How Many Onions Grow From One Bulb?
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How many onions grow from one bulb? For a regular bulb onion, the answer is one: a set, a transplant or a seed each makes a single plant and a single bulb. The exceptions are multiplier onions, potato onions and shallots, which split underground, so one planted bulb gives a cluster of smaller ones. An onion bottom from your kitchen is a third case, and for most people it ends in green shoots rather than a new bulb.
| What you plant | What you get | Time to harvest | How to plant it |
|---|---|---|---|
| Onion set (small bulb), transplant, or seed | 1 bulb per plant | 85–120 days to mature bulbs (UMD Extension) | Sets: pointed end up, covered with 1 inch of soil; 4–6 inches apart (UF/IFAS) |
| Multiplier / potato onion | A cluster of smaller bulbs from each one you plant | No extension figure; one specialist grower gives 70–100 days for spring planting | Pointed end up, 4–6 inches apart; fall or spring (UMD Extension) |
| Shallot | A cluster; bulbs are sold with 2–6 cloves depending on variety (UMD Extension) | UF/IFAS gives 50–60 days for Florida; other regions will differ | About 6 inches apart for small bulbs, growing point left exposed (UF/IFAS) |
| Egyptian walking onion | Plants itself: bulbils form at the top of the stem and root where they fall | Grown mainly for greens (UMD Extension) | 9 inches apart (UMD Extension) |
| Kitchen onion bottom | Green shoots, most reliably. New bulbs are reported by some bloggers; I found no extension support. | Shoots in 2–3 weeks in soil (one gardening blog) | Cut a thick slice off the root end, plant root side down in damp soil |
Why one bulb gives one onion
A bulb onion is a biennial that gardeners treat as an annual: it needs two years to finish its life cycle, and the bulb you eat is the end of year one (UMD Extension). A set is a small bulb that you plant to grow on into a full-size one. That is why planting a whole store-bought onion does not behave like planting a seed potato. A regular bulb onion normally grows as a single plant, not as a clump.
This also explains the sets’ main failure. Planting too early, with exposure to cold temperatures, can make onions send up a seed stalk (UMD Extension), and the plant then stops swelling the bulb. BBC Gardeners’ World notes that most sets are heat-treated to make bolting less likely. Sets being small, partly grown bulbs is the usual explanation; I did not find that spelled out in the extension pages I read, so treat it as common practice rather than a quoted rule. A mature pantry onion has already done its year of bulb growth, so I would not expect a bigger onion from it.
Multipliers, potato onions and shallots: the ones that do multiply
These belong to a group sometimes labelled Allium cepa var. aggregatum, and UMD Extension lists shallots and multiplier onions (also called potato onions) as perennial types that produce a cluster of small bulbs. Seller names vary, so read the variety description rather than relying on the word “multiplier” alone.
How many bulbs you get is less certain than most pages pretend. Extension sources say “cluster” and give a clove count for shallots (2–6). One long-running potato onion grower reports averaging about five new bulbs per planted bulb, and says small planted bulbs tend to make a single larger bulb instead of a nest (one grower’s experience, not extension). Treat any “you will get N” figure as a range for your own soil and variety.
The planting math that follows from this. If your clusters average four bulbs, you plant one bulb, harvest four, and keep one as next season’s seed stock, leaving three to eat. To eat 30 bulbs a season at that ratio, you would plant 10. Replace the four with whatever your first harvest shows. You also need the spacing in the table: crowding a multiplier is the quickest way to get a clump of marble-sized bulbs.
The practical bit is that these types can be saved and replanted every year. UMD Extension says to dig them when the tops die back and fall over, divide and store some to replant in fall. If you want to start with the real thing, look for shallot bulbs sold for planting, which come labelled with a variety, rather than guessing with grocery-store shallots.
Regrowing an onion from the kitchen: what to expect
Cutting an onion leaves the root end with its flat base plate. Set it root side down in damp soil and it will usually produce roots and green shoots. A gardening blog that tried this reports that water rots the cut base before roots form, so it plants in soil, and it estimates that about half of its attempts form bulbs. That is one blog’s figure and I could not find a university source backing new bulbs from a cut base, so plan on greens.
Greens are still useful. You can snip them like scallions, and that account recommends cutting the base about a half inch to an inch thick. Do not confuse this with regrowing store-bought green onions, which is a different plant: bunching onions that regrow leaves from their roots.
How many onions per square foot: spacing sets the size
This is where “how many” really gets decided. UMD Extension says onions should be thinned to 2 inches apart if you want green onions, 4 inches for moderate bulbs and 8 inches for large bulbs, with block planting at 4 × 4 inches. The table converts those spacings into plants per square foot (12 ÷ spacing, squared).
| Spacing (each way) | Goal (UMD Extension) | Plants per sq ft | In a 4 × 8 ft bed |
|---|---|---|---|
| 2 inches | Green onions | 36 | 1,152 |
| 4 inches | Moderate-size bulbs | 9 | 288 |
| 8 inches | Large bulbs | about 2 | 72 |
Two things matter here. First, closer is not “more onions” in any useful sense: you trade bulb size for count. Second, a bed that is crowded invites disease. BBC Gardeners’ World says not to overcrowd because onion-neck rot can be a problem in wet summers. For beds, see the best vegetables to grow in a raised garden bed and raised garden bed benefits. If you are buying for a specific spacing, onion sets are cheap to over-buy, so work out your plant count before you order.
Mistakes that cost you the onions
- Wrong day-length type. Long-day varieties set bulbs at about 15–16 hours of daylight and short-day ones at about 12 (UMD Extension). UF/IFAS tells Florida gardeners to choose short-day varieties for fall planting. For warm southern regions, including southern California, where my garden is, check the day-length label on the variety before you buy. A mismatch tends to give you tops and little bulb.
- Hilling soil up around the bulbs. UMD Extension says not to, because it encourages stem rot, and that bulbs emerge above the soil as they enlarge. A bulb sitting on top of the soil is normal.
- Skipping early feeding. Onions have a high nutrient requirement. UMD Extension says to fertilize before planting and side-dress one to two weeks after bulb enlargement begins. See plant food vs fertilizer for how to read a label.
- Letting weeds in. Onions compete poorly with weeds because their roots are shallow (UMD Extension), so hand-pull instead of hoeing deep.
- Using one harvest trigger everywhere. Sources disagree: UF/IFAS says harvest when about a quarter of the tops have fallen over, UMD says about two-thirds. Check your own county Extension office for the trigger in your climate.
FAQ
How many sets do I need for a 10-foot row? A 10-foot row is 120 inches, so about 30 sets at 4-inch spacing or 15 at 8 inches (calculated). UMD Extension gives an approximate yield of 20 to 25 pounds per 10-foot row, with 12 to 24 inches between rows.
Can I split a shallot or multiplier cluster and replant it? Yes, that is how they are propagated. Kentucky Living’s gardener advice says you can separate the bulbs from the clusters at storage time after curing, and UMD Extension says to divide and store some to replant in fall.
Is a multiplier onion the same as a potato onion? UMD Extension uses the two names interchangeably (“multiplier onions, aka potato onions”). Sellers use the labels loosely, so check the variety description.
Why did my onion make a flower stalk and a small bulb? That is bolting. UMD Extension links it to planting too early and exposing sets to cold; BBC Gardeners’ World says to remove flower heads because they divert energy from the bulb. Plant later, and if you can find sets labelled heat-treated (a UK source mentions this), they are less likely to bolt.
Related reading: best vegetables for a raised bed · plant food vs fertilizer.
