Starting and growing onions  

Onions are fun and easy to grow. Watching a tiny onion plant turn into a large bulb is a very rewarding gardening experience for anyone. January is a great time for us central Kansas gardeners to make a plan for our onion crops since onions are cool season plants that will be some of the first to go into the garden as spring weather arrives.

How to start onion seeds

It can be difficult to find specific onion varieties in sets or transplants, so for some gardeners, growing onions from seed may be a preferred option.

Onions are one of the first plants to be seeded for transplanting because this crop takes a significant amount of time (6 to 8 weeks) to reach transplant size and because they can be set out relatively early (late March in central Kansas). With this in mind, we want to start onions in mid- to late-January.

Onion seed should be placed ½ to 3/4 inch apart in a pot or flat filled with a seed starting mix. Place the container in a warm (75 to 80 F) location until young seedlings emerge. Using a seed starting heating mat under the pot or flat can help speed up germination.

Move the pot or tray to a cooler location (60 to 65 F) when the seedlings are 1 to 2 inches tall. Make sure they have plenty of light, using florescent or LED lights if needed.

Start fertilizing when the seedlings reach 2 to 3 inches tall using a soluble fertilizer with each or every other watering.            

Onion seedlings tend to be spindly with the remains of the seed sticking to the end of a leaf for several weeks. Encourage stockiness by trimming the ends of the leaves when the plants reach 4 to 5 inches tall.

Start hardening off the onions in early March by moving the plants to a protected outdoor location. You may have to move them inside temporarily to protect them from any extreme cold snaps.

Onion plants

If starting onion seeds isn’t for you, don’t worry. Purchasing onion plants is also a great option.

Onion plants are started and grown in southern areas in the fall or winter, bundled 50 to 100 plants per bunch, and sold in the spring. Some greenhouses offer market packs of onion transplants similar to other bedding plants. Onion plants usually are well identified as to variety. They make excellent mature bulbs, allowing gardeners to choose varieties adapted for Kansas conditions. Choose healthy, green, and fresh transplants, setting them 1 to 1½ inches deep in rows 12 to 16 inches apart (or wider) in the garden. Onions should be spaced 2 to 4 inches apart, depending on the size of the mature bulbs.

Onion Varieties

Regardless of whether you start from seed or not, many gardeners wonder which varieties of onions may be good for Kansas. The below chart will provide a good starting point. Remember that in Kansas it is generally recommended to choose intermediate day onions. Intermediate day onions initiate bulb formation at 12–14 hours of daylight and produce best in zones 5 and 6

Plant VarietiesColorNotes
BermudaYellow or WhiteSmaller, flattened, bulbs, mild flavor, poor keepers
CandyYellowGlobe, sweet, intermediate-day variety
Early HarvestYellowMedium, round to flat shape
GranexYellowFlattened, crisp and sweet
Red Candy AppleRedFlattened, sweet, intermediate-day variety
Super StarWhiteGlobe, sweet, intermediate-day variety

To learn all about growing onions in your garden, the below K-State Research and Extension publication is a wonderful resource.