Trimming or pruning leaves and excess growth from tomato plants helps produce sturdier plants and higher fruit quality. There are two types of tomato plants: Indeterminate tomatoes that produce tomatoes most of the growing season and determinate tomato plants that grow to a certain size and produce fruit during a period of about four weeks. Since the determinate type has a determined growing period, the only leaves that are usually trimmed are the lower leaves up to the first blossom cluster. However, you need to trim the leaves from indeterminate tomato plant types several times during the growing season.
Look at the tomato plant when it is 12 to 18 inches tall and decide what size is best for your growing space. Large indeterminate tomato plants need 36 inches to spread and most grow at least 48 inches tall, but you might allow it to grow taller or train it into a shorter plant. Even with trimming or pruning, the plants will need the support of a trellis, stake or tomato cage.
Look for new growth that is growing out of the areas where the leaf sets join the main stem. These areas are called "crotches." The leafy growth growing from a crotch is called a "sucker." By managing the growth of the suckers and leaves you channel the plants energy into creating more fruit rather than more leaves. The best way to remove the leaves and suckers is to thump them off with your thumb and forefinger. This prevents disease from spreading from one plant to another because you are not touching the stem. Another way is to pinch them off with your thumb and forefinger, but wash your hands between plants to avoid spreading disease.
For a sturdy plant, you will trim the new growth of the tomato plant so there are three or four main branches coming off the main stem. This is done by allowing three or four of the suckers growing from the plant's crotches located near the bottom of the plant to remain and grow. This will create a strong compact bush that is easier to manage and prevent the plant from drooping when heavy with fruit. When the plant droops it allows the fruit to touch the ground where it can become diseased or eaten by insects. Do not trim the main stem, or leader, at this time.
Remove or trim the leaves from the bottom of the plant up to the point where your main stems are located. Keep any new growth in this bottom area removed during the growing season. The bottom leaves are where insect and disease problems usually begin because they are located close to the soil where insect and disease pathogens live.
Continue to allow the growth of the main stem and the branches you have chosen, removing any new leaves or suckers, until the plant reaches its desired height, usually 4 to 5 feet. When the plant reaches the desired height, pinch off the tops of the main stems and any new leaders and suckers that try to form along the stems. After trimming the top off the plant a few times the plant will stop attempting to grow any taller and begin bushing out, producing a large amount of bloom clusters. Allow the bloom clusters to form but continue to thump out any suckers that attempt to grow during this time. If you cannot tell the difference between a sucker or a potential bloom cluster, allow the sucker to grow a set of leaves or two before removing, so you can be sure if it a bloom cluster or not.