About a month ago I posted on starting seeds indoors. Seedlings develop quickly and mine will soon have true leaves. Basically, most seedlings start out looking the same to me with two smooth leaves. The next sets of leaves to develop are called true leaves and they look more like the leaves of the specific kind of plant. When your seedlings have reached this stage, it is time to transplant them into a larger pot. 
My seedlings aren’t quite ready yet this year (in this picture you can see the beginning of their true leaves, but I want until they are fully developed) so I’m using pictures from a couple of years ago to go along with the instructions of what to do next.
Reasons to transplant:
- Stimulation of roots and room for the seedling to grow
- Richer soil (you can use a commercially prepared soil mix for growing vegetables)
- Selection of the fittest (if you have multiple seedlings growing in the same pot you need to select the strongest one to keep and cut the other one off using scissors. Note that you should not pull out the weaker one because it may damage the other seedling)
How to transplant:
- Prepare the containers, labels and moisten the soil (for tomatoes use anything from a 2 to 6 inch pot) – it is best to have all the supplies you may need on hand so read ahead before you start
- Lift out the seedling using a piece of cutlery, a popsicle stick, pencil, screwdriver or similar tool – the trick is you want to be able to lift the seedling while causing the least damage to its roots – if you are going to hold the seedling by its leaves, hold the first (round) ones rather than the true leave (the pointy ones) and be sure to plant one at a time to make sure the roots do not dry out
- Replant the seedlings into the larger containers by placing some soil in the pot while holding it at an angle, then while holding the seedling with one hand fill soil around it with the other, when you are done pat the soil down gently so that the seedling has some support but the soil will not have all the air compressed out of it. With tomatoes only you can plant them as deep as the roots will allow you (make sure they aren’t touching the bottom of the pot or are coiled) and you can actually bury a part of the stem because new roots will grow from the stem, which is especially useful if your seedlings have become really tall. The stem of the tomato plant will grow roots once buried, but this isn’t the case with other types of plants.
- Watch your seedlings and keep them out of light and in a cool place at first. If there is any sign of wilting and they are already well watered you can set up a “greenhouse” but placing a seedling in a plastic bag to help rebalanced the moisture until they perk back up
After the transplant and a couple days of recovery there are a few things to keep in mind:
- Allow the soil to slightly dry before watering your seedlings, it helps prevent water logging the plants and promotes better root growth – if possible keep watering from below by adding water to the tray rather than pouring water on top of them
- You may want to consider fertilizing depending on the type of soil you used, but proceed with caution because seedlings can be sensitive (consider mixing fertilizers to half strength)
- The seedling will need plenty of light (at least 6 hours) – if you are using a window and you notice the seedlings reaching for the light occasionally rotate them
- If the seedlings are getting really tall really fast this is called getting “leggy” and the stem may be too weak – it may be caused by insufficient light, high temperatures, or crowding of plants – if you notice this problem or others, such as dropping of leaves, leaf curl, or discolouration, check out this great resource available online: http://www.green-seeds.com/pdf/seed_starters.pdf
The next stage will be “hardening off” the seedlings, which means gradually taking them outside for longer and longer periods after the last frost (for Toronto it is around May 9th) but before planting them outside permanently or sharing them with your friends and colleages.