Answer: Pruning tomato plants is completely optional. There is no law in gardening that mandates that the pruning of all tomato plants. Although there certainly are advantages to pruning tomatoes, there are some disadvantages as well. Tomato plants produce growth called "suckers" which emerge between the main stem and a main branch. These suckers will eventually develop flowers and fruit but because they are positioned between a regular branch and the stem, they sometimes become so heavy they can break off the plant. I usually just leave my plants alone and allow them to grow at will. To keep them upright I either cage them or tie the branches to a wire fence or trellis.
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 you can take. 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.
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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