Trimming back tomato plants, better known as pruning, promotes healthy and productive plants. Both determinate and indeterminate tomato varieties require it, although the way you prune changes for each. Some stages of pruning go hand-in-hand with staking your growing tomato plants. Properly pruned tomato plants grow more vigorous, edible fruit more quickly. They also present greater resistance to disease in addition to the aesthetic benefits of pruning.
Establish whether your plant is an determinate or indeterminate tomato plant. Determinate tomatoes require less pruning because they have been created to adhere to exacting specifications regarding fruiting and stem growth. Indeterminate tomatoes require pruning throughout the season. Semi-determinate tomatoes are a hybrid of the two types, and require a median amount of pruning throughout tomato season.
Observe your tomato plant growth. When the first fruit clusters appear, cut any side stems growing below the first cluster of fruit on all types of tomatoes. Use a sharp razor blade or your fingers to trim the clusters. Removing these side stems causes the main stem to become stronger and sends more sugar to the growing fruits. Additionally, it reduces competition among your tomato's leaves for sunlight, as there will be less of them.
Stake your tomato plants once the vines begin to grow along the ground. Leaf exposure to soil promotes disease and spotting. Additionally, all tomato leaves should get some sun for optimal growth.
Train your tomato vines to stakes using soft, thick twine or rope. In a pinch, tights or pantyhose will work as well. Whatever you use should be thick, as thinner string will cut fragile tomato vines.
Prune indeterminate plants as soon as you see side stems and suckers starting to come off the main stem. Allow up to three or four central stems in total per plant, but any more than that will sap the strength and disease-resistance of your tomato plants.
Prune the tomato plants one last time about a month before the first frost. Take off all leaves above the topmost fruit clusters. This sends all the plant's remaining vigor into the fruits, making them grow and ripen much more quickly. This is your best chance of getting one last tomato harvest in before the end of the growing season.