How to Grow Onions from Sets for Reliable Bulbs

Pick a site that gets at least 6 hours of direct sun and drains within 24 hours after a heavy rain. To test drainage, dig a 6-inch-deep hole, fill it with water and time how long it takes to clear: good drainage clears in under 24 hours. If water stands longer, plant in a raised bed or mound the soil to improve runoff.

Test soil pH with a home kit or lab test; onions prefer pH 6.0–7.0. To raise pH by about 0.2–0.5 points on a 100 sq ft bed, work in 1 to 2 pounds of agricultural lime a week before planting; to lower pH, apply 1 to 2 pounds of elemental sulfur per 100 sq ft and wait 4–6 weeks before planting. Incorporate 2–3 inches of well-rotted compost into the top 8 inches of soil to improve structure and supply nutrients.

If your soil is heavy clay, mix in 20–30% coarse sand or 2–3 inches of compost to get a friable texture so roots can spread 6–8 inches. For a raised bed 4 inches deep, add 2 inches of amended mix across the surface and fork it in to a 6–8 inch working depth.

Buy firm, disease-free onion sets about 3/8 to 1/2 inch in diameter for reliable bulbing. Keep sets cool and dry before planting; you can hold them for up to 6 weeks at 40–50°F. Discard any soft, moldy, or split sets.

Plan rows 12–18 inches apart. Use a length of twine and stakes to mark a straight row; mark planting points every 4 inches for small varieties and 6 inches for large types. For a 10-foot row at 4-inch spacing you will plant approximately 30 sets; at 6-inch spacing you will plant about 20 sets.

Plant each set pointy end up so the top 1/2 to 1 inch of the bulb is at or just below soil level; burying deeper than 1 inch can delay bulbing. Make a shallow furrow 1 inch deep with a hoe or dibble, set bulbs at the marked spacing, and firm soil around each set by pressing with the heel of your hand or the flat of a trowel.

Water gently after planting to settle the soil: apply roughly 0.2–0.4 gallons (about 0.8–1.6 liters) per plant. During active growth supply about 0.5–1 inch of water per week (roughly 0.3–0.6 gallons per square foot), more in sandy soils and less in clay; measure rainfall with a gauge and adjust irrigation accordingly.

Side-dress at 3 weeks with a balanced granular fertilizer at about 1 ounce per linear foot of row (roughly 30 grams per meter). If using a soluble feed, apply at half-strength every 2–3 weeks until bulbing begins; once necks start to swell, reduce nitrogen and switch to a low-nitrogen formula to encourage skin formation.

Thin sets when tops reach about 6 inches tall. Use scissors to cut out the weakest plants at soil level rather than pulling to avoid disturbing neighbors; retain thinnings as green onions and use them within 3–4 days. Aim to leave remaining bulbs 4–6 inches apart so each plant gets sufficient space for a 2–3 inch bulb diameter.

Mulch with 1–2 inches of straw or shredded leaves after plants are established to keep soil moisture even and suppress weeds. Because onion roots are shallow, avoid cultivating inside the 2-inch band around each bulb; hand-weed or shallow rake instead.

Monitor pests and diseases weekly. For thrips, look for silvery streaks on leaves; spray insecticidal soap every 7–10 days until damage subsides and consider floating row cover for 2–6 weeks in heavy-pressure years. For cutworms, install a 2-inch-tall paper collar or dust a 1/8-inch band of diatomaceous earth around each plant base; rotate onions out of the bed every 3 years to reduce buildup of white rot and other soil-borne diseases.

Harvest when 60–80% of tops have fallen over and necks feel soft, typically 90–120 days after planting for most sets. Use a fork to lift bulbs gently from a 6–8 inch radius to avoid nicking skins; leave roots attached during curing to reduce tears.

Cure bulbs in a single layer out of direct sun in a dry, well-ventilated space for 10–14 days; aim for temperatures around 70–85°F and low humidity. Turn bulbs once during curing so all sides dry evenly; when outer skins are papery and necks are dry, trim tops to 1 inch and remove excess soil.

Store cured onions in mesh bags, crates, or slatted boxes at 32–40°F and 65–70% relative humidity for longest keeping. Under those conditions, well-cured long-day storage types can last up to 9 months while short-day types typically keep 4–6 months; check bulbs monthly and remove any with soft spots to protect the rest.

Container tip: use a pot at least 12 inches deep with a good-quality potting mix plus 20% perlite for drainage. Space sets in a single ring or row with the same 4–6 inch spacing; expect slightly smaller bulbs but a convenient, high-yield container crop for a balcony or small patio.

format_quote

“Start sets shallow and thin early: the faster you reduce competition, the larger the remaining bulbs will grow.”

—

Further reading: How to Grow Tomato Plants in Containers · How to Grow Vegetables You Can Harvest in a Month or Less
Sources: UMN Extension