By the end of the first week, your little pots should be crowded with tiny seedlings.
The first leaves you’ll see are oval, fat and bland. These are not “true” tomato leaves. Botanically speaking, these embryonic leaves are called cotyledons. Their job is to store food before germination, then do a little photosynthesis after the seed sprouts. Cotyledons are the reason seedlings don’t need fertilizer—they come equipped with a few day’s food on their own, just enough to give the seedling a decent start in life.
The early growth rate of your seedlings depends mostly on heat and the intensity and availability of light. If your seedlings are basking in lots of bright light, you might already be seeing the first tiny true leaves emerging by Day 8. In a chilly, darker room, however, it might take a little longer for the true tomato leaves to emerge.
As the pots fill up with seedlings, you’ll notice that some are healthier and stronger, while others are on the wimpy side. Somewhere in their second or third week of life (depending on their size), you’ll want to thin the seedlings to the two or three strongest plants. Competition among seedlings is not a good thing.
To thin seedlings, use scissors or a clean razor and snip off the seedlings close to the soil surface. Don’t pull the smaller seedlings out from the top! This can disrupt the soil and you might end up holding a clump with ALL your seedlings. Which would suck.
When you’re thinning seedlings, it’s OK to leave two or three in every small pot, but it’s also fine to just pick the healthiest seedling and snip the rest. Ultimately, you only want one healthy tomato seedling per pot—they’ll need room to grow when they’re adults.
Around Day 14, you might also need to step up the pot size, depending on how you started the seeds. If the seeds are in a flat or peat pellets, transplant them into 4″ Jiffy pots. When you transplant the young seedlings, use a razor or sharp scissors to snip off the bottom leaves, including the embryonic leaves (if they’re still attached). Plant the seedlings slightly deeper than they were before, actually burying part of the stem. New roots will actually sprout from the buried portion of stem, making for a stronger plant in the long run.
Next: Feeding Tomato Seedlings