Although they can be transplanted at any time after germination, a
common recommendation is any time after the first true leaves develop.
That's only because they are a bit easier to handle then. But as the
FAQ here on Transplanting says - any time after germination.

That's
assuming they were seeded individually. If multiple seedlings are in
the same small container then they need to be separated as soon as
possible before the roots becomes all tangled.

But it really
depends on the size of the containers who are talking about, the
container needs to be somewhat appropriate in size to the size of the
plant. In otherwise it isn't good to take a 1" tall plant and put it in
a 6" pot. If you use cell packs as most do then it is easy - they go
from 1" cells to 2" cells to 4" cells etc. as needed as they grow.

Dave

Thank you Dave. As I am new to gardening and even newer to starting seeds indoors, how will I know when the true leaves begin? For awhile there have been just two leaves and the 'stem' has grown to about 2 inches. Just recently there have been leaves developing from the center (between where the original 2 leaves sit). Are those true leaves? Should I wait until the true leaves grow to a specific size before moving to a larger container? thank you

I have transplanted Tomatoes when they have only had their seed leaves and I've transplanted them when they've had their first true leaves or even the second true leaves and all have done well. When planting in larger containers I alway sink them down to the leaves. Tomatoes have the ability to grow roots along their stem.

Peppers are a bit different. I transplant them when they have their first true leaf and plant them at the soil level that they were growing at.

How do I know which are the true leaves?

thank you

How many oz are the container you'll transplant to? Do you transplant just one more time after the true leaves grow and prior to taking them outside?

Ounces are not normally a measurement associated with seed starting. It isn't something most would know or pay any attention to. The measurement associated with seed starting and even growing on is inches, either the diameter or the width of the container used. So what is the diameter, or at least the width of your container if it is round? The other important measurement is the depth of the container used.

Whether you transplant just once or more often depends on many things - the size of the plant, the size of the container it is in, how long before it can be planted in the garden, etc. As I said above, seeds are normally started in very small containers - either one at a time in something like 1 1/2"-2" wide and 2" deep OR if you start multiples in the same container you can use something a little bigger bigger.

Have you ever bought a 6 cell-pack of transplants at a nursery? If so those plants were started 100's at a time in big 2" deep flats and when they were 2" or so tall and had 2 sets of true leaves they were transplanted into that pack you buy. When they get 6" tall and still haven't sold they get pilled from those cell pack and transplanted into 6" pots and offered for sale.

Dave

Thank you Dave. Once the plants have true leaves growing, what is the appropriate size container, in inches? Is it something like a 1-to-1 ratio of pot size to plant height?

No it is more like 1:2. But it isn't as if it is some absolute rule, just a general guide. EX: a 2" plant does good in a 3-4" pot, a 5-6" plant in a 6" pot. Much beyond that and they need to be planted where they will grow.

Dave

I transplant most veggies and flowers when they get their true leaves (aka the 3rd and 4th leaves) . There are a few exceptions for me like Marigolds and nasturtiums that grow aggressively. Those I pretty much move from peat pellets to pots as soon as they sprout. Dry Shelling beans and cucerbits never get put into larger pots. Theyre started directly in cow pots

Would moving the seedling from a one inch pot to a six inch pot hurt or stunt the seedling in any way?

"Would moving the seedling from a one inch pot to a six inch pot hurt or stunt the seedling in any way?"

Yes it can because a 6" container can hold far too much moisture for a seedling that is as small as those coming out of a 1" container. Plus in a 6" container the perched water table level is too deep for the small roots to reach. So it goes back and forth between overly wet or overly dry.

It can be done if the grower is very very careful with water applications OR if the container is only filled 1/3 to 1/2 full when transplanted and then more soil is added as the plant grows.

Dave

I knew there was a reason..... thank you :)