none of the listed companions is a legume.
Ground cover and root layers. Ground cover does apply: lettuce fills the gaps between plants, shading the soil, holding moisture in and smothering weeds until the main crop needs the room.
None of this replaces the basics. Spacing, watering, rotation and weekly inspection matter more than any pairing.
Plants to avoid near yellow onions
The plants to keep away from yellow onions are its own relatives. Yellow Onions belong to the Amaryllidaceae family, and plants in the same family share the same pests and diseases — here, aphids, slugs, cutworms — so problems pass easily between them and build up in the soil.
| Plant to avoid | Why |
|---|---|
| Garlic | Same Amaryllidaceae family; shares pests such as aphids, slugs, cutworms. |
| Leeks | Same Amaryllidaceae family; shares pests such as aphids, slugs, cutworms. |
| Shallots | Same Amaryllidaceae family; shares pests such as aphids, slugs, cutworms. |
| Scallions | Same Amaryllidaceae family; shares pests such as aphids, slugs, cutworms. |
| Red Onions | Same Amaryllidaceae family; shares pests such as aphids, slugs, cutworms. |
Keep these relatives in a different bed and rotate the Amaryllidaceae family to fresh ground each year. Others in the same family in our crop data include White Onions, Chives, Daffodils.
Planting layout and spacing
Space bulbs 4–6 inches apart, stop watering once the tops start to fall, and lift the bulbs to cure in the sun.
Give yellow onions their full spacing of 4–6 inches between plants. Crowded yellow onions mean poor airflow, more disease and smaller harvests, and no companion plant compensates for it. Work out how many plants fit your bed with the plant-spacing calculator or the square-foot garden planner.
Plant companions in the spaces the spacing leaves, not instead of it. Low companions go in the gaps and along the bed edges; taller ones go at the north side so they do not shade the yellow onions; scented herbs go closest to the crop, dotted through the row.
Yellow Onions are started indoors or sown direct: sow under cover where needed, expect germination in 7–14 days, and set plants out once the weather is right. Work backwards from your last frost date with the seed-starting calculator. Yellow Onions need full sun and soil with a pH of 6.0–7.0. Water needs are moderate, so keep the soil consistently moist with deep, regular watering rather than a light daily sprinkle, and mulch thickly to hold moisture in. The watering calculator gives a starting point for your bed size. Expect 90–120 days from sowing to maturity.
Common problems and fixes
The pests most often reported on yellow onions are aphids, slugs, cutworms. Companions may reduce pressure, but they do not prevent them — inspect leaf undersides and new growth weekly and act while numbers are low.
| Problem | What you see | First fix |
|---|---|---|
| Aphids | Sap-sucking insects on new growth; leaves curl. | Wash off with water; encourage ladybirds. |
| Slugs | Chewed leaves and silvery trails, worst in damp weather. | Beer traps or copper tape; clear debris. |
| Cutworms | Caterpillars that fell seedlings by chewing stems at soil level. | Cardboard collar around each young stem. |
Aphids
Wash them off with a strong jet of water and leave ladybirds and lacewings alone to eat them. Check the undersides of young leaves, where colonies start.
Slugs
Slugs feed at night in damp weather. Clear boards, pots and debris where they hide by day, and set beer traps at soil level near the worst-hit plants.
Cutworms
Push a cardboard collar an inch into the soil around each young stem. Check beds at night with a torch when seedlings start falling.
Unsure what you are seeing? Try the pest-identifier helper or the plant-disease checker. Remove badly affected leaves and rotate the Amaryllidaceae family next year.
Frequently asked questions
What is the best companion plant for yellow onions?
The most widely recommended companions are marigolds. Grown as a border or dotted through the bed; its roots are traditionally said to discourage some soil pests, and its flowers attract beneficial insects.
What should not be planted near yellow onions?
Avoid other members of the Amaryllidaceae family, including Garlic, Leeks, Shallots, Scallions. They share the same pests and diseases as yellow onions, so problems move easily between them. Rotate the family to a fresh bed each season.
Can I plant yellow onions with basil?
Yes. Basil is on the companion list for yellow onions. Traditionally said to improve flavour; its strong scent may confuse pests. Draws pollinators when it flowers.
How far apart should yellow onions and their companions be planted?
Space yellow onions plants 4–6 inches apart, and put companions in the gaps, along the edges and at the ends of rows. The plant-spacing calculator works out how many plants fit your bed.
Does companion planting replace pest control?
No. It may reduce pest pressure, but it does not replace inspection, spacing, watering, rotation and good airflow. Check plants weekly and act early.