The function of trimming back tomato plants is to save plant energy to produce larger fruit yields. Tomatoes grow rapidly and require an abundance of nutrients. Left untrimmed, plants quickly grow stems that deplete the soil of nourishment. There are fewer blooms and thus fewer tomatoes per plant without trimming. Do not be afraid to trim back tomato plants because if they could speak, the plants would thank you.
Pinch-off the lower 1/4 of plant leaves after the tomato seedling is 1 foot tall. Grasp individual leaves with your fingertips and pull straight out toward you to avoid stripping the tomato stem.
Remove any stems growing in the lower 1/4 portion of the tomato plant. Use sterile pruning shears to make a clean cut flush with the main tomato plant stem.
Trim away all smaller stems growing inward or crossing over larger stems. Remove up to 1/4 of the total plant stems. Make clean cuts flush with the main stem or the larger stems.
Wait a week or until the tomato plant produces flower buds. After the first flower bud opens into a flower, pinch-off any new leaves or stems on the lower 1/4 of the tomato plant. Trim away any weak, yellow or thin stems with no flower buds or flowers.
Water the base of the tomato plant after each trimming until water drips from the bottom of the container.