Preparing Onion and Leek Seedlings to Go Outdoors


I’m currently in the process of hardening off the first round of onion and leek seedlings in preparation for permanently planting them outside. To recap, here’s the planting calendar that I follow:

Start Seeds: Since I want the seedlings to have about 8+ weeks of growth before putting them out, I count backwards from the Planting Out date (below) to determine when I need to start them. This is usually early February or March in my region.

Planting Out: I try to get onion seedlings outside around or on the last frost (May 9 in Toronto). My ideal is 3-4 weeks before the last frost, but I don’t fret if I am late. In fact, I often do multiple rounds. I still have some underneath lights that will be going out after this first group.

Hardening Off: I try to give all seedlings about 2 weeks hardening off time. I find out I don’t have to be too delicate with onions, but like all seedlings, they stay healthiest when they’ve been given ample time to adjust, especially if they were grown underneath lights in a warm room and are going out before the last frost. My kitchen is a particularly cool room (except when baking), so I tend to keep them here while they go back and forth from indoors to out through the hardening off process.

Before trimming.

After Trimming

Trimming Leggy Onions and Leeks

Even under the very best growing conditions I find that many varieties of onions and leeks produce leggy growth in the 2 months that they are indoors. I trim them back intermittently to encourage strong growth and to prevent the plants from becoming a tangled mess. To do this, simply cut the green part with a pair of sharp scissors, trimming back to about 3-5 inches. Do the last trim before you begin the hardening off process.

Nothing Goes to Waste

Don’t throw the trimmings away! Eat the young, tender greens fresh on top of salads, eggs, potatoes, and meat, toss into meals in the final minute or so, or add to savoury baked goods such as cheese and herb scones. You can also mix them into creamed cheese or dips.

Gayla Trail
Gayla is a writer, photographer, and former graphic designer with a background in the Fine Arts, cultural criticism, and ecology. She is the author, photographer, and designer of best-selling books on gardening, cooking, and preserving.

Subscribe to get weekly updates from Gayla

20 thoughts on “Preparing Onion and Leek Seedlings to Go Outdoors

  1. Oh, yay! Thanks for this timely post. I’m growing leeks/onions from seed this year for the first time. I started them over a month ago and have been eyeing them thinking I really ought to be doing something with them. Now I know what to do! Thanks :)

  2. I’ve started leeks from seed many times and had great success. This is one crop that always impresses me with the amount of money saved by starting from seed–leeks are so expensive!
    This year is the first time I’m starting onions from seed. In northern WI, we won’t have spring for weeks, so I need to think about transplanting things to larger pots. Oh the interminable wait!
    Thanks, Gayla, for this and everything you do!

  3. Thanks for this. Have been thinking I should have a go at planting onions.

    Never tried leeks. But to be honest I have never been that keen on leeks, so may leave that for someone else to try!

    The onions look like they could be a fun side project though for the kitchen mantelpiece.

  4. Hi! My onions, shallots and leeks (leek potato soup mmmm) are on the back porch now, first step in hardening off. My advice includes the haircuts to keep them shorter but also (for all my seedlings) a modest fan – intermittent or ceiling fan – every day from the day they come up, several hours a day. The idea of “three days” to harden them off is baloney, and it’s better to start hardening off early and have them grow slowly. Easy to start, they are hardy little buggers, and you get a lot bigger bulb starting from seeds than from sets. If we can just quit having the snow, now….

    • I’ve never heard only 3 days to harden off — I always give them far more time than that, most especially if they are going directly from underneath lights to the still cool outdoors.

  5. I got some Walla Walla green onion seeds back in February and started them right away under lights, but unfortunately, they haven’t been growing wonderfully. They’ve come up, but they don’t seem anywhere near as nice and bushy as yours. I put two or three seeds per little seedling slot in the tray, but only one or two per slot came up, and they’re pretty small. Everything else I’ve been growing has shot up and is almost leggy – EXCEPT for the onions. Kind of sad about that. I guess maybe I’ll have to resort to buying seedlings for those ones this year. :(

    • I have had the same issue: my Ailsa Craig onions have been really spindly and fragile…I planted many (maybe 40?) and I suspect I will get no more than 5 or 6 onions out of them.

    • I’d say it could be the lights, soil, or cool temperatures, but if your other seedlings are thriving then it may just be down to the seed stock. Sometimes seeds just aren’t of very good quality.

  6. I think I may have done poorly by my onions – I put them straight out last weekend without hardening them off, and we’re expecting snow tomorrow! That said, they seem to be doing ok so far (although they have been spindly since the beginning; I knew I shouldn’t have let the hardware store guy talk me into purple grow-lights instead of fluorescents.) I am planning to plant my remaining seeds in a container in the fall and leave them out over the winter – does this seem wise?

  7. Thanks so much for encouraging to trim. I’d read both to trim and not to trim, but my shallot seedlings are starting to tumble over and in that area, are withering. I’ve already lost a whole planting of Ailsa Craig seedlings from damping off. Too much moisture and too little air movement and seed starting mix from last year. Sterilized new planting mix, got new seeds, and installed fan round the clock. These onions are thriving. Leeks are looking fine. Thanks again for the timely post. You just may have saved my precious shallot seedlings :)!

  8. I bought bunching onion seed and was planning to direct sew into containers… uh oh… maybe I have to wait until next year?? It seems like it is too late to start the seedling indoors at this point but maybe I’ll give it a go anyway. Any advice?

    • See the post I link to at the top of this page. I outline other times that I sow onions…I still do another batch direct sown around the last frost date. There’s still time.

  9. how have I never thought to trim my onions before? such a great tip. Any tips for keeping the squirrels out of them while they harden off?

  10. How long does it take you to grow leeks? 2 springs ago I planted some and they matured by next spring!! So long.

  11. Genius! It never occurred to me to trim the leggy growth off the onions. I just did this with mine and they look so much healthier (and I think they might be growing more leaves now). Assuming that onions are somewhat like grasses, trimming the top growth would promote fuller top-growth (called tillering in grasses).

Comments are closed.