What to Plant With Onions: Onion Companion Plants (and What to Avoid)

Onions are some of the easiest plants to fit into a mixed bed. They grow upright, take very little sideways room and go in early in spring, so they slip into the edges and gaps around larger crops. The catch is that the onion family has more listed conflicts than almost any other crop group: beans, peas and lettuce are all named as poor neighbors. Here is what extension sources list, and how to place onions so they help instead of crowd.

Good companion plants for onions

Tomatoes and peppers

Virginia Extension lists the onion family as a tomato companion, and University of Maryland Extension suggests alternating rows of peppers and onions. Onions go in weeks before either crop and are often harvested by mid to late summer, just as the tomatoes and peppers need more room. Together with basil, this is the classic salsa bed. See what to plant with tomatoes and what to plant with peppers.

Carrots

Carrots and onions are a listed pair and a very space-efficient one: both are slim, upright crops that can be grown in close rows. A carrot-and-onion strip makes a tidy edge for a bed. More in what to plant with carrots.

Cabbage family and beets

Virginia Extension lists the onion family as a companion for cabbage-family crops (cabbage, broccoli, kale, cauliflower) and for beets. All of these are cool-season crops planted in early spring, so they share a bed and a schedule well. See what to plant with broccoli for the cabbage-family side.

What not to plant with onions

Beans and peas

Virginia Extension lists the onion family, garlic included, as incompatible with both beans and peas. This is the most common keep-apart rule in vegetable gardening, so plan legumes and onions for different beds or opposite ends of a long bed. See what to plant with beans for where beans fit instead.

Lettuce

Lettuce is also on Virginia Extension's incompatible list for onions and garlic. Many gardeners grow the two close together without trouble, and research on most companion effects is limited, but if you are following extension lists, give lettuce its own row away from the onions. Our lettuce guide covers better lettuce neighbors.

The full list of pairs to keep apart is in our keep-apart chart.

Sets, seeds or transplants, and day length

UMN Extension describes three ways to start onions: seeds, transplants, or small bulbs called sets. Sets are the simplest; transplants tolerate light frosts; seed is the cheapest and offers the most varieties.

Day length matters too. Onions start forming bulbs when days reach a certain length. Long-day onions need about 14 or more hours of daylight and suit northern gardens; short-day (often sweet, mild) onions bulb with shorter days and are better suited to southern gardens, and UMN notes they tend to stay small in the north. Check the seed packet or set label against where you live.

Planting and spacing

• Seed outdoors: as soon as the soil can be worked in spring, sow in a two-inch-wide band a quarter to half an inch deep, in rows 12 to 18 inches apart, then thin to three to four inches apart.
• Seed indoors: start 10 to 12 weeks before planting out.
• Transplants: plant about two inches deep, three to four inches apart, in rows 12 to 16 inches apart.
• Sets: plant as soon as the soil is workable in spring.

A simple onion bed layout

1. Tomatoes or peppers along the north side.

   Both are listed onion companions and the tallest plants in the bed.

2. A row of onions in front of them.

   Planted early in spring, three to four inches apart.

3. Carrots next.

   Another slim crop that pairs with onions.

4. Beets or a cabbage-family crop along the south edge.

   Both are listed onion companions.

Keep beans, peas and lettuce in a different bed. Our raised bed layouts include a ready-made salsa bed with onions.

When to plant onions

Onions are an early crop. Sets and transplants go out about three to four weeks before your last spring frost, and seeds started indoors need a long head start. Our planting calendar turns your frost date into both windows. For dates by region and day length, see when to plant onions.

Watering, harvest and curing

Onions are shallow-rooted and need steady moisture. UMN Extension recommends soaking the soil at least once a week if rain does not supply an inch, more often in sandy soil, and stopping once bulbs reach full size and the tops start to fall. Hoe shallowly, since nicked bulbs are more prone to disease, or mulch three to four inches deep with weed-free straw or herbicide-free clippings.

Harvest when about half the tops have fallen over and dried. Lift the bulbs with a spading fork, then cure them in a warm (75 to 90°F), dry, well-ventilated place for two to four weeks, until the outer scales are papery and the necks are tight. Store cured onions somewhere cool and dry; UMN notes they start to sprout above 40°F.

Onions in containers

Green onions and scallions are an easy container crop because they are harvested young and need little room. Bulb onions can grow in a wide, well-drained container if they are spaced three to four inches apart and kept evenly moist, though bulbs are often smaller than in the ground.

Frequently asked questions

What grows well with onions?

Extension-listed companions for onions in our checker are beets, cabbage-family crops, carrots, celery, peppers and tomatoes. The table on this page explains why each pairing works.

What should you not plant with onions?

Our sources list asparagus, beans, lettuce and peas as poor neighbors for onions.