Onions of the right day-length type for where you live (the planner tells you which)
Garlic cloves from a garlic grower or seed company, ready to plant in autumn
Weed-free straw to mulch garlic over winter and keep weeds out of onions
A warm, airy, shaded spot to cure the harvest before it’s stored
Onions and garlic are close relatives, but they keep very different calendars. Onions go in early in spring, and choosing the right type for your day length makes or breaks them. Garlic goes in in autumn, sits through the winter, and is ready by early summer.
What family are onions and garlic in?
The onion is Allium cepa, and garlic is Allium sativum. Both are alliums, a group of related plants in the amaryllis family (Amaryllidaceae). The rest of the group is familiar:
Shallots are the same species as the onion, Allium cepa, grown for a cluster of smaller bulbs.
Leeks are Allium ampeloprasum. So is elephant garlic, which despite its name is a kind of leek.
Bunching onions and scallions are Allium fistulosum, grown for their green stalks.
Chives are Allium schoenoprasum, a perennial that comes back every year.
Being one group matters most for rotation, because the alliums share their soil diseases (more on that at the end).
Which onions should I grow where I live?
An onion starts forming its bulb when the days reach a certain length, and that length depends on the type. Short-day onions start bulbing at about 10 to 12 hours of daylight, intermediate-day onions at about 12 to 14, and long-day onions at about 14 to 16. That makes your latitude the key question:
If you skip this, the planting fails. The planner won’t allow it.
How it works: Bulbing fires when daylength crosses a cultivar's critical threshold (short ~10-12h, intermediate ~12-14h, long ~14-16h). A short-day cultivar grown too far from the equator crosses its low threshold early and bulbs prematurely on small plants, so bulbs stay small. A long-day cultivar grown too close to it never reaches its high threshold and fails to bulb. Changeover is around the 35th-38th parallel, north or south.
Evidence: checked against 2 sources in the research-based literature.
… onions will not form bulbs until daylength reaches the cultivar's critical threshold. Short-day onions bulb at approximately 10-12 hours of daylength ... Intermediate-day onions bulb at approximately 12-14 hours ... Long-day onions bulb at 14-16 hours of daylength.
Long day varieties are for northern growing areas (above the 35th latitude); short-day varieties are for southern. If short-day varieties are grown in our zone, they won't do well. They will not form bulbs of any decent size. And since the long day onions need more daylight, they will not do well
Remedy
Choose long-day onions far from the equator and short-day near it; the planner picks the group from your plot's latitude.
When you tell the planner where you garden, it says which onion country you’re in: long-day, short-day, or the changeover band between them, where intermediate-day onions are the safe choice. The frost dates, zones and day length guide explains why this comes down to latitude. Your hardiness zone doesn’t decide it.
Two things to watch when buying. Sweet, mild onions are usually short-day types, so far from the equator they tend to stay small. Onion sets (the little bulbs sold by the bag) can be short-day too: in Minnesota, most of the sets on sale are short-day types, even though long-day onions suit the climate there. Check the type on the label.
When and how do I plant onions?
Onions are hardy, so they go in on the planner’s early window, weeks before the last frost:
Cold-hardy annuals can be sown or transplanted before the last spring frost (hardy crops about three weeks before, half-hardy about two weeks before). Tender crops wait for the frost itself.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Cold hardiness. Hardy crops tolerate hard frost and half-hardy crops tolerate light frost, so they can occupy the ground weeks before frost-tender crops are allowed in. The offset is measured from the last spring frost.
Evidence: checked against 2 sources in the research-based literature.
Seed - sow as soon as the soil can be worked, 6–12 mm¼–½ inch deep in a band, then thin the seedlings to 8–10 cm3–4 inches apart.
Transplants - bought, or raised indoors from seed 10 to 12 weeks ahead. They take light frost. Set them 5 cm2 inches deep and 8–10 cm3–4 inches apart.
Sets - plant as soon as the soil can be worked.
Where winters are mild, onions can also be planted in autumn or late winter and grow on through the cool months. Don’t plant too early in autumn, though: a plant that gets big before winter can bolt (send up a flower stalk), and a bolted onion makes a small bulb with a thick neck that won’t cure well.
Onions have shallow roots, so they need steady moisture. If a week goes by without about 2.5 cman inch of rain, soak the bed thoroughly; sandy soil needs it more often. Stop watering once the bulbs are full size and the tops start to fall. Weed with a shallow hoe, because a nicked bulb is more likely to rot, or mulch with weed-free straw. For feeding, go by a soil test. Too much nitrogen gives soft bulbs with thick necks that don’t store well.
When do I plant garlic?
Garlic works the other way round from onions. You plant it in autumn so the winter cold can do its job:
Garlic is planted in autumn, not spring. The cloves go in the ground several weeks before it freezes, root, and overwinter. The weeks of winter cold below about 40 °F vernalize the clove, and that cold is what triggers it to form a bulb the following spring as the days lengthen. Planted in spring, garlic misses the cold and bulbs poorly.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: A garlic clove will not form a bulb until it has been vernalized, held for roughly six to eight weeks below about 40 °F. The bulb itself does not develop during that cold; it is only induced by it, and forms in spring as days lengthen and temperatures rise. Fall planting delivers the cold naturally over winter with the clove already rooted, so growth resumes early and a full bulb sizes up by early summer. It is the same biennial vernalization that prematurely bolts a spring-sown cool-season crop, but with the sign reversed. There the accumulated cold buttons the plant into an early flower and ruins it; here, in a crop sown in autumn to be harvested as a bulb, that same cold is the intended trigger.
