Grow Onions From an Onion: What Really Happens
Plant a sprouted kitchen onion and something will definitely grow — just not what most people picture. Within 1 to 2 weeks you’ll have fresh green tops to snip for the kitchen. What you almost certainly won’t get is a new full-size storage bulb to replace the one you planted. An onion is a biennial: a bulb that has already sprouted is in its second year of life, and its biological job now is to flower and make seed, not to build another bulb. This page covers what actually happens when you plant one anyway — week by week — and what to plant instead if real bulbs are the goal. (If you just want the planting method itself, our guide to growing onions from an onion you already have covers it step by step.)
Growing From an Onion at a Glance
| What you plant | What you actually get | How long it takes |
|---|---|---|
| A sprouted whole onion | Green tops to snip; often a flower stalk later | Greens in 1 to 2 weeks |
| An onion bottom (root end) | Greens; sometimes a cluster of small bulbs if shoots are separated | Greens in 1 to 2 weeks; small bulbs take months |
| An unsprouted grocery onion | It may sprout and give greens; same limits as above | Greens in 1 to 2 weeks once it sprouts |
| Onion sets (for comparison) | Real, full-size storage bulbs | About 2 months |
| Onion seed (for comparison) | Real, full-size storage bulbs, widest variety choice | Up to 5 months |
Why a Sprouted Onion Won’t Rebuild a Storage Bulb
This is the most misunderstood part of the whole topic, so it’s worth being plain. In its first year, an onion plant stores energy in a bulb. In its second year, that same bulb spends the stored energy on flowering and setting seed. When you plant a sprouted grocery-store onion, you are planting a second-year plant. Its growth is real — roots, shoots, sometimes several shoots from one base — but the destination is a flower stalk, not a bigger bulb. Grow Organic’s growing guide makes the same point: these plants can grow, but they don’t reliably reach full-size bulbs, and starting from seed or sets gives far more dependable results for a storage crop.
That doesn’t make planting one pointless — just a different project: free greens, a possible seed harvest, and an interesting experiment, as long as you expect those rather than a pantry full of onions.
What Actually Happens, Week by Week
Weeks 1–2: greens. The sprout already on the bulb keeps growing, and fresh leaves push up within 1 to 2 weeks of planting. This is the reliable part of the experiment — snip the tops for cooking and the plant keeps replacing them for a while.
Weeks 3–6: the base divides. On a planted onion bottom especially, several shoots often emerge from the one base. Left together, they compete and stay small — this is the stage where the outcome is decided, so separate the shoots once each has its own roots.
Months 2–4: small bulbs or a flower stalk. Separated shoots, given a full season, can form a cluster of small bulbs — small being the honest word. Unseparated or whole planted onions are just as likely to send up a thick central flower stalk instead. Once it appears, the plant’s energy goes into seed and bulb growth is effectively over.
The One Documented Example — and Why It Isn’t the Baseline
One home grower documented the best case on video: a single onion bottom planted out, the shoots separated at about day 40 and replanted, with harvest beginning around day 120 and finishing with 21 small bulbs from that one base. The same grower found an unseparated cluster produced much smaller onions, while a whole bulb planted alongside grew faster and bigger than either.
That’s a strong result from careful separation and a long season — one grower’s documented outcome, not a controlled trial or a baseline to plan around. Most people get greens, a handful of small bulbs at best, and a flower stalk if the season runs long. Plan for that, and treat anything more as a bonus.
What You Do Get
- Kitchen greens for weeks. The dependable return. Snip the tops as you need them, the way you’d use green onions.
- Seed, if it flowers. A bolted second-year onion that goes to flower can give you seed for next season — the plant completing the cycle it was trying to finish.
- A cheap experiment. It costs nothing but a pot and a sprouted onion, and it teaches you more about how onions grow than any diagram.
The Method in Brief
Use a container with at least 6 in (15 cm) of soil. Set the onion or base in root-side down, cover the roots and base with soil, and keep the shoot above the surface. Water evenly, give it full sun, and separate divided shoots once each has its own roots. The full walkthrough, including rooting the base in water first, is in our step-by-step method for growing onions from an onion. For greens alone, you can also skip soil entirely and regrow green onions in water on a windowsill.
If You Want Real Bulbs, Plant This Instead
If the goal is a harvest of storage onions, a sprouted kitchen onion is the wrong starting material, and the three standard methods all beat it:
- Sets are the fastest and easiest route — small dormant bulbs that plant and go, ready in about 2 months.
- Transplants give reliable, large bulbs without starting seed yourself, in about 2 to 4 months.
- Seed is the slowest at up to 5 months, but the cheapest per plant and by far the widest choice of varieties — see our guide to growing onions from seed.
Our homepage’s complete onion growing guide compares all three side by side.
Frequently Asked Questions
Can you grow a new onion from a sprouted onion?
You can grow the plant, but rarely a new full-size bulb. Expect green tops within 1 to 2 weeks, sometimes a cluster of small bulbs from separated shoots, and often a flower stalk — it is a second-year plant heading toward seed, not bulking up.
How long does it take to see growth from a planted onion?
Fast — leaves appear within 1 to 2 weeks, and you can snip greens as soon as they’re long enough to use. Anything resembling a bulb takes months, not weeks.
Why won’t my planted onion make a big bulb?
Because onions are biennials. A bulb forms in year one as an energy store; a sprouted bulb is in year two and spends that store on flowering and seed. No feeding or watering routine changes which year the plant thinks it’s in.
How many onions can grow from planting one onion?
No predictable number. A base commonly divides into a few shoots, occasionally a dozen or more. One documented grower got 21 small bulbs from a single base after separating shoots at about day 40 — a strong outcome, not a typical one.
Should I separate the shoots that come up from one onion?
Yes, if you want any bulb size at all. Once each shoot has its own roots, gently pull the cluster apart and replant the shoots a few inches apart. Left together, they compete and every bulb stays small.
Can I save seed from a planted grocery-store onion?
If it bolts and flowers, yes — let the flower head dry and collect the seed. Bear in mind that seed from a store onion may not grow back identical to the parent, so treat it as an experiment, not a seed supply to depend on.
Is it better to start the onion base in water or soil?
Water is convenient for initial rooting and for short-term greens. Soil gives the plant nutrients and support, so anything you want to keep growing beyond a few weeks does better moved into soil.
Can you plant an onion that hasn’t sprouted yet?
You can — it will usually sprout once planted and give you greens. But the same biennial limits apply: a grocery onion is a mature bulb, not a young plant, so greens and possibly seed are the realistic outcome.
What should I plant instead if I want a real onion harvest?
Sets for speed and simplicity (about 2 months), transplants for reliable large bulbs (2 to 4 months), or seed for the widest variety choice at the lowest cost (up to 5 months). All three are first-year starting points — which is exactly why they make bulbs.
Final Thoughts
Growing an onion from an onion is worth doing — for the greens, for the possible seed, and for the lesson in how onions actually live. It just isn’t a way to restock your storage crop, because a sprouted bulb is finishing its life cycle rather than starting one. When you want a real harvest, start from sets, transplants, or seed, and let the kitchen-scrap experiment run alongside for fun.
How This Guide Was Checked
Written by Ada Jones. The biennial growth pattern and the realistic outcomes described here are checked against Grow Organic’s growing guide; the 21-bulb result is drawn from one grower’s own documented video and is presented as a single example, not a verified average. Spot a mistake? Contact us and we’ll correct it.
