Answer: I'll try to explain the process to you. 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.
As a tomato grows, side shoots, or suckers, form in the crotches, or axils, between the leaves and the main stem. If left alone, these suckers will grow just like the main stem, producing flowers and fruit.
Suckers appear sequentially, from the bottom of the plant up. The farther up on the plant a sucker develops, the weaker it is, because the sugar concentration gets lower as you move up the plant. On the other hand, side stems arising from below the first flower cluster, although stronger, compromise the strength of the main stem. For a multi-stemmed plant, your aim is to have all stems roughly the same size, although the main stem should always be stronger, because it has to feed the entire plant for the next five or six months. Here's how I achieve this.
I keep tomatoes free of side stems below the first fruit cluster. When trained to one vine and left free-standing, tomato plants develop strong main stems. To encourage a strong stem, I remove all suckers and I don't tie plants to their supports until the first flowers appear.
Determinate tomatoes need no pruning other than removing all suckers below the first flower cluster, because pruning won't affect their fruit size or plant vigor. If you do any pruning at all above the first flower cluster on determinate tomatoes, you'll only be throwing away potential fruit.
Indeterminate tomatoes can have from one to many stems, although four is the most I'd recommend. The fewer the stems, the fewer but larger the fruits, and the less room the plant needs in the garden. For a multi-stemmed plant, let a second stem grow from the first node above the first fruit. Allow a third stem to develop from the second node above the first set fruit, and so forth. Keeping the branching as close to the first fruit as possible means those side stems will be vigorous but will not overpower the main stem.
Hope this sheds some light on the debate about whether or not to prune suckers off tomato plants!
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