Crop guide · Care

Onion plant care: spacing, depth and watering

Cited onion care here is thinning, water, and mulch — not a spray calendar and not a fertilizer program. Harvest/cure notes below are a source comparison only. This page does not prescribe products.

Growing guidance is general information. Timing, spacing, and care depend on cultivar, weather, soil, and local conditions. Check the sources and adjust the plan for your garden.

Timing lives on when to plant onions. Types live on the onions hub.

Cited thinning (not a BioArva average)

Separate source assertions — not a BioArva average.

Maryland Extension (Maryland): onions should be thinned to 4 inches apart if you intend to harvest moderate-size bulbs, and 8 inches apart for large bulbs. Thinning on their sheet — not a BioArva average.
Clemson HGIC 1314 (South Carolina): their sheet lists thinning onions in spring to four inches apart. Thinning on their sheet — not a BioArva average.
Illinois Extension (Illinois): to produce dry onions, plant the smaller sets 1 inch deep, with 2 to 4 inches between sets; allow 12 to 18 inches between rows. Spacing on their sheet — not a BioArva average.
UMN Extension (Minnesota): after seedlings emerge, thin to 3 to 4 inches apart. Thinning on their sheet — not a BioArva average.
Clemson HGIC 1314 (South Carolina): transplants should be spaced 3 to 4 inches apart in the row. Spacing on their sheet — not a BioArva average.

Cited water notes (not a BioArva schedule)

Separate source assertions — not a BioArva average.

Maryland Extension (Maryland): ensure ample moisture, especially after bulbs begin enlarging. Watering on their sheet — not a yield claim.
Clemson HGIC 1314 (South Carolina): their sheet describes discontinuing watering a week before harvest in the context of protective scales. Source-specific harvest-adjacent context—not a BioArva irrigation schedule or storage promise.
UMN Extension (Minnesota): onions are shallow-rooted and require constant moisture for good growth; if the planting does not receive one inch of rain each week, soak the soil thoroughly at least once a week. Watering on their sheet — not a BioArva schedule.
UMN Extension (Minnesota): if your soil is sandy, it is important to water more often than once a week. Watering on their sheet — not a BioArva schedule.
UMN Extension (Minnesota): their sheet describes stopping watering when tops begin to fall. Source-specific harvest-adjacent context — not a BioArva irrigation schedule or storage promise.
Utah State (Utah): moisten the soil thoroughly to a depth of 12 inches and use drip irrigation if possible. Watering on their sheet — not a BioArva schedule or product SKU.

Cited mulch notes (not a BioArva how-to)

Separate source assertions — not a BioArva average.

Maryland Extension (Maryland): do not hill up the soil on onions; maintain a weed-free bed and apply an organic mulch to maintain soil moisture. Care note on their sheet — not a BioArva how-to.
Clemson HGIC 1314 (South Carolina): cultivate shallowly as often as needed to control weeds, but do not hill up soil around the plants. Care note on their sheet — not a BioArva how-to.
UMN Extension (Minnesota): mulch with herbicide-free grass clippings, weed-free straw, or other organic material to a depth of 3 to 4 inches to help prevent weed growth and reduce the need for frequent cultivation. Mulch note on their sheet — not a product ranking.
Utah State (Utah): their sheet lists organic mulches for water conservation and weed reduction. Mulch note on their sheet — not a fertility claim or product ranking.

Cited harvest/cure comparison (not a protocol)

Separate source assertions — not a BioArva average.

Maryland Extension (Maryland): bulb onions should be harvested when about two-thirds of the dried tops have fallen over; onions may be pulled and left in the field for several days to dry, then cured, with tops intact, in a well-ventilated attic or porch out of the direct sun for 1 to 2 weeks. Cure context on their sheet — not a storage promise.
Clemson HGIC 1314 (South Carolina): harvest bulb onions when about three-fourths of the tops have fallen over; thoroughly air-dry bulbs in a shaded area before storage. Cure context on their sheet — not a storage promise.

How far apart do you put onions?

Maryland Extension (Maryland): onions should be thinned to 4 inches apart if you intend to harvest moderate-size bulbs, and 8 inches apart for large bulbs. Thinning on their sheet — not a BioArva average.

How many onions can I plant in a 4x8 bed?

This guide does not calculate a bed count.

Do onions need to be watered every day?

This guide does not set a daily watering schedule. Maryland Extension (Maryland): ensure ample moisture, especially after bulbs begin enlarging. Watering on their sheet — not a yield claim.

What depth of soil do onions need?

This guide does not state a soil-depth requirement.

What is the secret to growing big onions?

There is no single trick. This page does not prescribe fertilizer.

Can you just leave onions in the ground?

This guide does not tell you to leave onions in the ground. Harvest/cure notes on this page are a source comparison only — not a protocol.

What not to plant next to onions?

This guide does not provide companion-planting guidance.

What to record in the Field Book

Save the bed with the crop and the date. You may add a photo and notes. Compare entries you have recorded with each other — not with a diagnosis from this page. If you want an identification, bring the record to local Extension.

How we know — methodology. Ranges and assertions are cited per source, not blended.