Determinate tomato plants grow to a predetermined size, typically 1½ to 2 feet, and produce fruit at one time. Indeterminate plants continue to grow throughout the summer, reach heights of 4 to 6 feet and set new fruit until frost. These massive plants require pruning to maintain shape and prevent fruit from setting on late in the season when it will not have time to mature.
Prune tomato plants early in the morning to prevent stress from the afternoon sun and allow the cut to dry and heal during the day. Vines snap easily in cool morning temperatures making a clean break and reducing the chance of disease.
Remove new shoots as they appear between the Y formed by the main stem and branches. Pinch or break shoots free of the stem or they will create a large sprawling plant. Pruning directs the plant's energy to producing fruit.
Monitor plant growth and prune new shoots before they reach 4 inches in length to keep the plant in shape and encourage fruit production.
Prune blooms that appear late in the season by breaking the flowering tips. Although the blooms will produce fruit, they will not have time to mature and ripen. Pruning them sends energy to the existing fruit, increasing size and quality.
Control fruit production on clustering tomatoes by pruning all but three or four of the healthiest fruits to provide room for fruit to grow and ripen. Use care not to damage existing fruit as a tiny break can develop into a large scar when the fruit matures.