Traces of onion have been found in Bronze Age settlements dating as far back as 5000 BC (about the time the potato was being developed on the other side of the world; because what is a potato without an onion?). It is uncertain if these traces were of "cultivated" onion; true cultivated onions can be traced to about two thousand years later in ancient Egypt, leeks and garlic also had their beginnings at this time. Ancient Egyptians believed the onion's spherical shape and concentric rings represented eternal life. Traces of onion have been found in Egyptian burial chambers and it's believed that the workers who built the pyramids ate onions and radishes. In Greece, onion was believed to lighten the balance of the blood. in Roman culture, gladiators were rubbed with onion to firm up their muscles. In the Middle Ages they were so important they were used to pay rent, and given as gifts. Medicinally onions have been used to treat constipation, erectile dysfunction, infertility, headaches, coughs, snake bites, and hair loss. Columbus brought cultivated onions with him to the Americas, only to find out later that the natives were already using wild onion for everything from food, to dyes, to toys.
You should never give onion to pets or other animals as it may poison them.
Onions contain chemicals known to have anti-inflammatory, anti-cholesterol, anti-cancer, and anti-oxidant properties (in humans). Studies have shown that eating onions may lower the risk of head and neck related cancers. The more pungent an onion is, the greater its ability to inhibit cancer cell growth. Onions may also be beneficial in reducing the risk of osteoporosis in women.
Lacrimatory Factor, or LF, is the gas created when an onion is cut into or bitten that causes your eyes to tear up. This can be reduced by using a sharp knife to cut them, or by chilling or freezing them before use, or by cutting them under running water or in a bowl of water.
Onions are propagated by seed, or by bulb set. Onion sets are small bulbs of onion grown from seed the previous year, planted in the ground these will grow into full sized bulbs in a shorter time frame than seeds. Sets are reputed to grow bulbs with less durability than seeds though.
Onions that produce seeds are "day-length sensitive", this means they require a certain number of daylight hours before they will begin to bulb. Long-day onions require 15 or more hours of daylight, intermediate day onions require 12-13 hours, and short day onions require 9-10 hours. Often, short day onions are planted in the fall in mild climates and will bulb in early spring.
Green onion, or scallion, refers to either an immature onion that has not yet bulbed, or to the Welsh onion, an Allium species which does not bulb at all. You can differentiate between a Welsh onion and a common onion scallion by the cross section of the leaves; Welsh onions leaves are flattened on one side, while the common onion leaf is round. Tree onions, top onions, topsetting onions, walking onions, or Egyptian onions refer to the species Allium proliferum; this onion plant forms bulblets on top rather than a flower and seeds. These bulblets can then be planted to grow more onions, or eaten like a normal onion bulb.
Green onions and leeks should be refrigerated after harvest. Regular bulb onions can be stored at room temperature in a cool, dry, dark, ventilated area. At room temperature onions have a shelf life of 1-5 weeks depending on variety; in the fridge this can be extended up to 4 more weeks. In storage, onions can absorb the smells of apples and pears; they should be stored away from moister vegetables as they can draw that moisture from the air and will spoil sooner.
Onions generally come in 3 colors: red, white, and yellow. Yellow onions are full-flavored and are usable for just about any culinary purpose. Red onions are best chopped fresh, or as grilled or char-broiled onion. Red onions have a sharper flavor. White onions are traditionally used in Mexican food, they sautee to a golden color and develop a sweet flavor when cooked.
Small bulbed onions are used for pickling, or eaten as vegetables; these include pearl, boiler and pickler onions. Onion seeds can also be eaten as sprouts, similar to alfalfa sprouts.
For longer season onions (those that make a regular bulb) you should plant seeds in the fall; or plant sets instead in cold climates. Short season onions like scallions and pearl onions can be planted early in the spring. Starting them inside can help you get ahead of the game when the weather stays cold late in the season. Seeds can be planted very close together (my pearl onion seed pack said to plant up to 100 seeds per 4"-6" pot) then thinned after sprouting. Thin to about 1-2" apart, further for larger onions. Sets can be put into the ground as early as January, or as late as April depending on variety and climate. Tops will grow up, when tops die back the bulbs are ready for harvest. The whole onion can be harvested at any time too, for use as scallions. Don't waste your thinnings! Chop them up into a recipe that calls for green onion.
Onions are a great addition to any garden, they can deter some pests, look pretty both in growth and in bloom, take up very little space, and can be added to nearly any recipe. If you're going to grow tomatoes, you'd better grow onions too. You can't make salsa or spaghetti sauce without them. I have never heard of a pest or disease problem with onions either, though I may be mistaken (Note: the SFG book says onion fly maggot can be a problem). My only issue with them in years past has been that they just don't want to bulb, so this year I'm giving them a head start in the house, both the pearl onions and the sweet Spanish white onions, in the hopes that I'll actually have some onions for all my summer recipes. In an SFG garden plant 16 onions to a square foot.
After harvesting lay onions out in a single layer in a warm dry place, this will allow them to cure and the skins to dry, ensuring they will last longer in storage.
My onion growing knowledge is kind of slim, so if anyone can add to this please feel free to let me know.