Yes, you can, and while some do....I don't. My soil is clay based however, and my own experience is that my perennials do better with a little more developed root system than what I would find at the first true leaf stage.

I use 5" minimum soil depth when sowing seeds and leave them to grow some roots before planting out (some I even pot up individually till Fall - I've got time to take care of pots over summer, not everyone does)...but you will know your own soil and what works best for you.

And just to confuse you, I don't wait. I don't pot up to another container. I take the smallest seedlings I can handle and plant directly into the bed. I've got some in the beds now that are as small or smaller than those I will plant long before our last frost and sometimes, between snowfalls. It worked for me last year. But my soil is heavily amended where I do this. In heavy clay, I would probably rethink it. In fact, I have some areas that are still pretty bad and will wait a bit before planting in those. Maybe once they have three sets of leaves.

I've had great results with a lot of seedlings planted out when they are about 1-2 inches in size. I often plant in groupings of 3-7 seedlings or so, between 6-12 inches apart. When planted out early, perennials are much more likely to bloom a little in the first year. It might not be much, but at least you get to see what the blooms look like.

Slugs can be a problem with small seedlings though, so I usually pull any mulch back away and sometimes use Sluggo, if I remember. Also, be diligent about watering small seedlings, every day or two if it doesn't rain.

One nice thing about WSing is that you get lots of sprouts, so that if you lose a few, you still get some survivors.

Thank you, all! Like you, morz8, I have clay soil. It warms more slowly and is a mess in spring rains, so I don't know how much luck I'll have in planting early. Just the thought is exciting because I never have everything planted before the hot summer weather sets in. This could give me an extra month or more! Even if I wait until 3 or more leaves, as Token suggests, that would still have me ahead of my usual time table.

I'm not sure which is the greater hazard for me: slugs or rabbits. I'm going to go with rabbits. Too bad Sluggo won't work on them!

I haven't gotten a single pot ready or a single seed sown yet, but I am so excited about this thanks to a friend who talked me into trying it. I saw all her babies this past spring and even brought some home to plant in my own garden. This weekend I'll get busy with it all. Right now I'm still trying to get organized.

Thanks for the answers!

The earliest perennials to sprout are usually those which remain evergreen or semi-evergreen during winter, so they're adapted to harsh weather. I plant out ASAP. Do you know what happens when you don't plant out ASAP? You have a ton of flats to plant all at once before hot weather takes over--which means daily watering chores to keep them going in late spring and summer. Ick.

Do yourself a favor and plant out seedlings as soon as you can.