Proper onion spacing is essential to promote healthy plant growth and high yields in a vegetable garden. Plant and row spacing depends on the type of onions you’re growing (green onions, small bulbs, or full-sized bulbs), as well as the planting technique (direct seeding, transplanting seedlings, or planting sets). Keep reading to learn more about how far apart to plant onions.
Why is onion spacing important?
Proper plant spacing is essential when growing vegetables. Those planted too closely compete for water, sun, and nutrients, but planting too far apart wastes garden space and reduces yield. Tightly packed plants also limits air circulation which increases the risk of plant diseases. Getting the spacing right means healthy plants and larger harvests. Spacing information is typically provided on seed packets, but you can also find it in a book like my award-winning book The Year-Round Vegetable Gardener.
Types of onions
There are a lot of types of onions you can grow in your vegetable garden. My favorites include leeks, shallots, chives, scallions, walking onions, and bulbing onions. In this article I’m going to focus on bulbing onions which can be harvested as green onions, small bulbs, or mature bulbs for storage. When flipping through the onion section of your favorite seed catalog you may notice there are three categories of onions: long-day, short-day, and intermediate-day onions. This is important because day length (hours of daylight) influences when an onion plant starts to form its bulb and reach maturity. To be successful growing bulbing onions you need to choose the right type for your geographic location.
- Long-day varieties are grown in northern regions above 37 degrees latitude. The bulbing process begins when the day length reaches 14 to 16 hours. Because I live in the north, I grow long-day onions.
- Intermediate-day or day-neutral onions are those grown in latitudes of 32 to 42 degrees and begin to bulb when there is 12 to 14 hours of day length.
- Short-day varieties are grown in the south, below 35 degrees latitude. Short-day onions initiate bulbing when there is 10 to 12 hours of light.
What influences spacing for planting onions?
Plant spacing for onions can be determined by several considerations including planting technique (direct sowing, transplanting, planting sets) as well as how you intend to harvest the crop (green onions, small bulbs, full-sized bulbs). Below you’ll learn how far apart to plant onions based on both of these factors.
Another point to consider is that onions can be planted in rows or in a grid pattern. Planting in a grid is particularly useful when gardening in raised vegetable beds or containers. It maximizes space without sacrificing bulb size.
3 ways to plant onions
Onions grow best in a site with full sun and fertile, well-draining soil. Amend the bed with an inch of compost or rotted manure before planting. If your soil isn’t particularly fertile, you may wish to add a slow-release organic fertilizer when you prepare the garden bed. If planting onions in pots, choose a container or fabric planter that holds 7 to 10 gallons of potting mix and offers good drainage. Now that we know where to plant them, it’s time to look at the three most common planting techniques because these play a part in plant spacing for onions: direct sowing outdoors, starting seeds indoors and then moving the pencil-sized seedlings into the garden, or planting from sets. There are benefits and drawbacks to each planting technique.
Direct sowing onion seeds
The biggest benefit to growing onions from seeds is variety. Seed catalogs offer dozens of variety options versus just a few for onion sets. If you wish to direct sow onion seeds in the garden, they must be planted in early spring to ensure they have enough time to size up before bulb formation begins. My northern garden doesn’t offer a long enough growing season for me to grow bulbing onions from seeds.
Transplanting onion seedlings
My preferred technique is to start onion seeds indoors under grow lights or in a sunny window about 10 to 12 weeks before the last expected spring frost. As with direct sowing, the advantage to this method is that you can choose from the many varieties of onion seeds available in seed catalogs. An easy way to start onions seeds for healthy transplants is to winter sow. Learn more about how to winter sow in this in-depth article. If you don’t have time or space to start your onion seeds indoors you can also buy sturdy seedlings from a local gardening center in spring.
Planting onion sets
Onion sets are immature bulbs sold in bags or bulk each spring. Each small bulb is about a half inch to three-quarter inches in diameter and grown from seeds planted the previous season. Sets are a convenient way to plant onions, but you’re limited in variety selection. Most garden centers sell sets for red, white, and yellow onions, but not specific varieties. If you decide to grow onions from sets, plant only the smallest sized sets, those less than three-quarters of an inch in diameter because these result in larger bulbs.
Onion spacing based on planting technique
As noted above, there are three ways to plant onions. Once you’ve decided which ones works for you follow these spacing recommendations below:
Onion spacing when direct seeding
I’ve had great success direct seeding scallions and salad onions which are relatively quick to grow, but prefer to transplant onion seedlings when my goal is large bulbs. This is because my growing season isn’t long enough to mature direct seeded long-day bulbing onions. If you wish to try your luck at direct sowing onions, plant in early to mid-spring when the soil temperature is 50 F (10 C).
Sow the seeds a quarter of an inch deep and a half inch apart. When the young plants are several inches tall, thin them to 1 inch apart for green onions, 2 inches apart for small to medium-sized bulbs, or 4 to 6 inches apart for large bulb onions. Plant in a grid pattern or space rows 12 to 18 inches apart.
Onion spacing when planting seedings
This is my go-to technique for growing onions. I start the seeds, like for these yellow sweet Spanish onions, under my grow lights in late winter and transplant the pencil-sized seedlings 3 to 4 weeks before my last frost. Set seedlings 1 inch apart for green onions, 2 inches apart for small to medium-sized bulbs, or 4 to 6 inches apart for large bulbs. Plant in a grid pattern or space rows 12 to 18 inches apart.
Onions spacing when planting sets
It’s quick and easy to grow bulbing, or storage, onions when using sets. Again, be selective when planting sets and only use those that are no larger than three-quarters of an inch across. Like seedlings, plant sets 3 to 4 weeks before the last expected spring frost. Plant each set 1 inches deep and 2 inches apart for small bulbs or 4 to 5 inches apart for large bulbs. Plant onion sets in a grid pattern or in rows spaced 12 to 18 inches apart.
How to multisow onions
Recently, I tried a new technique for growing onions called multisowing or multiplanting. This is essentially growing onions in clumps or clusters of three or four plants and is an easy way to increase yield in a home vegetable garden. To multisow onions, start the seeds indoors 10 to 12 weeks before the last frost. Sow seeds in a high quality potting mix in cell packs, like 806’s or 1004’s. An 806 tray has eight packs each with six cells. A 1004 has 10 packs each with four cells. Any container or cell that is about 3 inches across is fine.
Sow 4 to 5 seeds per cell. As they grow, thin the clump to 3 to 4 seeds each. About a month before the last expected spring frost, harden off the seedlings and transplant the entire clump into the garden – don’t separate them. Space each clump of seedlings 6 inches apart in a grid pattern. If planting in rows, space each clump 6 inches apart with rows 12 to 18 inches apart.
Caring for onions
Onions are a shallow rooted vegetable and require regular moisture to grow well and yield large bulbs. Mulching the soil with straw reduces the need to water. It’s also important to remove weeds that sprout up between the plants as excessive weeds can reduce bulb size. Use a collinear or scuffle hoe for shallow cultivation to uproot weeds or pull weeds by hand.
Harvest bulbing, or storage onions when about one-third of the leaves have turned brown and the tops have fallen over. They need to be cured so they can be stored for months. Curing is easy to do, but takes several weeks. Read more about curing onions in this detailed article. Once cured, store onions, as well as related crops like garlic and shallots, in a cool, well ventilated spot in mesh bags or crates.
Learn more about growing onion family vegetables in these articles:
- How to grow green onions
- 6 types of perennial onions
- Planting garlic in the spring
- How to grow garlic in pots
- How to harvest chives
- Growing ornamental alliums
Do you consider onion spacing when planting seeds, seedlings, or sets in your garden?