Secret #2: Stop watering your onion crop before harvest. When you notice the leaves on the first few plants start to fall over, stop watering and leave the onions in the ground for 7 to 14 days (depending on how dry or humid your climate is) to allow them to finish maturing.
Water thoroughly after planting, and regularly thereafter, about once a week if there is no rain. Onions have shallow roots, so don't let the soil at the base of the plants dry out and crack. Overwatering is equally problematic. If leaves develop a yellow tinge, cut back on watering.
When the ground starts to crack as the onions push the soil away, the bulbing process has begun. Stop fertilizing at this point.
Onions are finished curing when their outer skins turn papery, the foliage at the neck constructs, and the foliage shrivels and turns brown. Test one by cutting the stem about an inch from the bulb. The center of the cut area should not show any green. If it does, then the onions have not cured completely.
After harvesting, dry or cure the onions in a warm, dry, well-ventilated location, such as a shed or garage. Spread out the onions in a single layer on a clean, dry surface. Cure the onions for two to three weeks until the onion tops and necks are thoroughly dry and the outer bulb scales begin to rustle.
Selecting the right spot to cure your onions is really important in your onion drying process. You don't want it to be in direct sunlight, even though we want them to dry. If they're in direct sunlight, then you run the risk of actually sunburning your onion.
As a general rule of thumb, long-day varieties store longer than short-day varieties (the “long” in long day refers to the onion needing 14 to 16 hours of sun to develop properly), and pungent onions store longer than mild onions.
Onions should be stored in a cool, dry, well-ventilated space out of direct sunlight. A garage or unheated room in the house is ideal. Don't store any thick-necked or soft bulbs; use these up as soon as possible instead. Individual onions can be placed – carefully – into net bags.
Onions are most often grown from bulbs, or “sets”, in the spring but can usually make a successful crop when fall planted. One bulb will produce one onion; sets may be planted 2” apart if harvested as scallions or 'green onions', or 4”-6” apart if allowed to mature into a full-sized cooking onion.
In late winter, give autumn-planted onions a nitrogen-rich fertiliser, such as sulphate of ammonia, at a rate of 35g (1oz) per square metre/yard. This not only enhances growth but can also suppress premature flowering. Alternatively, use dry poultry manure.
Onions require a high source of nitrogen. A nitrogen-based fertilizer (ammonium sulfate or ammonium nitrate) should be applied at the rate of one cup per twenty feet of row. The first application should be about three weeks after planting and then continue with applications every 2 to 3 weeks.
Ensure immature bulbs stay covered with light mulch to protect them, retain moisture, suppress weeds, and allow air circulation. Do not cover emerging onions. Fertilize every few weeks with nitrogen to get big bulbs. Stop fertilizing when the onions push the soil away, and the bulbing process has started.
Steam from a kettle, hot running water or pan of water will draw away the sulfoxide (gas) from the onion keeping your eyes safe and tear-free.
As Kate Winslow and Guy Ambrosino explain in Onions Etcetera, both cold temperatures and water make syn-propanethial-S-oxide less volatile. For the same reason, some people suggest keeping a damp towel or bowl of water near your cutting board as you work.
Long-day and short-day onions
Onions (Allium cepa) have relatives that include garlic, chives, leeks and shallots. Storage onions grown in Minnesota generally are “long-day” types that require 14 or more hours of daylight to form bulbs. All onions require full sun for best growth.
Keep storage onions in a cool, dark, and dry place. Moisture and light lead to mold (ew) and sprouting (annoying, though not a deal-breaker), so stow your storage onions (red, yellow, and white as well as shallots and the diminutive pearl and cippoline) in a dry, well-ventilated basket, bin, or large bowl.
To dry onions in the oven, spread them in a thin layer on a baking pan covered with parchment paper. Bake at 140°F until they are completely crunchy dry. It is suggested to stir and re-spread the onion bits every 30 minutes as you go.
Drying and Storing Onion Bulbs
Once harvested, storing onion bulbs becomes necessary. Onions must first be dried before they can be stored. To dry onions, spread them out on a clean and dry surface in a well-ventilated location, such as a garage or a shed.
If you plan to store the pungent bulbs for winter use, curing onions is a necessary step in the process. Without proper curing, onion bulbs turn soft and mushy. Their dry, fully cured skins protect them from desiccation and keep pathogens from entering the bulb, causing rot, and developing a foul smell.
Cover with a tight-fitting lid, place over a very gentle heat and allow the onions to 'sweat'. 2 Check the onions occasionally, especially if a lot of steam is escaping. If the cartouche is dry, re-dampen it and return it to cover the onions. If any onions have browned on the bottom of the pan, don't stir them in.