For square foot gardening onions, plant 9 bulbing onions per square at about 4-inch spacing, or 16 green onions per square at 3-inch spacing. That simple grid gives each bulb enough room to swell while packing a surprising harvest into a single foot of soil.
The catch is that spacing only works if the rest of the plan fits. Pick the wrong day-length type and even perfectly spaced onions stay small.
I’d sort your sets or onion seeds by purpose first, then mark each square with plant labels so you know which ones are for bulbs and which are for scallions.
How Many Onions Fit in One Square Foot?
One square foot holds 9 bulb onions or 16 green onions. The count changes because bulbing onions need room to spread sideways, while scallions grow straight up and get pulled young.
Red onions and yellow onions follow the same spacing; color doesn’t change bulb width much.
I like to keep a packet of onion seeds or a bag of sets next to the bed with a few plant labels, so each square is tagged before anything goes in.
Onion seedlings look identical early on, and a label saves you from pulling bulbs too soon.
Use a Nine-Plant Grid for Bulb Onions
Bulbing onions do best in a 3 x 3 pattern. Divide each side of the square into thirds, which puts each plant about 4 inches from its neighbors.
Here’s the easy way to mark it:
- Press two fingers into the mix to make lines 4 inches in from each side.
- Do the same in the other direction to form nine small boxes.
- Poke one planting hole in the center of each box.
If you want extra-large slicing onions, drop to 4 per square at 6-inch spacing. For medium bulbs that store well, nine per square is my pick.
Use a 16-Plant Grid for Green Onions
Green onions and scallions fit 16 per square in a 4 x 4 pattern, about 3 inches apart. They’re harvested before they crowd each other, so the tight spacing works.
You can also mix purposes in one square. Plant 16, then pull every other onion as a scallion once the tops reach 6 to 8 inches tall. The remaining eight have room to form small bulbs.
Which Onion Varieties Suit Your Region and Harvest Goals?
Day length decides whether your onions form bulbs at all, so match the type to your latitude first. After that, choose sweet onions for fresh eating or storage onions if you want bulbs to last into winter.
Match Short-, Intermediate-, or Long-Day Onions to Your Location
Onions start bulbing when daylight hits a certain number of hours. Each group is bred for a different part of the country.
| Onion type | Starts bulbing at | Best US region | Example varieties |
|---|---|---|---|
| Short-day | 10 to 12 hours | South, about zones 7 to 10 | Yellow Granex, White Bermuda, Red Burgundy |
| Intermediate-day | 12 to 14 hours | Middle of the country | Candy, Red Amposta, Sierra Blanca |
| Long-day | 14 to 16 hours | North, Midwest, Northeast, Pacific Northwest | Yellow Sweet Spanish, Walla Walla, Cortland |
A rough dividing line is the 35th parallel, which runs near the North Carolina and Tennessee borders. Long-day onions grown in the Deep South never get the bulbing signal. Short-day onions grown up north bulb too early and stay tiny.
If you garden near a boundary, intermediate-day types are the safest choice.
Choose Between Fresh-Eating and Storage Varieties
Sweet onions taste great raw but keep only 1 to 3 months. Storage onions are more pungent and last 3 to 9 months in a cool, dark spot.
Most dependable storage onions are long-day types, which suits northern gardeners well. Southern gardeners get excellent sweet onions and should plan to eat them sooner.
Shallots are worth a square too. They split into clusters, so plant them at 4 per square, and they store well once cured. If you like the allium family in tight beds, growing garlic in a square foot garden uses the same 9-per-square grid.
When Should You Plant Onions?
Plant onion sets and onion seedlings 2 to 4 weeks before your last spring frost in most of the country. In mild southern winters, fall or winter planting gives short-day onions a longer head start.
Spring Planting Before the Last Frost
Onions are a cool-season crop. They grow leaves in cool weather, then switch to bulbing as days lengthen. More leaves before bulbing means bigger bulbs, since each leaf adds a ring.
My timing advice for spring:
- Sets: plant as soon as the soil can be worked, 2 to 4 weeks before the last frost.
- Seedlings: set out about 2 weeks before the last frost, after hardening off.
- Seeds indoors: start 8 to 10 weeks before the last frost.
In much of the Northeast and Midwest, that lands in late March to April. Late plantings produce small bulbs or none at all.