Evidence: checked against 3 sources in the research-based literature.
Plant cloves ... in the fall (October to early November)
content.ces.ncsu.edu
Vegetable GardeningNC State Extension, Extension Gardener Handbook, chapter 16
Garlic | B=180-210 | 4-6 in
Remedy
Plant cloves in the fall, roughly late September through November in most regions, several weeks before the ground freezes so they root before winter. Set them pointed end up, about 1 to 3 inches deep and 4 to 6 inches apart, using the largest cloves for the largest bulbs. Once the ground has chilled, mulch with a few inches of weed-free straw to blunt winter heaving and injury. Match the type to the winter. Hard-necked garlics prefer cold-winter regions, and soft-necked are better adapted to mild-winter areas.
In a cold-winter place like Minnesota, that means planting about a week or two after the first killing frost. The cloves put down roots before the ground freezes, and the shoots usually don’t come up until spring. Split the bulb into cloves a day or two before planting. Set each clove pointed end up, with its base 5–8 cm2–3 inches below the surface and 10–15 cm4–6 inches apart, then mulch with 8–10 cm3–4 inches of straw or leaves.
Plant cloves from a garlic grower or seed company. Supermarket garlic is mostly a softneck type grown in mild climates, and it often does poorly in cold-winter gardens like Minnesota’s. Plant only firm, healthy cloves, because many garlic diseases come in on the cloves themselves. After your first crop, you can save your own: keep the biggest cloves to plant the next autumn.
What is the difference between hardneck and softneck garlic?
Garlic comes in two main types, and the planner knows both:
Hardneck garlic (Rocambole, Purple Stripe, Porcelain) grows a flower stalk called a scape. Snap the scapes off just after they start to curl, and eat them. Hardnecks are the type for cold-winter regions.
Softneck garlic (Artichoke, Silverskin) doesn’t make a scape, usually has more cloves, and is easy to braid. It’s better suited to mild winters.
When are onions and garlic ready, and how do I cure them?
Onions are ready when about half the tops have fallen over and dried. Loosen them with a garden fork and lift them.
Garlic is usually ready between late June and late July, depending on the type and your climate. Start when the lower leaves have turned brown but about half of the upper leaves are still green, or pull one bulb and cut it in half: if the cloves fill their skins, the crop is ready. Dig too early and the bulbs are small; too late and the cloves start to split apart. Stop watering garlic about two weeks before you harvest it.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: At harvest the bulb's neck and outer scales are still moist living tissue and the neck is an open channel into the bulb, so curing in warm moving air dries the neck shut and the outer scales to a papery protective skin, removing the wet tissue that storage rots colonise and sealing the neck they enter by; an uncured bulb goes into storage with a moist open neck and soft scales, and decays.
Evidence: checked against 2 sources in the research-based literature.
Allow bulbs to dry until the neck is dry and the outer skin is papery, approximately two to three weeks
Remedy
After lifting, spread the bulbs in a single layer or hang them in bundles in a warm, shaded, well-ventilated place out of direct sun and rain, and leave them until the neck is dry and tight and the outer skin is papery. That takes roughly two to four weeks for onions (a warm 75-90 °F speeds it) and two to three weeks for garlic. Only then trim, and keep the cured bulbs in mesh bags or shallow boxes in a cool, dry, well-ventilated spot. Handle gently throughout, as bruising also seeds storage rot.
Once cured, onions can be braided or trimmed and stored somewhere cool and dry, but not freezing. Above 4°C40°F they start to sprout. For garlic, cut the stems about 1–2.5 cm½–1 inch above the bulb and trim the roots close. The harvest and keeping guide covers storage for the whole garden.
Where do onions and garlic go next year?
Onions, garlic, leeks, shallots and chives all share the same soil diseases, so rotate them as one group:
Do not plant alliums in ground that held alliums within 4 years, and know that the worst allium disease outlasts any rotation.
If you skip this, you lose real yield. The planner warns you.
How it works: Fusarium basal rot (Fusarium oxysporum f. sp. cepae) and pink root survive in soil as spores and on other plants' roots, and their inoculum builds where alliums repeat; a four-year break starves them. White rot (Sclerotium cepivorum) is allium-specific and its sclerotia stay viable 15 to 40 years, so rotation reduces but cannot clear it.
Evidence: checked against 4 sources in the research-based literature.
White rot survives in soil in the form of sclerotia, which are dense masses of fungal tissue. ... They may remain viable in soil for 15 years or more without allium hosts.
Remedy
Plant alliums in different ground, or wait until 4 years have passed since this ground last held them. If white rot has ever shown here, treat this ground as lost to alliums and grow them elsewhere.
The one disease rotation can’t fix is white rot. If it has ever turned up in a bed, grow your alliums somewhere else from then on. The crop rotation guide helps you fit a four-year break into the beds you have.
Further reading
Good places to read more about the technique on this page. The grade on each card comes from that card’s own sources.
