Tomato plants bear most heavily when unnecessary shoots and branches are cut back, directing the plant's energy toward producing tomatoes rather than foliage. Prune plants back several weeks after they are established, again mid-season at the height of growth, and a third time several weeks before season's end to make your plants their most productive.
Examine plants for shoots growing low on tomato stems during the first month of growth. Cut off any long shoots or sprouts coming from ground level or up to 6 inches along the main stem. These shoots, commonly called suckers, sap the energy of the main stem and branches without ever gaining enough strength on their own to bear fruit. Above 6 inches on the stem, determine whether such shoots are part of main branch clusters or likely to remain solitary and spindly.
Prune plants again when they are close to mature height, blooming, and setting fruit. Remove or shorten any branches whose length and weight could split or break the plant in case of heavy rain or wind. Cut such heavy branches that are already setting fruit back close to fruit clusters.
If possible, rein in the sprawl of these trimmed branches within a tomato cage or anchor them to a stake with flexible ties.
Prune plants again several weeks before the end of the growing season. Cut back the tops of plants that show abundant leaves but few blossoms--even though you hate letting those blossoms go. Remove leafy branches that show no signs of flowers or fruit. During this final growth period, plants need to put waning energy into the growth and ripening of established fruit.