There are three classifications of onions, pertaining to when they will start to bulb, or enlarge. It is determined by day length (number of light hours), there is Long day (best for Northern area summers, as they have longer days), Short day (best for extreme southern area winter production, like Texas and Florida), and Intermediate day (best for transition zones, like Virginia, or the middle of the country latitudinally). As I said Intermediate day types would be the best choice for us in Virginia (earliest harvest around June or July), but Long day types will work as well, they just require a longer growing season here, and will be a later harvest (usually August if planted in late March).
There are also many schools of thought on the best way or method to plant your onions out in the garden. I will share the method I was taught, and the way in which I have had the most success here, in our Virginia clay soils. Onions require impeccable drainage, and prefer a loose, friable (crumbly) soil that is rich with organic matter (will require less additional fertilizer this way), so the best way I have found to give them all of this is to create a mounded or raised row planting system. I dig the soil and mound it into a raised row (either in fall, or if soil is workable in spring, before planting) to a height of around 4 to 6 inches and somewhere around 18-24 inches wide (enough to fit 2 rows of onions staggered) this helps with the diversion of water. The loose soil helps with the bulbing process and lets the bulb reach full size potential. I space the plants staggered in the row at 6 inch spacings/intervals (in the row) and about 12-18 inches apart (across from each other) and down the center of the bed I dig a shallow trench and fill it with a slow release organic fertilizer (I have seen great results with Bulb-tone, an Espoma product), but you could also use a synthetic fertilizer high in phosphorus as well. I also lightly spread out fertilizer over the whole row after planting, and side-dress occasionally. You will notice in the picture at the bottom, almost the entire bulb is above the soil surface (how onions naturally want to grow) this is ultimately helped along by the raised row system, but not burying the sets/starts too deep at planting is crucial as well.
I have talked mostly about mature bulb culture, primarily because that is what I am after, but there are also scallions and shallots that are essentially just early-harvested sets. Closer in-row spacing can yield good quantities of these while thinning and leave sufficient space for your mature bulbs to form.
We carry a multitude of onion varieties here at Strange's, but the types I have had success with and would recommend highly are Candy (an intermediate day, yellow onion), Red Candy Apple (your best chance at success with a red onion in VA), and Superstar (only white onion to win All-America selection and an intermediate day type).
Whatever onions you wish to grow, I hope this helps demystify the onion growing process, and leads to more gardening success!
|'Candy' onions grown at the West Broad vegetable test garden in 2012|