For monster onions, plant smaller sets
Q: I have planted onions in my garden every year for the past three but they never get very big, maybe 2 or 3 inches across. I start them from sets that I buy at the grocery store every spring and plant them about an inch deep. We have pretty good soil in our garden, but maybe they need something extra• Suggestions?
A: I think you have several issues to deal with before you can grow the monster onions you're hoping for. First and foremost, though grocery store onion sets may be the least expensive, they also may be varieties that are totally inappropriate for our climate.
All onions fit into one of three categories:
Short day -- these varieties begin to set bulbs when the days reach 10 to 12 hours in length. Meaning that for Pennsylvania gardeners, they'll finish growing when the days reach 12 hours way too early in the season. If you grow them here, they'll be puny even when fully mature. These are good choices for Southern gardeners where they are grown when the days are shorter. Vidalia, White Bermuda and Southern Belle are examples of this type.
Long day -- These onions bulb when days reach 14 to 16 hours in length. They are perfect for Pennsylvania and everyone who lives all the way up into Canada. In the South, they'll never reach maturity. Long day varieties include Yellow Spanish, Walla Walla and Red Zeppelin.
I ntermediate day (also called day neutral) -- these types are best for the middle of the country. They bulb when days are 12 to 14 hours long. Though they'll do OK here, they aren't the best choice and, in many cases, these are the ones grocery stores (and many local nurseries) carry. Superstar and Candy are two examples.
So, purchase long day varieties only for your biggest bulbs. Contrary to popular belief, smaller sets yield significantly bigger onions. Plant the smallest sets for your main onion harvest and use the larger sets for scallion (aka green onion) production. Harvest these when they reach pencil size.
Also, you should know that planting onions from seeds started indoors 10 to 12 weeks before planting outside, or by purchasing transplants from a mail order source or local nursery, nearly always result in larger onions than sets. If you do grow your own seeds, you'll have a greater choice of varieties and you'll get more for your money. Grow them under fluorescent lights in a quality seed starting mix. Leave the lights on for a maximum of 12 hours per day to prevent early, and puny, bulb formation.
And lastly, feed your onions by working compost or well-aged horse manure into the planting area every year. A few handfuls of bone meal or rock phosphate also can be added to ensure there is ample phosphorus for good bulb development.