It is important to cut back tomato plants to ensure they stay healthy, maintain their size and encourage new fruit growth for the future. There are two types of tomato plants: indeterminate and determinate. For vine tomatoes that grow on a trellis and reach more than 9 feet tall, choose indeterminate plants. Determinate plants are compact and bushy, growing to 4 feet. Each requires pruning with simple tools.
Clean your pruning scissors thoroughly with an old rag dipped in rubbing alcohol to sanitize them. This is important because dirty pruning equipment can spread harmful bacteria to the tomato plants, causing disease.
Cut back any dead or broken branches. Make sure they are removed all the way to the trunk of the tomato plant.
Pinch off the tips of the tomato branches throughout the growing season to encourage fruit production and healthy, full growth. Each time you pinch a tomato branch, only trim off the last set of two leaves.
Cut back tomato branches that are not blooming or that you have noticed aren't producing any fruit. Also look for branches that are crowding each other, criss-crossing or intertwining. You don't want any tomato branches to be on the ground; they should all be suspended from the plant or a trellis.
Cut back to the trunk every third branch if there is a particularly crowded area. Don't thin out the plant too much because the fruit needs shade. You want the tomato plant to look full of healthy branches but not overcrowded to the point where you can't identify individual branches.
Use the pruning scissors to remove all leaves, suckers and small stems before the first flower set on the tomato branch. A well-pruned tomato plant should present all its leaves up toward the sun.