I pull out excess seedlings to thin them, but wait until the seedlings have three leaves to transplant them to roomier quarters. The first leaf withers pretty early and I snip it off. I also keep the seedlings trimmed back to 3-4 inches, which helps them grow stockier, with better light penetration. I don't want to have to pot them up but once since it's so time-consuming.

I had to pot up artichoke seedlings last week, and I had already oven-processed some old potting soil to use, then dumped it in a bucket that froze hard outside. Set next to the wood stove, it took that bucket 18 hours to thaw!

Good advice by Planatus. My question is do you really need to pot them up? Could you grow them in the soil you germinated in until they are ready to go in the ground? Thats how I do it with only some fish emulsion for extra nitrogen now and again.

If you need to pot up, my simple and cheap soil recipe is sifted compost, peat moss and sand. About a 8:2:1 ratio.

I thought you had to pot up because seed stater mix doesn't have enough nutrients.

I thought you had to pot up because seed stater mix doesn't have enough nutrients.

I thought you had to pot up because seed stater mix doesn't have enough nutrients.

Sorry about the duplicate posts I'm posting from mt phone

I don't use that stuff. I make my own because it's much more cost effective.
If you need to buy soil mix, find one that's not heavy on wood debris and consider adding sifted compost. Onions like nutrient rich soil to grow strong till transplanting.
If I only need to germinate and plan on potting up (tomatoes, peppers) I use less compost or used soil. I never found a need for 'soil-less mix'. I think it's unnecessary.
If you did use starter mix, you will need to feed heavily or pot up. As you said, that stuff doesn't have any nutrients. As far as pot size, with good soil you can cram quite a lot of onions into a small container as you can see in my photo, 1/4" apart is fine.
Next time, consider starting in good soil and avoiding a step. Don't let this (or me) discourage you, I'm sure they will do great!

Here's how I grow mine:

This post was edited by madroneb on Sun, Feb 3, 13 at 1:40

Wow Madroneb that is a lot of onions!! I plan to pot up in a few weeks once the seedlings have a few more leaves. I only got a 50% germination rate so I started more last night.

Do you think If I fertilized I would not need to pot up?

That could work if the seedlings have enough space to grow. I think about 1/4" between plants should be enough.

Usually fish emulsion works wonders for me.

I've been pricking out month-old onions this week, and some of the roots are 6 inches long (most shorter). They were already spiraling in the bottom of the seedling containers. Left any longer, they would have tied themselved in knots.

Planatus, were you potting up?

I moved shallots and two varieties of onions into individual cells in a plastic thingie with 4-inch deep, 1.5 inch wide "cells" for each seedling. By the time it's time to transplant, the roots will fill the cells completely. I have another tray of 96 to do, but it's too cold to work outside right now.

I change to regular organic potting soil rather than seed starting mix as I pot up the onion seedlings. The fresh medium provides ample nutrients for a couple of weeks, delaying the need to start feeding.

I also use the bottoms of waxed half-gallon milk cartons, planting four onion seedlings per pot, and other random containers that I can't recycle. Onions are not picky about containers as long as they provide ample rooting space. Square milk carton bottoms are great because they don't tip over.