How to pot up Tomato Seedlings
Most gardeners start seedlings in small pots or trays and then "pot up" (transplant to a bigger container) when the plants are larger. I usually pot up my tomatoes 3-4 weeks before they go out to the garden. You could just start your plants in large pots but I don't have space under my lights until the broccoli and other cool weather seedlings have moved out to the porch.
After Starting Tomatoes from Seed May 25, I watch eagerly as the seeds start to sprout. If any seedlings show signs of disease, remove the whole seed block before it can infect the others. (You can see where I culled a sickly looking Opalka) If more than one seed sprouts per block, I keep the stockiest looking one and snip out the other with scissors.
April 14 - ready to pot up. Most seedlings have two sets of true leaves and the roots have filled the soil blocks.
I pot up into pint sized plastic containers. Many gardeners use red solo cups for tomatoes but I prefer to wash and reuse my collection of nursery pots. I use a mix of mostly potting soil and a little compost. Remember to add drainage holes if you are using cups.
Bury the seedlings deep - roots will sprout all along the stem. I prefer to pinch off the cotyledons rather than bury them. If your seedlings are really leggy, you can bury all but the top set of leaves.
Below: April 26 - only ten days of growth in the big pots! I have to raise the lights a little almost every morning they are growing so fast.
Frost will kill your tomato plants, and extended night temps below 50 will make them sad. If your weather is warm enough, plants this size can be very successfully planted outside after a few days of "hardening off" (acclimating to the brighter sunlight and wind in the shelter of part shade). My weather can't be trusted, so I will keep these in under lights until at least mid-May. In about a week, I'll top off the pots with some purchased compost to give them a boost.