Answer: Whether or not you can prune depends upon the type of tomato you're growing. Tomatoes have three different growth habits: determinate, indeterminate, and semi-determinate. Determinate types are relatively small, often referred to as bushy or compact. Each short branch ends in a flower cluster and the plant does most of its growing before any fruit are set. The tomatoes tend to ripen at once, usually over a short 2 to 3 week period. After most of the tomatoes are harvested the plant yellows and additional production is limited. If grown upright, the plants are seldom more than 2 - 3 feet tall. Some examples of this type include Pik-Red, Early Girl, and Super Bush. In general, many of the earliest varieties are determinate types.
Contrast that growth habit to indeterminate types. These are the traditional, large home garden varieties. They produce plants as large as you will allow them to grow. They have many widely spaced branches, numerous suckers, and produce tomatoes all season long. These are the varieties that can be manipulated in all sorts of fashion to make the plants conform to your needs. Examples of this type of growth habit include Big Beef (most of the beefsteak types), Supersonic, and Big Boy. In between these two are the semi-determinate types. They will produce suckers like indeterminate types but not as many and the plant will grow between 3 and 5 feet. Examples of semi-determinate types include Celebrity and Mountain Pride.
Knowing the growth habit of the variety you are growing is critical in determining whether a plant can be pruned and the level of pruning. Indeterminate varieties will have many suckers and branches, each producing many flowers and eventually fruit. These can be pruned and pruned severely. Determinate types, however, are pruned slightly if at all. Any pruning done on a determinate removes a finite number of blossoms and fruit. If you prune all the suckers on a determinate type you will have a small plant, few fruit and lots of sunscald due to a lack of foliage and shading. You will also dramatically reduce your yield. Semi-determinate types can be pruned but not nearly as much as indeterminates.
What's a gardener to do? Remember, you can grow perfectly fine fruit without pruning your plants. But if you want to prune, here are a few guidelines. For determinate types, there is no need to prune at all. For indeterminate types, allow one, two, or three suckers to grow from the base of the plant. Each of these will become a main stem with lots of flowers and fruit. Prune off all the others suckers and provide the plants with strong support. Research has shown that the best time to remove suckers is when they are about 3 to 4 inches long. For the semi-determinate types, limit your pruning. When the plant is 8 - 10 inches high, look carefully and observe the first flower cluster on the stem. Remove all the suckers below the flower cluster except for the one immediately below the cluster. You may have to go back and give these a second pruning 7 to 10 days later. Remove no more than that or you run the risk of pruning too much. The amount of pruning among these varieties to produce optimum yields varies. Some varieties would do better if you left 2 suckers below the flower cluster. Experiment and see which works best for the variety you are growing. Hope this is helpful.
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