How to Grow Onions from Seed: Sowing Date and Seedling Size

Fix the sowing date to within a few days

The window for sowing onions is surprisingly narrow compared with other autumn-sown vegetables. A week either side changes the result. Sow too early and the seedlings get too big, so they bolt in spring. Sow too late and the roots never establish before winter and the plants do not survive it.

Work backwards. Aim to sow 55 to 60 days before you intend to plant out, and plant out about three weeks before your first frost, so the roots have time to move. Find your average first frost date, then set the planting date, then the sowing date.

Even within one district, a few hundred feet of elevation moves the date by several days. When onion plants appear at your local nursery is a good reading of the right window where you live.

Choose the variety before you sow

Early varieties want a slightly later sowing and late maincrop varieties a slightly earlier one, so read the date on the packet variety by variety. Sow an early and a late on the same day and one of them will be wrong.

Variety choice also depends on your latitude, because onions bulb in response to day length. A long-day variety grown far south never bulbs properly, and a short-day variety grown far north bulbs before it has made enough leaf. Buy varieties described as suitable for where you garden.

If you want onions that keep, choose mid or late maincrop. The very earliest varieties are satisfying because you eat them in spring, but they barely store at all and start sprouting about a month after lifting.

Do not use last year's leftover seed. Onion seed is short lived and germination drops off noticeably after a year.

The seed bed and how to sow

Put the seed bed somewhere sunny and free draining, away from the plot where the crop will grow. A compacted strip along a path gives you thin plants because the roots cannot get down.

1. Level the bed Make a seed bed two to three feet wide, break the soil down fine and level it. Hollows collect water, the seedlings there stretch, and you end up with uneven plants.
2. Sow thinly in drills Draw shallow drills four inches (10 cm) apart and sow about one seed to every half inch (1 cm). Sowing thickly is the single biggest cause of drawn seedlings, and thinning does not undo it.
3. Cover with a quarter inch Just cover the seed and firm it with the back of a board so it is in contact with damp soil. Cover it deeply and germination is uneven, and uneven seedlings get discarded at planting.
4. Do not let it dry before germination Cover with row cover or a light mulch and water in the morning until the seedlings are through, which takes seven to ten days.

What to watch while the seedlings grow

Once they are up, take the cover off and give them full light. Over the eight weeks or so of raising, the thing to watch is not the height but the thickness of the white base. Too much water and feed gives you tall plants that fall over, and those disappear over winter.

1. Uncover them early Once eight out of ten are through, take the cover off during the day. Left covered they are short of light, and one week is enough to ruin them.
2. Thin to half an inch At two true leaves, pull only where the leaves touch, to leave about half an inch (1 cm) between plants. Thinning hard all at once leaves the rest liable to flop.
3. Feed very lightly A pinch of fertilizer between the rows, or a liquid feed diluted below the label strength. Overfeeding gives you a thick base, which is exactly what you do not want.
4. Keep prolonged rain off In a wet spell, put a cover or a low tunnel over the seed bed. A waterlogged seed bed is where downy mildew starts, and the roots stay shallow.

Downy mildew that starts in the seed bed travels to your plot on the plants. Pull anything with vague yellow patches on the leaves and get rid of it.

What a good plant looks like, and why fat is dangerous

A plant ready to go out is eight to ten inches (20 to 25 cm) tall with a base about a quarter of an inch (5 to 7 mm) thick, roughly a pencil lead. It should feel firm between your fingers and carry three or four leaves.

A base thicker than about a third of an inch (8 mm) means the plant will be too large going into winter, and it will form a flower bud and bolt in spring. A bolted onion runs a hard stem up the middle, the bulb never fills, and it will not store. Thickness is not something to be proud of, it is a warning.

Thin plants under an eighth of an inch (3 mm) are the opposite problem: frost heave lifts them and they die. In a year when that is all you have, plant them closer together and accept that you will harvest whatever survives.

Do not mix thick and thin plants in the same row. Growth becomes uneven and every later decision about feeding and harvest becomes a compromise. Sort by thickness and plant them separately.

If you miss the window

Once you know you have missed it, the options are few, and they have one thing in common: do not try to force the plants back to the ideal. Accept the condition you have and pick the one move that still works.

1. You sowed late Do not delay planting to let the seedlings fatten. Plant them small, within the window. Whether the roots get down while there is still warmth in the soil is what decides survival.
2. You sowed early Stop feeding and cut back the watering to slow them down. Plant at the late end of the window and put the thick plants in a separate block.
3. You are short of plants Bought plants will make up the numbers. What is on sale locally is matched to the local window, so check the type and buy to fill the gap.
4. The ground is not clear Plant into whatever is free as it becomes free. Waiting until the whole bed is available means the last row misses the window badly.