Grow file · Yellow Sweet Spanish (long-day)
Spanish Onion
Allium cepa
A long-day storage onion sweet enough to sweat down without scorching. The aromatic base under every tomato sauce on this site — and the earliest sowing on the calendar.
| Days to maturity | 110 days from transplant |
|---|---|
| Days to germinate | 7–14 days at 70 °F |
| Germination soil temp | 65–75 °F |
| Start indoors | 12 weeks before last frost |
| Transplant | 4 weeks before last frost |
| Spacing | 5″ between plants |
| Sun | 6+ hours direct |
| Container size | Ground or raised bed only |
| Realistic yield | 0.75 lb per healthy plant, per season |
Your dates, by zone
Built from each zone's average last frost and this cultivar's 110-day maturity. Zone averages are not your garden — enter your own frost date in the builder to shift every row.
| Zone | Sow indoors | Transplant | First pick |
|---|---|---|---|
| Zone 3 | Feb 20 | Apr 17 | Aug 5 |
| Zone 4 | Feb 17 | Apr 14 | Aug 2 |
| Zone 5 | Feb 5 | Apr 2 | Jul 21 |
| Zone 6 | Jan 27 | Mar 24 | Jul 12 |
| Zone 7 | Jan 9 | Mar 6 | Jun 24 |
| Zone 8 | Jan 3 | Feb 28 | Jun 18 |
| Zone 9 | Dec 6 | Jan 31 | May 21 |
| Zone 10 | Nov 8 | Jan 3 | Apr 23 |
| Zone 11+ | Oct 23 | Dec 18 | Apr 7 |
Tight means first pick lands within two weeks of that zone's average first fall frost. It is doable, with zero slack in the indoor start.
The first thing you sow
Onions from seed need twelve weeks indoors and go in the ground weeks before your last frost. That makes them the earliest thing on the whole calendar — typically January in Zone 5 or 6. If you are building a sauce garden, onions are the one crop where “I’ll start next weekend” costs you the year.
Seed, not sets
Onion sets — the little dry bulbs sold in mesh bags — are convenient and they bolt. A set is a second-year onion that has already been cold-shocked once; a meaningful share of them respond by flowering rather than bulbing, and a bolted onion will not store.
Seed takes longer and gives you bigger bulbs, better storage life and a bolting rate near zero. For an ingredient you want to still have in February, it is not close.
Day length is a hard requirement
Onions bulb on photoperiod, not on age.
- Long-day (like this one): bulbs at 14–16 hours of daylight. Correct above roughly 36° N — most of the US.
- Short-day: bulbs at 10–12 hours. Correct in the deep South.
Plant a long-day onion in Florida and it will grow beautiful green tops and never make a bulb. This is not a soil or feeding problem and no amount of fertiliser fixes it.
Growing and curing
Transplant seedlings about four weeks before last frost, five inches apart, in ground or a bed — onions are shallow-rooted and want horizontal room rather than depth, so they are a poor container crop.
Harvest when the tops fall over on their own. Do not bend them over yourself; that is folklore and it shortens storage life. Cure in shade with airflow for two to three weeks until the neck is papery and tight, then store somewhere cool, dark and dry.
About three-quarters of a pound per plant. A 12-lb marinara batch wants roughly 1 lb of onion, so two plants covers it with margin.
How this crop actually fails
Named, specific, and in the order you're most likely to meet them.
Plants send up a flower stalk and the bulb never sizes
Cause Bolting, usually from cold-shocked transplants or from planting sets rather than seed.
Fix Grow from seed, not sets, and transplant when soil is genuinely workable rather than as early as possible. A bolted onion is still edible but will not store.
Tiny bulbs at the end of the season
Cause Wrong day-length type, or crowding. A long-day onion grown in the deep South never receives its bulbing trigger.
Fix Long-day above roughly 36° N; short-day below it. And thin to a real 5 inches — crowded onions stay small no matter how well fed.
Bulbs rot in storage within weeks
Cause Not cured, or necks were still thick when pulled.
Fix Pull when tops fall over naturally, cure in a dry shaded spot with airflow for two to three weeks until the necks are papery and tight, then store cool and dry.
What spanish onion needs
Yellow Sweet Spanish Onion Seed
A long-day storage onion with enough sugar to sweat down without scorching. The aromatic base under every tomato sauce on this site.
$4.94Buy →
Organic Seed Starter Potting MixMust have
Seed-starting mix is not potting soil with a nicer label — it is finer, lighter, and low in nutrients on purpose, because a seedling's first roots want air and moisture, not fertiliser.
$15.82Buy →
72-Cell Trays with 1020 Bottoms, 5-PackMust have
The cheap black nursery trays crack after one season and flex enough to break roots when you lift them. These do not. Over five seasons the price per use is lower than the throwaways.
$66.99Buy →
T8 Full-Spectrum Grow Lights, 1 ft 4-packMust have
A south window in March delivers a fraction of the light a pepper seedling needs, and the plant tells you by stretching. Two inches of LED above the canopy for 16 hours a day fixes it permanently.
$38.69Buy →
Rapitest Soil Test Kit
In-ground growers are guessing until they test. pH outside 6.0–6.8 locks nutrients out no matter how much fertiliser you add, and no amount of feeding fixes that.
$15.99Buy →
Pure Organic Worm Castings, 15 lb
Not a fertiliser so much as a soil-biology inoculant with a mild nutrient charge. A cup in each planting hole is a cheap, unglamorous improvement you can measure by August.
$26.75Buy →
Garden Harvest Basket with Tool Caddy
A harvest basket that doubles as a colander means the hose rinse happens in the garden, and the dirt stays outside. On a 20-pound tomato day that is a genuine time saving.
$40.39Buy →
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Sauces this plant is for
Last reviewed 2026-09-01. Numbers are cultivar-typical, not guarantees —tell us if yours ran different and we'll say so on the page.