Using paper egg carton flats as a planting “grid” is a slick way to space onion sets evenly, keep weeds down, and get a tidy bed like the one in your photos. The cartons act like a biodegradable mulch and spacer—then break down as the onions grow.

Below is everything you need to do it successfully, without extra fluff.

What this method is best for

Onion sets (small dormant bulbs) for full-size onions
Scallions/green onions (you can plant closer or harvest early)
Small garden beds where neat spacing and weed control matter
Skip this method for plastic/foam cartons. Use only plain paper/pulp cartons that tear easily.

Materials

Paper egg carton flats (the 30-egg “trays” are perfect, but regular 12/18 work too)
Onion sets (or small transplants)
Compost (or aged manure) + basic garden soil
Rake/hoe/shovel
Watering can or hose with gentle spray
Optional: balanced fertilizer or a nitrogen-forward fertilizer for early growth
Optional: mulch (straw/leaves/grass clippings) for bed edges
Step 1: Choose the right onion type (quick but important)

Onions form bulbs based on day length. If you buy locally, you’re usually fine—but if ordering online:

Long-day onions: best for northern areas (roughly above 36–38° latitude)
Short-day onions: best for southern areas
Day-neutral: flexible, good “safe choice” for many places
Also: smaller sets are better for bulbing onions. Very large sets tend to bolt (send up flower stalks) and make smaller bulbs.

Step 2: Timing (when to plant)

Cold-winter climates: plant sets early spring, as soon as soil can be worked (often 2–4 weeks before last frost).
Mild-winter climates: short-day onions are often planted fall to winter for spring harvest.
Onions like cool starts and steady growth. Heat later is fine—especially once bulbs begin forming.

Step 3: Prep the bed (this matters more than the cartons)

Onions want loose soil and good drainage.

Clear grass/weeds from the strip (a trench along a fence line is totally fine).
Loosen soil 6–8 inches deep.
Mix in 1–2 inches of compost.
Level the surface so the trays sit flat.
Avoid heavy, soggy soil. If water puddles there after rain, raise the bed a bit or add more compost.

Step 4: Prep the egg carton trays

Use paper/pulp trays only (no shiny coatings if possible).
If they’re stiff, lightly wet them so they mold to the ground.
Lay flats end-to-end where you want the row.

Optional but helpful

Tear or poke a few extra holes in the bottoms to help roots punch through faster.
If wind is an issue, sprinkle a little soil on the trays to hold them down before placing sets.
Step 5: Place onion sets in the cups

Put one onion set per cup, pointy end up.

Spacing rules (use the trays to your advantage)

Full-size onions: aim for 4–6 inches between plants
With egg trays, that often means planting every other cup (or choosing a tray style with wider spacing).
Scallions / smaller bulbs: 2–3 inches is fine (plant more cups).
If you plant every cup in a tight tray, you’ll get lots of greens and smaller bulbs unless you thin/harvest some early.

Step 6: Cover correctly (depth makes or breaks onions)

This is the #1 mistake people make.

Cover sets with about ½ to 1 inch of soil
You should still be able to feel the set isn’t buried deep
Don’t mound heavy soil over them
Onions like to “sit up” a bit as they bulb. Planting too deep encourages rot and slows bulbing.

How to do it cleanly:

Toss a thin layer of soil over the trays and sets (like in your photos), then gently rake or pat so the soil fills the gaps without packing hard.

Step 7: Watering (steady, not soggy)

Right after planting: water thoroughly to settle soil.

After that:

Aim for about 1 inch of water per week (rain + watering)
Keep moisture consistent, especially during leaf growth
Reduce watering a bit once bulbs are close to harvest and tops start falling
If the bed stays wet for long periods, onions can rot—better to water deeply but less often than to sprinkle daily.