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I checked the map and the
I checked the map and the border of southern Idaho is 42 degrees North latitude. That means that long day varieties will perform best in your area, You can find those varieties at many websites. Since it’s too late to start plants from seed I would suggest you order plants of long day varieties. I would also google for the “Average Last Day of Frost” for your city or Zipcode, or call your local agriculture extension. At 6100ft. elevation that may be around the end of May, but you should know what it is. Once you have that information you should plan your planting date for plants one month prior to the ALDF. About growing conditions and fertilizer, you won’t be able to grow large onions without the proper amount if nutrients and regular, even moisture. I would suggest 10-15-10 applied at 3 to 4 week intervals. If your soil is poor, you may have to fertilize every 2 weeks. Good Luck and may your onions grow large and sweet!