Prune tomato plants for better health and better fruit by Frank Ferrandino
A properly pruned and supported single-stem tomato plant presents all of its leaves to the sun. Most of the sugar produced is directed to the developing fruit, since the only competition is a single growing tip. The result is large fruits that are steadily produced until frost. If more stems are allowed to develop, some of the precious sugar production is diverted from fruit to multiple growing tips. Fruit production, although slowed, never stops. The result is a nearly continuous supply of fruits throughout the season. In general, more stems means more but smaller fruits, which are produced increasingly later in the season. (This is much less applicable to determinate plants, due to their shortened growing season and better-defined fruiting period. Therefore, determinate plants require little pruning. See "Indeterminate vs. Determinate," below.
Indeterminate vs. determinate
Indeterminate tomato plants continue to grow, limited only by the length of the season. These plants produce stems, leaves, and fruit as long as they are alive.
Determinate tomato plants have a predetermined number of stems, leaves, and flowers hardwired into their genetic structure. The development of these plants follows a well-defined pattern. First, there is an initial vegetative stage during which all the stems, most of the leaves, and a few fruit are formed. This is followed by a flush of flowering and final leaf expansion. Finally, during the fruit-fill stage, there is no further vegetative growth. As the tomato fruits ripen, the leaves senesce and die. Commercial growers favor this type of tomato because all the fruit can be mechanically harvested at once.