This post may contain affiliate links. As an Amazon Associate I earn from qualifying purchases. You can read my full disclosure statement here.
This article will teach you how to plant onion seedlings for a healthy robust onion harvest in late summer. To learn how to grow your own onion seedlings, please see my How to Plant Onion Seed article from my Seed Starting 101 Series. In that article I discuss the pros and cons of planting onions by seed or by sets, and talk about different onion varieties you can choose to grow. If you have any other questions after reading, please feel free to comment below!
Onions are a cold hardy crop, so they can be planted much earlier than your warm season crops. Here in the PNW, I like to get mine planted in early April. Make sure that your soil is workable and not too wet by doing the squeeze test. Grab a bit of soil in your hand and squeeze. If the soil stays in a muddy ball when poke, then the soil is too wet. If it crumbles and falls apart when poked, you are good to go. Planting seedlings in soil that is too wet, will cause them to rot. When determining where to plant your onions, choose a sunny location with loose, well draining soil. Remove any weeds from the area prior to planting. Please don’t use commercial weed killer, you really don’t want that stuff anywhere near you or your food!
If you are new to gardening or starting a new garden, it is probably a good idea to test your soil to see where it is lacking in nutrients. I follow a no till method in my garden, so all supplements and organic matter are just added in layers above the soil. Onions are heavy feeders, so first I add a good sprinkling of organic vegetable fertilizer. I try to choose one with a higher phosphorus level, as onions are a root crop, and phosphorus aids in root formation. This year I used Jobes Organic Vegetable & Tomato Fertilizer which is rated 2-5-3. The first number is nitrogen, the second is phosphorus and the third is potassium. This is what I mean by choosing a fertilizer that is higher in phosphorus, see how the phosphorus number is the larger number of the three? Than I add a couple inches of home made organic compost and water this all in a few days prior to planting.
Prior to planting the seedlings, they need some preparation too. First, they should be big enough. I like for them to be at least 1/8″ in diameter. This means they are hardy enough to go outdoors. Smaller than this, they may just rot and succumb to the elements. Second, I trim them down if they are long by giving them a haircut, reducing them to only 3-4″ tall. Save these trimmings, and use them as you would chives, or add to salads or stir fry with other veggies. Or dehydrate and save for later use. Don’t toss them! Then, right before planting, as I go along, I also trim the roots a bit, by pinching off the long scraggly roots, keeping them maybe 3/4″ -1″ long.
When everything is ready for planting, take a chopstick or pencil, and poke holes into the growing bed. I use a metal chopstick, poke it into the soil about half it’s length. Then turn it around and around in about a 1″ diameter. I tend to do about 25-30 holes at a time, and then take one seedling at a time, and poke it down into the hole, one at a time. Don’t pat them in, just poke the seedling in and move on to the next one.
After planting them, I take a hose and water them in gently with a gentle spray nozzle. This not only waters them, but allows some of the compost and soil to fall into the planting hole. Water them well this way. After the seedlings have become established for about a month, you should mulch with another layer of compost and feed some organic fertilizer once a month for the first few months. Onions are heavy feeders, so providing the extra compost and fertilizer helps them grow large healthy bulbs. They also like to be watered regularly, so don’t neglect them during dry spells or consider using soaker hoses.
You can harvest onions when they are young for green onions. To harvest when they are full size, wait until the bulbs are large in size, and the majority of the onion greens begin to flop over late in summer, this is when it is time to harvest them. Gently lift them from the ground using a pitchfork, not by pulling the greens. Shake off the extra dirt and let dry in a warm dry area with good circulation for a couple of days until the roots shrivel, but not in direct sunlight.
I hope this article has successfully taught you how to plant onion seedlings. Please feel free to ask any questions in the comments! Happy Planting!