How to Plant Onion Sets

Our garden experts are constantly answering your garden questions (that’s what they’re here for!), so were plucking out the popular ones, right when they’re asked the most. On the Docket: When you should be planting onion sets along with how to do it.

Find Spring Onion Sets at Prairie Gardens every season around the first week of March. 

Ask an Expert: When can I plant Onion Sets?

As long as the ground is workable, you can get planting onion sets. If the ground is frozen (which is not an issue this time of year) or too wet, you should wait to plant. That typically means you can start planting onion sets right around now or late-March in our area.

— Mary Ann Metz, Prairie Gardens Plant Expert


Plant your onion sets about 1” deep with about 2-4” spacing. Leave about 12-18” in between rows. If you’d like to grow some as green onions, you may plant your sets closer and harvest every other one, leaving some for larger onions.

Grab a clump of dirt in your hand and squeeze. If the dirt stays clumped together once you open your hand, it’s too wet to plant.

— John W, Prairie Gardens Plant Expert


Fertilize your onions with a nitrogen-based fertilizer (ammonium sulfate or ammonium nitrate), with your first application being about three weeks after planting. You can then continue fertilizing about every couple weeks, stopping fertilization completely once the neck of your onions begin feeling soft.  Make sure to water after fertilizing.

Weed Control

Applying a pre-emergent herbicide prior to planting can help prevent weeds. Always be cautious with edible plants, making sure to follow label instructions.)

Disease/Insect Control

Blight (leaves will turn pale-green and then yellow) and purple blotch (leaves will acquire purple lesions) are the most common diseases that affect onions. Prevent disease by using well-drained soil. Multi-Purpose fungicides, like Daconil, can be applied according to label instructions if disease occurs

In case of insects, an insecticidal soap of biological insecticide may be used according to label instructions. Do not apply any insecticide within 7 days of harvest.


When your onion tops have fallen over, they are fully mature. Pull them from the ground and allow them to dry, clip the roots and then cut the tops back 1 inch.


Keep your onions cool, dry and separated. The best way to store onions is in a mesh bag or nylon stocking, placing the onions in the bag and trying a knot at the end.

Any more questions? Give our Plant Experts a call, they’ll be happy to help: (217)356-6532

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