True or False: Onions are biennials.

[DWSmith]

I've read that one should only plant onions as seeds and not use onion sets unless you are growing them for the greens. The reason give is that onions are biennials and onion sets have completed their first year so the size of the bulb won't grow as large. When planted from seeds the onion puts its first year of growth into the bulb.

True of false?

[Kirk]

They are biennials, the problem with a set is "Has the onion registered the growth to set size as the first year or not?"

If it has not then the first year you/we put them in the ground they consider as their first year and so build up bulk = form an onion.

If it has then the first year you/we grow them the onion considers it as their second year so form a seed head = bolts.

One aspect is to plant the smaller onion sets. as they are most likely not to have registered the seed to set period as the first of the 2 year cycle.

[Thelma Sanders]

Onions are indeed biennial. They get leaves and start to grow bulbs in the first year, then grow fatter bulbs til they flower in the 2nd year (if left in the ground for long enough.)

If the plants are dried out at the end of the first year, they are used as seed sets for planting - their 'second year' they grow some more and the bulbs get fat. If left to grow long enough they will also flower.

Some varieties do get big enough in just one year of growing that they are dried and eaten at the end of the first year. If you didn't dig them up they would still flower in the second year.

I've certainly grown some very big Ailsa Craig from seed in one year, but I've also had big ones grown from sets.
Onions, from sets or seeds, will continue to grow their bulbs until conditions are right for them to flower, which will be at some time in their 2nd year So they are biennials.
I've noticed you are in the USA so Day Length onion rules apply

If you plant a long-day variety in a southern region your crop will always fail to set bulbs; your crop can be used for scallions (green onions).

Bulb Onion Growing: Day Length and Temperature - Harvest to Table
We only grow one kind of onion so I can only say what happens here oops!

[DWSmith]

Many thanks to both of you!

[rary]

When onions from sets start to bolt, can you store the onion if you remove the flower stem early enough?

[veggiechicken]

Bolting onions should be used first - they don't store.