Determinate tomato plants are those which develop to a predetermined size, produce a harvest of fruit and perish. Indeterminate tomato plants continue to grow, producing new stems and fruit during the end of the season. Indeterminate tomato plants are the only ones which need regular pruning. Before beginning pruning your tomato plant, you need to understand the nature of the suckers that develop between the primary stem and the leafy side shoot. Each sucker has the potential to develop into another main stem which produces fruit and leaves. Should you prune off each the suckers, you will have a strong, vigorous plant with fewer but bigger fruit. Most gardeners compromise by removing all but three to five suckers so the plant can create plenty of fruit without extreme leafy growth.
Snip away all the clusters of flowers that appear before the plant is 12 to 18 inches tall with pruning shears. Following the plant reaches this height, then allow the next cluster of blooms to develop. Snip off all lateral branches below the growing cluster of blooms.
Cut the suckers just over the second leaf in the underside. Leaving a couple of leaves on each stem helps shade the fruit and raises photosynthesis. Leave three to five suckers to develop into main stems.
Eliminate broken, diseased and damaged stems when you can. Damaged areas provide an entry point for infection, and diseases spread quickly when contaminated tissue stays on the plant.
Prune a last time about a month before you anticipate the first autumn frost. Eliminate each the growing tips to divert the sugar which the plant produces to the fruit and encourage ripening.