It depends on the plants. I planted the poppies HOS, just divided the container into four clumps. I did the same with my marigolds, which sprouted copiously.

For the cape daisy, I separated as best I could. Did the same with the dahlia (only 3 of them) and the aster. That and the dill (HOS) and some lettuce are the only things planted out so far.

Caryl

When I want to separate seedlings, I just water well the day before, then pop the whole thing out of the container and they usually fall apart easily. Just pick them up by their leaves and not by way of the stems :)

Vera

Pretty soon I'll be planting out the first container - sweet william seedlings which are about 2-3 inches tall. I usually wait until they're at least an inch or two in size. A few are planted HOS but most are planted individuallly in small groupings about 6 -12 inches apart.

I try to remember to water regularly and keep mulch back from the seedlings to discourage slugs, until they are around a foot tall. Also use sluggo or Escargo sprinkled around most seedlings, esp. Salvia, Zinnias, milkweed etc. which the slugs love and will easily decimate small seedlings.

I sow my veggies singly (tomatoes, peppers, etc.)- or really what I do is space the seeds apart in the container. I will divide the container in 4 (or 8 depending on the size) and plant two seeds per quarter, and then thin if necessary. It's pretty easy to scoop out the individual seedlings that way.

For most of my flowers, though, I prefer the hunk of seedlings method. I usually break it into 4 pieces and plant them a little spaced out so I get an even bigger clump once they're established.

I lost some salvia which I had separated out.
I think bec. I forgot to go back and water.
Usually I water everyday, after planting or separating, but I've got this health issue which is causing me to forget things.

oh well, I've got a lot more various kinds of salvia in my containers: Blue Angel Salvie, Fairy Queen Salvia, Mealy-cup Salvia. - all blue.