Time to plant onions outdoors

Bulbing onion flowering in late spring

Sunday, March 9, 2025

It is time to plant onion sets and onion transplants in the garden to get full size bulbs for harvesting this summer. 

You can order sets on-line, get them in big box stores, local nurseries, local farm supply stores and local hardware stores at this time of year.  Your local stores should have the type of onions that will bulb properly for your latitude.  Onions set bulbs based on the hours of daylight.

If ordering on-line, know what daylight zone you are in to get the ones that will develop into full bulbs for your zone.  They start forming bulbs when daylight hours hit a minimum. For long day onions, it is 14 hours. For intermediate, it is 12-13 hours. Short day onions are 9-10 hours.  You also need to get them planted this month to insure optimal size for harvesting.

 I would have thought that long day onions would be for the South, but this is wrong.  The North gets the really long summer days (think of Alaska in June with no darkness).  Long day onions should be planted in states north of the Oklahoma/Kansas border (approximately 36 degrees latitude).  Intermediate day onions are planted in the middle of the US and short day onions in the South (like Vidalias).

I live in Kentucky so right in the heart of intermediate day onions.  I can plant long day onions, but should have gotten them in the ground as soon as the soil could be worked last month so that get a good root system for making large bulbs when peak daylight occurs in late June.

This year, I bought sets from a big box store and started seeds of interesting intermediate types.  For sets, the bulbs need to be firm to still be viable.  I planted my sweet onion sets yesterday and will plant my seedlings in a couple of days.  I have the seedlings hardening off on the back patio to get used to the cooler temperatures and increasing time in unfiltered sunlight.

If ordering sets or plants on-line, they will send them to you when it is time to plant in your area.

For planting, onions like loose, rich soil.  I dug a trench 2" deep, put blood meal in the bottom, added an inch of soil, then planted my sets 3-4" apart in rows 1 foot apart.

I planted potato onions and Egyptian walking onions last fall to give them the winter to develop good root systems for harvesting this spring and summer.

For more on onions, see 

Onions-everything you need to know to grow 'em