Fall and Winter Planting in the South
In zones 8 to 10, short-day onions go in during fall or early winter. Many southern gardeners set transplants out from October through January for a late spring harvest.
Seeds for these crops can start indoors in late summer or December, depending on your zone. A layer of mulch protects young plants during cold snaps.
In colder zones, some gardeners plant sets at least a month before the first fall frost to overwinter under mulch. Hardy varieties work best for this.
How Do You Prepare a Square Foot Bed for Onions?
Onions want full sun, loose mix, and steady fertility. Good prep in the square fixes most bulb-size problems before they start.
Provide Sun, Drainage, and the Right Soil pH
Give onions at least 6 hours of direct sun, and more is better. Bulb size depends on how much energy the leaves collect.
Well-drained soil is a must. Onions rot fast in soggy ground, which is one reason raised beds suit them so well.
Aim for a soil pH of 6.0 to 7.0. A simple home pH test tells you where you stand. Add lime to raise pH or elemental sulfur to lower it, following the package rate.
Refresh the Growing Mix with Compost
The classic mix is equal parts compost, peat moss or coco coir, and coarse vermiculite. Onions only need about 6 inches of depth, since their roots stay shallow.
Before planting, work a trowel or two of compost into each onion square. That gives you nutrient-rich soil without tilling.
Growing onions in a square foot garden works best when the mix stays loose and crumbly. Fluff the top few inches with your hands and remove any old roots or weeds first.
How Do You Plant Sets, Seedlings, and Seeds?
Sets are the easiest start, seedlings give the biggest bulbs, and seeds offer the widest variety choice. Whichever you use, onion square foot gardening comes down to marking the grid and planting at the right depth.
Mark the Grid and Plant Sets or Seedlings
Start by sorting sets by size. Sets smaller than a dime make the best bulbs. Larger ones tend to bolt, so I’d use those as green onions.
Steps for planting:
- Mark your 3 x 3 or 4 x 4 grid with your fingers.
- Push each set in pointed end up, about 1 inch deep, with the tip just at or below the surface.
- Set seedlings 1 inch deep, deep enough to stand up.
- Firm the mix gently and water well.
- Add a plant label with the variety and date.
Don’t hill soil over bulbing onions later. Bulbs should sit near the surface as they grow. For a deeper look at the trade-offs, see my notes on vegetable garden seeds vs plants.
Start Seeds and Thin Them to Their Final Spacing
Sow onion seeds ¼ to ½ inch deep. Indoors, use a shallow tray of seed-starting mix in a bright, warm spot.
Trim seedlings to about 4 inches tall with scissors to keep them sturdy. Harden them off for a week before moving them to the bed.
Outdoors, drop 2 or 3 seeds in each grid spot, since germination is patchy. Once they’re a few inches tall, snip the extras at soil level so one remains per spot. Pulling them disturbs the roots of the keeper.
How Should You Lay Out Onion Squares and Nearby Crops?
Square foot gardening puts plants close together, so onions need sunny squares away from tall crops. Pick low-growing neighbors that won’t spill into the onion grid.
Keep Taller Crops and Trellises from Shading Onions
Put trellises and tall crops on the north side of the bed. That keeps their shadow off the shorter rows to the south.
Onions belong on the south or middle squares. Trellised tomatoes, corn, and climbing crops stay north. If you grow pole beans in square foot gardening, keep their trellis well away from onions, since beans and onions are a poor pairing anyway.
Leave 2 to 3 foot paths between beds so you can weed onions without stepping in the mix.
Choose Neighbors Without Crowding the Grid
Good square foot gardening onions neighbors are short and tidy. My favorites:
- Carrots, which share the cool season and benefit from onion scent masking them from carrot rust fly
- Lettuce and spinach, which grow fast and leave before onions bulb
- Beets and chamomile
- Brassicas like cabbage and broccoli
I like growing carrots in a square foot garden right beside onion squares. Fast radishes in a square foot garden also fit nearby and clear out in a month.
Many gardeners keep beans and peas away from onions, though the evidence is limited. More pairings are in my guide to companion planting in square foot gardening.
How Much Water and Fertilizer Do Onions Need?
Onions need about 1 inch of water a week and regular feeding during leaf growth. Both should taper off as the bulbs finish.