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.
Step 8: Feeding onions (simple schedule)
Onions need nitrogen early to build lots of leaves (each leaf = a ring in the bulb).
- At planting: compost is usually enough to start
- When shoots are 4–6 inches tall: feed lightly with nitrogen (or a balanced fertilizer)
- Repeat every 2–3 weeks until you see bulbs starting to swell
Once bulbing really begins (thicker bases, plants looking “oniony”), stop heavy nitrogen or you’ll delay maturity and reduce storage quality.
Step 9: Weed control and why the cartons help
Onions hate competition because they have shallow roots.
The trays help by:
- blocking light to many weed seeds (like a paper mulch)
- keeping spacing consistent so you can weed quickly
Still:
- Pull weeds early and often
- Weed shallowly (don’t chop onion roots)
If you want extra insurance, mulch the paths/edges (not directly burying the onion necks) with straw or dry clippings.
Common problems (and easy fixes)
1) Bolting (flower stalks)
Causes: sets too large, cold stress after planting, variety mismatch
Prevention: use smaller sets, plant at the right time, choose correct day-length type
2) Small bulbs
Causes: too much shade, overcrowding, low nitrogen early, wrong variety
Fix: plant sunnier, give 4–6″ spacing for bulbs, feed early
3) Rot or mushy bulbs
Causes: poor drainage, overwatering, planting too deep
Fix: raise bed, water less often, cover only ½–1″
4) Thrips (silvery streaks on leaves)
Fix: strong water spray on leaves, keep plants unstressed, consider insecticidal soap if severe
Harvesting
For green onions
Harvest anytime once they’re pencil-thick—often 30–60 days depending on variety and weather.
For bulb onions
You’ll know they’re ready when:
- 50%+ of the tops fall over and yellow, and
- bulbs look full at the soil line
Pull them, shake off soil (don’t wash), and move to curing.
Curing and storing (don’t skip this)
- Cure in a warm, dry, shaded, ventilated place for 10–14 days (longer if humid).
- Tops should be papery and necks tight/dry.
- Trim tops to 1–2 inches (or braid if you prefer).
Store in a cool, dry spot with airflow (mesh bags, crates). Don’t seal them in plastic.
Quick success checklist
- ✅ Paper/pulp trays only
- ✅ Loose soil + compost, good drainage
- ✅ Correct onion type for your region
- ✅ Sets planted shallow (½–1″)
- ✅ Space for bulbs (often every other cup)
- ✅ Steady water, early nitrogen, stop heavy feeding at bulbing
- ✅ Weed early, cure well, store dry
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