When to transplant onion seedlings – get the timing and method right

If you are after bigger onions, then it may be worth experimenting with onion seeds as they tend to be larger than those grown from sets come harvest time. A key part of this process will be judging when the time is right to transplant your onion seedlings.

Growing onions has been an annual task for me in the kitchen gardens I worked in. I have always sowed onions from seed every year and had to pick when to transplant my onion seedlings out in the vegetable garden. This decision is dictated both by the size of the seedlings and the weather. Some years the seedlings are large enough but a cold snap delays the transplanting. Others the weather will be perfect earlier, but the seedlings might not be large enough and you have to wait. And then there’s those years where everything aligns perfectly.

Learning to identify the signs they are ready, and knowing how to safely transplant them without damaging your precious seedlings, are two great skills to have for any vegetable grower.

Drew Swainston

Drew is a former professional gardener who specialized in growing vegetables. He worked in kitchen gardens for the National Trust in the UK and ran a productive walled garden growing vegetables, fruit, and herbs for a high-caliber restaurant in England. 

When to transplant onion seedlings

There are two ways to grow onions for your vegetable garden, and that is by onion sets or onion seeds. When growing onions from seed, sow undercover in mid-winter (January and February) either in a heated greenhouse, propagator, or on a warm windowsill in your house. Onion seeds need around 70˚F to germinate efficiently. Once the seeds have germinated, artificial lights – such as these LED propagation grow lights from Nature Hills – can be used to supplement the lack of light around during winter time if necessary.  

When onion seedlings get to around 6-8 inches and have three leaves it is time to start preparing them to transplant. Typically it can take around 50-70 days to go from sowing the seed to the seedlings being the right size to transplant. When to plant onions grown from seed is always dependent on when they were sowed and also the heat and light you are able to give the seedlings.

• Buy onion seeds from Burpee
• Buy onion seeds from Amazon

How to transplant onion seedlings outside

Onion seeds can be sown in either trays or modules – I tend to prefer using modules as it makes the process of transplanting the seedlings simpler. My onions were typically sown during January in a heated polytunnel and then would be planted out in the vegetable garden around three months later. Once the small onions are ready for transplant, they do need to be hardened off first. This attunes the seedlings to colder temperatures in preparation for the big day when they are transplanted outdoors.

Jen McDonald, certified organic garden specialist and co-founder of Garden Girls, normally starts her onions indoors 12-14 weeks before the last frost date for where she is based in Houston. She hardens them off over an extended period. She explains: ‘Introduce them to outdoor temperatures a little bit each day, adding time as you progress, for about 2-4 weeks. As they acclimatize, you’ll eventually reach the milestone of leaving them out overnight. Then they’re ready.’

The method of transplanting will differ depending on if your onion seedlings are in trays or modules. Either way, seedlings should be transplanted 1 to 1.5 inches deep and each transplant needs to be 3 inches apart. The onions can be planted in rows 12-15 inches apart.

When transplanting from a tray, use a fork to gently lever the seedling out of the soil. Be super careful not to damage the roots and only handle the seedling by the leaves and not the stem. Place the seedling into a good-sized hole, fill gently with soil, and pat the onion in. For seedlings grown in modules, it is best to gently squeeze the container to encourage the seedling and soil to come out. Make a larger hole than you would for a seedling from a tray and place the young plant in, patting it in. Water the seedlings lightly to avoid harming the shallow roots.

Transplanting onion seedlings into containers

Onions can be transplanted into pots and containers, whether for temporary purposes or as their final position. Any seedlings grown too large for their module can need potting up into a larger pot to grow on. Smaller modules can result in onions quickly using all the nutrients up in the soil available. If the seedlings start to look yellow and unnourished, or lots of roots protrude out the bottom of the tray, it may be time to pot them up into a larger container to grow on. Squeeze the seedlings out of its module and place into a 1 inch hole in a pot filled with soil. Water well and place somewhere light to grow bigger ahead of planting outside.

Growing onions in containers is a fantastic way to get a good onion harvest and seedlings can be transplanted into their final position in a pot or container. A suitable container for onions needs to be at least 10 inches deep and 24 inches wide to allow onions to develop to their full size. To transplant onion seedlings into containers, follow the same spacings and steps outlined above, whether the young plants are in trays or modules.