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Usually when my store bought onions starts to sprout, I plant them in a plastic container filled with water and get some green onion.

This time I decided not to wait until onion starts to sprout. I chose quite a large onion bulb and put it into a plastic container filled with water.

I keep water level just below the bulb, so that roots are submerged, but the bulb itself is above the water.

After quite some time (about two or three weeks) it started producing roots. Lot's of them.

After another few weeks I've noticed that the onion bulb itself has elongated a bit towards water. Obviously there's some "growing" happening.

So, lot's of roots, change in size, but no sprouts.

Why?

Could it be something to do with the onion bulb being very large? Or is there not enough humidity and warmth to start sprouting? Or perhaps the season is wrong (planted around October, and it's January now)? Or something else?

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Onions are an important staple crop; a lot of research has gone into why they sprout in storage, since once sprouted they lose a lot of value in a marketing context. The more they can be engineered to prevent sprouting the better for growers, so genetics will play an important role. You may have a variety that just behaves that way as part of its natural life cycle.

On a more mundane level, onions can sometimes grow large when multiple smaller bulbs decide to share the same root base. Examine the root ring to see if there is evidence of multiple bulbs - it might just be one of two that has been activated. As growth progresses, since there is no green at this stage then all the substantive growth in the roots has to come from the closely packed "leaves" in the bulb. These are becoming deflated (less turgid) and this can leave space inside the bulb - if the new expanding leaves do not have enough energy to break open a tight cluster of dry scales then they can continue to expand but become doubled up inside the bulb as the deflating scales make room.

So growth might actually be happening, but the new leaves are just shy. Ahh.

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It may be that the onion you used is the normal, spring planting, crop later in summer type, so it's too early for it to produce green growth - in the northern hemisphere, light levels will be particularly low right now, especially indoors, and that onion knows its not time to put out anything green yet. You could take another onion that hasn't yet sprouted, plant it in potting soil so its covered in soil with the tip near the surface, stand it on a sunny windowsill and wait... that might work better at this time of year. Or not...

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