Repotting seedlings in general is fairly straightforward. Just plop the plant out, put it in a new container, and refill with soil.
But tomato seedlings are unique in that they like to be buried deeply. They have the ability to form new roots along their stems, so they can be repotted up to their lowest set of leaves. A bigger root system means a healthier and more robust plant.
Tomato seedlings are ready to be repotted when they are at least 3 inches tall, and have a couple sets of “true leaves,” the second and subsequent sets of leaves that appear. The first leaves that sprout, cotyledons, are not leaves at all, but embryonic structures from the seed that provide nutrition until the seedling can make its own food.
Start with clean 4-inch pots and pre-moistened potting mix. If several seedlings are growing in the same pot, some people will snip off the extras and keep only the strongest seedling, so as not to disturb the roots during transplant. But if you repot them before they become rootbound, it’s quite easy to separate the seedlings without damaging the roots.
Water your seedlings to loosen up the potting mix and keep the roots moist while you work. If you started your seedlings in newspaper pots, unroll them. The roots should be fairly developed and free.
Separate the seedlings. Always handle them by their leaves, not their delicate stems. If a leaf pulls off, chances are it will grow back. But if the stem snaps, your seedling is done.
Separate the seedling by gently pulling on its leaves and wiggling it away from the potting mix. The roots should release easily. Let the moist potting mix cling to the roots to protect them from drying out.
Fill the new pot with fresh potting mix, and sink in the seedling to its lowest set of leaves. Pat down the mix gently and water.
Keep your newly repotted seedlings out of direct sunlight for a day or two. Tomato plants are especially susceptible to overwatering, so keep the potting mix barely moist at all times. Water deeply to reach the roots at the bottom (or soak from the bottom up), and only water again when the first inch of the mix feels dry.
Tomato plants can be repotted two or three times before they go in the garden. Moving them into larger containers each time keeps them happy and gives their roots room to grow. If you are repotting a second time, you should pinch off the bottom two or four branches and sink the plant even more deeply into the pot to encourage new root growth along the stem.
And all that recycled newspaper you unraveled? You can recycle them one more time by adding them to your compost!