Tomato plants come in two types. Determinate varieties only reach about 4 feet tall, then they stop growing upward. Indeterminate varieties continue to gain in height throughout summer and into fall, reaching heights of 6 feet or more. When trellising or staking indeterminate tomatoes, this height can be an issue, as they overgrow their supports. This can also lead to breakage and damage to the stem as the plants become top-heavy. Topping the plants encourages more lateral branching and helps prevent the plants from growing too tall.
Remove the suckers on the tomato plant so it only produces one to two main stems. Pinch off the shoots that form between where the leaf joins the main stem. This prevents them from growing into multiple main vines and makes the tomato easier to tie to a stake or trellis.
Prune off the top of the main vine when it reaches the desired height for the plant. Use clean, sharp pruning shears to make the cut so the plant isn't damaged.
Pinch off the any new suckers that form where you topped the plant, as these can become new stems. Pinch them off where they form between the junction of the leaf and the topped stem or where lateral stems join the main stems.