Not every onion grows the same. These are its main types (its cultivar groups), and the planner sizes, times and supports each one by its own numbers.
Onion — bulb, spacing in row 10 cm4 in, mature height 45 cm18 in, mature spread 15 cm6 in, days to maturity 90–120
Shallot — bulb, spacing in row 15 cm6 in, mature height 45 cm18 in, mature spread 15 cm6 in, days to maturity 90–120
Numbers we’re less sure of
Many of our estimates come from an extension page we’ve read that gets close without pinning the number down. Others are still our own best guess, and on some the sources disagree. Treat them all as a starting point.
Shallot: days to maturity — an estimate
Shallot: mature height — an estimate
Shallot: mature spread — an estimate
Soil pH (in water) — an estimate
Days to maturity — an estimate
Mature height — an estimate
Mature spread — an estimate
Monthly temperatures it grows in — an estimate
Rules that apply to onion
Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles. They are still not "low-pressure" and need their own rotation. A good hunch
Alliums and carrot family are unrelated to nightshades/cucurbits/brassicas and can break those families' rotation cycles. They are still not "low-pressure" and need their own rotation.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: They host none of the big-three families' soilborne pathogens, so following those crops with an allium or apiaceous crop breaks the host cycle. They are not inherently low-pressure, though. Allium white rot (Sclerotium cepivorum) is allium-specific, its sclerotia persist 15+ years, and it cannot be eradicated once established; apiaceous crops carry their own soilborne diseases.
Evidence: checked against 1 source in the research-based literature.
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.
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.
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. Promising
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.
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.
If you skip this, the planting fails. The planner warns you.
How it works: On a clear, still night leaves lose heat to the open sky, so a leaf can fall to freezing and ice can form in it while the thermometer, a couple of meters up, still reads a few degrees above.
Evidence: checked against 7 sources in the research-based literature.
Frost occurs when air is warmer than 32°F but plant tissues drop to 32°F or below because plants radiate (lose) heat into the atmosphere, especially during cool, clear nights.
Radiation frosts occur on calm clear nights that lack cloud cover to hold in heat.
Remedy
The evening before, cover the crops at risk with a sheet or frost cloth, never plastic, reaching the ground and loose at the bottom so the soil's warmth can rise under it, and take it off in the morning. A cover helps least in a long, hard freeze.
If you skip this, it still grows, just less well. The planner suggests it.
How it works: Soil warmed by the sun by day is the heat source at night; a covering low to the ground traps that heat around the plants on calm, clear nights, keeping them a few degrees above the open air through the light frosts at each end of the season.
Evidence: checked against 6 sources in the research-based literature.
The ground and frame of the cold frame are heated by the sun during the day and radiate that heat during the night, keeping the inside warmer. They are used to extend the growing season by two to four weeks in both the fall and the spring, allowing you to start plants outdoors earlier and continue growing them later in the season.
In trials in Fort Collins, Colorado, a plastic cover on a frame typically provides 3°F to over 6°F of frost protection. It works well for cool season crops that are somewhat tolerant of frosty nights and adds two to six weeks or more on both ends of the growing season.
Soil warmed by the sun in the daytime, is the source of heat for frost protection at night. Moist, smooth soil absorbs more heat. To trap heat from the soil around young vegetables at night, place a covering that is low to the ground and spreading. To recharge the heat source for the next night, any covering must allow sunlight to shine through to the soil or must be removed in the daytime.
Low = Plant 1–2 weeks before suggested planting date; Medium = 2–4 weeks before suggested planting date; High = 4–8 weeks before average planting date
hort.extension.wisc.edu
Floating Row CoverUniversity of Wisconsin-Madison Extension, Wisconsin Horticulture, Susan Mahr
Depending on the weight of the covering you choose, you can gain between 2 and 8 degrees of frost protection or warm the local environment sufficiently to harvest certain crops a week or two early.
Clear plastic is useless in providing frost protection because the radiant energy released from the soil on those frosty nights passes right through the covering and never warms the air around the plants.
Remedy
Sow or set out cool-season crops under the cover that many weeks ahead of the open-ground date, and keep autumn crops covered past the usual last call. Vent on sunny days, keep plastic off the leaves, and close the cover before sunset.
If you skip this, you lose real yield. The planner suggests it.
How it works: A crop grows and makes its harvest within a band of warmth: in months colder than its band it grows too slowly to finish, and in months hotter than it a cool-season crop bolts and turns bitter while a warm-season crop drops its flowers.
Evidence: checked against 9 sources in the research-based literature.
Hold the Tomatoes!UC Master Gardeners of Monterey, San Benito and Santa Cruz Counties, Jazmin Lopez
Average monthly temperatures for cool-season crops during their growing period: 60-65F. This includes, carrots, parsnips, spinach, lettuce, broccoli, and garlic to name a few.
Group D: These vegetables thrive in areas where the mean monthly temperatures range between 15 and 18°C. They are intolerant to temperatures between 21 and 24°C, and tolerate weak frosts. This group includes spinach, lettuce, broccoli, beetroot, Brussels sprout, cabbage, radish, rhubarb and watercress.
In the Top End climate lettuce plantings are restricted to the cooler months from April till July. Hot weather makes the plants to bolt, as they do not form a head but flower instead.
If you skip this, you lose real yield. The planner warns you.
How it works: A hardy or half-hardy crop's tissue survives the ice of a light freeze down to about 28 °F (-2 °C), so its season ends at the first night that cold, which comes after the first 32 °F frost, and later still where winters are mild.
Evidence: not yet confirmed. It is cited to 8 sources, which a person has yet to read.
Tender crops will be damaged or destroyed by a light frost, semi-hardy crops can tolerate temperatures in the upper 20s°F, and hardy crops will survive temperatures in the mid to low 20s°F.
This temperature threshold kills some types of commercial crops and residential plants, while temperatures below 28°F for an extended period of time can kill most types of commercial crops and residential plants.
If you skip this, you lose real yield. The planner suggests it.
How it works: A crop grows at the pace its days to maturity assume only in months at least as warm as the low end of its band, and in cooler months it grows too slowly to finish, so a frost-free season of cool months can end before the crop is ready.
Evidence: not yet confirmed. It is cited to 8 sources, which a person has yet to read.
Hold the Tomatoes!UC Master Gardeners of Monterey, San Benito and Santa Cruz Counties, Jazmin Lopez
Average monthly temperatures for warm season crops during their growing period: 65-80F. This includes melons, corn, eggplant, peppers, squash, and tomatoes.
Eggplant needs warm conditions, and will not thrive during a cool season. There are varieties available that thrive in Minnesota’s short summer season.
Most seed packets and variety descriptions in catalogs estimate the days from planting to maturity. Add about 10 days to this estimate because of the cooler, shorter days in the fall.
Plus one working note on how the sources were read.
Remedy
Grow it under cover, in a greenhouse, tunnel or cloche; start it early indoors and choose its quickest variety; or grow a crop whose months these are.
A garden planner built on graded evidence. Every claim states its evidence grade and links its sources on the page. Cite it with the grade, and add the date you read it.
