You'll need to learn about transplanting seedlings because after just a few days under the light, your young tomato plants will be four inches tall, have outgrown their cells and be ready to be transplanted to small pots. In fact, some experienced gardeners believe that tomatoes thrive when transplanted to successively larger containers. It may be because of the fresh nutrients in the soil mix along with the added room for root growth. Follow these steps:
Choose three- or four-inch plastic pots.
Fill the pots with a soil mix similar to that used for seed germination or regular potting mix.
Remove the seedling carefully from the seed flat. Cut out with a spoon or dull knife if necessary to keep the root ball intact. A small Rumford seedling tool may come in handy for lifting seedlings from flats. Rumford makes several small quality tools for handling plants in small containers.
Plant the tomato seedling in its new pot, holding by its leaves or root ball, rather than its delicate stem.
Press down on the soil gently around the rootball.
Water well then allow the plant a couple hours to adjust before setting back in the sunlight or under the lights.
Fertilize, water, and provide light as before until the plants are eight to twelve inches high and seven to eight weeks old.
The tool I'm using is part of the The Rumford Gardener Mini Indoor Stainless Steel 3-Piece Garden Tool Set which is available at Amazon.com.
If the plants appear to be getting too leggy, pinch back the tops. This will force the plants to grow stockier and sturdier, and also results in more blooms per plant.
When the average frost free date has arrived, and the soil temperature has reached 50 degrees F it's time to move your tomato plants outdoors. One week before, it's a good idea to "harden off" the young tomato plants by placing them in a protected spot like a cold frame. A wheelbarrow or toy wagon can also be used to move the young plants outside during the day and into the garage at night. This process will ideally firm up the plant tissues to withstand unpredictable and unfavorable weather conditions.
Now that you have made a great start on growing tomatoes from seed into healthy young plants, it's time for planting tomatoes into the garden or outdoor container!