Onions grow well here in Anacortes WA, which enjoys cool but not cold winters. We almost never get below 20F and that means we can plant onions now, in early December.
We like raised beds. Then the soil is what we make of it. Our beds are about five years old and are replenished and amended each year. Here are some tips if you're just beginning:
Location: If you are in Zone 5 as we are, you want sun. Make sure you have the most sun your yard will allow. A southwest facing slope is ideal, and good drainage helps most crops. (I am using the Sunset zones, not the USDA ones, which take only low temperatures into account.)
Size: You can make a bed as small as 2'x2'. Or make long boxes to fit your terrain. Stick with 4' foot widths so you don't have to step on the soil inside the box to plant or weed or harvest. Or start with containers, but in the winter they will freeze sooner and all year around they dry out fast.
Making boxes: All you need for a 2'x2' is a single 8' board. We like the sturdiness of 2" stock. Make 3 cuts, one every 2 feet, then build a square. We use corner braces to secure the pieces. Our boxes are 12" high, but you can get away with less. The higher you make them, the deeper the soil you can accommodate. We finish the boxes with white barn paint (because it contains no toxins) to help keep them from rotting. You could use cedar or treated lumber but then it's more expensive and we don't know what is going to seep into the soil.
Soil: The basis of our boxes is top soil we purchased. We had a truckload brought in, back when we didn't have anything planted and the delivery wouldn't destroy anything. It was very sandy, really a disappointment. We have amended it in several ways.
Our favorite 'outside' amendment is coir, which is made of ground-up coconut shells. It holds an amazing amount of water and doesn't break down readily. So garden boxes we established four years ago with a portion of coir are still our best producers. Back then we could find only small bricks of coir and thought of it as an extravagance. Now I have been able to find larger bricks online at a fraction the cost. Coir holds an amazing amount of water and so will help keep crops happy during our summer dry spells. To use it, soak a brick, then use it when filling a box. We soak ours in a kiddy pool: one brick when wet fills the pool, and makes a good amount for a 12'x4' garden box.
We also use our own compost. We put all our table scraps in a back corner of the back yard. More on compost in a later post. Every lawn mowing contributes to the pile, along with every veggie scrap. Whatever we have is divided into our garden boxes. There's no such thing as too much compost and we have neighbors who happily contribute their (untreated) grass clippings to the pile.
We don't use peat moss, which is not a renewable resource. We also don't use vermiculite, which is expensive and blows around and gets wasted. We do bring in rotting horse manure from a friend's paddock and have at times gotten straw from a neighbor. Straw can be treacherous though if it is fact hay and still has hay seeds in it. They will turn your garden into an intractable lawn.
Planting. Start small. Read about Square Foot Gardening, in which each plant is treated as an individual. You don't need massive amounts of food because in SFG you don't have a lot of waste.
Advice: Just begin. Then grow it from there. More to come....
Onions are great fun to grow and store well in the winter. You can eat the greens along the way and never have to run out. They are just one of the crops you can start right in the garden in December in Zone 5. What varieties will you try this year?