How Deep to Plant Onion Sets
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Onion sets are small, dormant bulbs, and the planting depth is the rule most people want to know. For how deep to plant onion sets, two university extension pages give close answers. Iowa State says 1 to 1 1/2 inches. Illinois gives 1 inch for dry onions and 1 1/2 inches for green onions. Both pages also tell you to sort the sets by size before planting, because the size decides whether the bulb grows large or the set runs to a flower stalk.
Quick answer
- Plant sets 1 to 1 1/2 inches deep. Illinois uses 1 inch for dry onions and 1 1/2 inches for green onions.
- Sort the sets into two sizes first. The large sets are for green onions, and the small sets are for dry bulbs.
- For dry onions, space sets 2 to 4 inches apart in rows 12 to 18 inches apart.
- Plant in early spring, as soon as the ground can be worked.
How we chose these sources
We used two extension pages from two separate universities. Iowa State’s Yard and Garden page answers a single question about planting sets. Illinois Extension’s onion page covers the whole crop. Both were opened on 2026-10-29 and read in full. Neither is a retail page. The onion sets question involves no food-safety claim, so two sources meet the standard.
Sort the sets by size before planting
Both pages start with sorting. Iowa State says to separate the bulbs into two size groups before planting, those smaller than a nickel in diameter and those larger than a nickel. Illinois uses a dime as the cut-off instead, and says that large sets, larger than a dime, are best used for green onions, while the small sets produce the best bulbs for large, dry onions. The two cut-offs are close, but they are not the same, so a set that sits near the line should be judged by the purpose you have for it.
The reason for sorting is bolting. Illinois says large sets may bolt and form flower stalks if they are allowed to grow. Illinois also says that bulb formation starts when day length is of the proper duration, and that varieties differ in the day length they need; for the general idea, see how plants sense day length. Iowa State says the larger sets often bolt and do not produce good-sized bulbs. The small sets usually do not bolt, according to Illinois. For a dry onion crop, the small sets are the ones to keep.
Planting depth
Iowa State gives one depth for all sets: 1 to 1 1/2 inches. Illinois splits the rule by use. Large sets for green onions go in 1 1/2 inches deep, close enough to touch one another. Small sets for dry onions go in 1 inch deep. The two pages agree that the range is shallow, and Illinois’s 1 inch for dry onions sits at the shallow end of Iowa’s range.
Depth is not the only variable. Illinois says that round sets produce flat onions, while elongated or torpedo-shaped sets mature into round onions. Planting depth is therefore one part of the picture, and the shape of the set also matters for the bulb you get.
Spacing and rows
Spacing depends on whether you want green onions or dry ones. Iowa State gives rows 12 to 15 inches apart, and says that dry onions need 2 to 3 inches between sets. Illinois says 12 to 18 inches between rows, and 2 to 4 inches between small sets for dry onions. Its green onion rule is closer: plant the large sets touching one another, because green onions are pulled before crowding becomes a problem.
Illinois adds a practical option when sets are placed 2 inches apart. It says to harvest every other plant as a green onion, so the remaining sets have room to form bulbs. That approach lets one row provide two products without moving the sets.
Care after the sets go in
Planting is the start of a short list of tasks. Illinois says onions have shallow roots and compete poorly with weeds and grasses, so timely shallow hoeing and cultivation are important, especially while the onions are small. The same page says to keep onions free from weeds by shallow cultivation and hoeing.
Hilling depends on the use. For green onions, Illinois says to slightly hill the row when the tops are 4 inches tall, which develops long, white stems. It warns against hilling onions that are meant to be dried and stored, because hilling may cause the necks of the stored bulb to rot. Above 40 degrees north latitude, the same page says, root maggots may attack the roots of onion plants. The Iowa State page does not cover these steps, so the points in this section rest on the Illinois page alone.
When to plant
Both pages tie planting to the spring thaw rather than to a calendar date. Iowa State says to plant as soon as the ground can be worked, which it places at late March to mid-April in Iowa. Illinois says onions can be planted as soon as the garden can be tilled in spring, usually late March or early April in the prime regions for producing onions.
Cold weather adds a risk. Illinois says that extremely cold weather during early season growth may condition onions from sets to flower. For the general idea of cold exposure driving flowering, see what is vernalization in plants. That is the same problem that the bolting section describes, so the cold snap and the set size both matter. Planting too early in a cold spring is not a risk the pages quantify, so the draft does not give a cut-off date.
What we could not verify
The two pages answer the question on depth and spacing. They leave several gaps that this guide does not fill.
What the sources do not say
- Neither page gives a soil temperature or calendar date that marks the cold risk for bolting.
- Neither page says what happens when sets are planted deeper than the stated range. The draft does not predict the result.
- The nickel and dime cut-offs differ, and neither page explains the difference. The draft reports both.
- Iowa State gives no spacing for green onions. Illinois gives it, so the green onion spacing in this guide comes from Illinois only.
- Neither page compares sets with transplants or seed for yield or bulb size.
Frequently asked questions
How deep should I plant onion sets?
The answer depends on the use. Illinois says 1 1/2 inches for large sets used as green onions and 1 inch for small sets used as dry onions. The Iowa State page gives a single range for all sets, 1 to 1 1/2 inches.
Which onion sets should I use for green onions?
Illinois says the larger sets, those larger than a dime, are best used for green onions. Iowa State says the same larger sets are for green onions.
Why did my onion sets form flower stalks?
Illinois says large sets may bolt, and that extremely cold weather early in the season can condition sets to flower. The sorting advice above covers the size side of the problem.
How far apart should I plant onion sets?
For dry onions, Iowa State says 2 to 3 inches apart and Illinois says 2 to 4 inches apart in rows 12 to 18 inches apart. For green onions, Illinois says to plant large sets close enough to touch.
When is the right time to plant onion sets?
Iowa State says as soon as the ground can be worked, typically late March to mid-April in Iowa. Illinois says usually late March or early April in its prime regions.
Sources
- Iowa State University Extension and Outreach, Yard and Garden, “What is the proper way to plant onion sets?” (last updated July 23, 2024): https://yardandgarden.extension.iastate.edu/faq/what-proper-way-plant-onion-sets (opened 2026-10-29)
- University of Illinois Extension, “Onion,” Home Vegetable Gardening: https://extension.illinois.edu/gardening/onion (opened 2026-10-29)
