When I see plants that appear healthy, but seem too small, my first question has to be about temperature. Cool temps makes for slow growth.

It looks very wet. I'll bet that the seed starting mix that you bought has that "moisture-holding" capability that makes life very difficult! I would certainly try re-potting them in your new mix. It's not too late to start extra lettuce, since it's such a fast grower (normally).

They do look rather weak for being started over a month ago. It may be too late to re-start the broccoli up there? but personally I'd plant more lettuce seed directly in the garden soil now.

So, I started these under lights. I keep them in the basement. My basement is 70 degrees and I did use a heating pad when I started pepper and tomato seeds, but didn't think cold crops needed more heat than 70 degrees. I had a thermometer right next to them, so I know the temps. I also used a fan to keep the air moving. I watered them when dry. They were sitting on the backsteps yesterday in the sun and when I went to check them, they were so dry I was surprised they were not wilted. So they were just watered late yesterday. I've grown seedlings in this way multiple times before in these same circumstances and not had any problem.

And I agree that these commercial seed starting mixes have way too many ingredients in them. That's why I always try to mix my own. I am kicking myself that I didn't make enough last fall to stock for winter sowing and seed starting this winter. Oh well, live and learn, I'm done buying commercial seed starting mix.

Yes, I plan to start seed directly this weekend. We just finished turning over all the beds. I may try some broccoli and other things and I might try to buy some broccoli starts. Darn, I bought so many varieties of broccoli seed that I wanted to try. I have others, Eastern Magic and Green Magic from Johnny's. I might try planting a couple of these weak seedlings directly too.

Will fertilizing them help at all, or should I just let them get acclimated to being repotted?

Thank you!

They don't look weak to me, but a month after seeding, they do look sorta small, though brassica seedlings do grow slowly. MUCH more slowly than tomatoes or cucurbits. I'd assume they're going to make it, but yes, you might repot.

Dan, if I repot should I give them some Liquid Fish Emulsion?

The lettuce look ok to me but the brassicas definitely not. The cotyledons shouldn’t be purple.

I regularly get purplish cotyledons when I germinate collards. Some say too much light. Others point to a phosphorus deficiency. They get over it. Once they're up, and developing, it's time for some fertilizer.