I never knew it until I tried planting tomatoes, but tomato plants are fast growing, almost weed-like in their enthusiasm. If you have any intention of getting crops of good, healthy, sweet tomatoes, you had better know how to prune them. I learned how to do so by viewing this video (2:51). Although it is titled How to Prune Tomatoes, it could also have been called Take Out Those Suckers! It’s a brief and handy rundown by a seasoned tomato grower.
There are two types of tomato plants, according to Howard Anderson of Johnny’s Selected Seeds (johnnyseeds.com) in Albion, Maine who is interviewed in the video by Growing Wisdom’s David Epstein. The two types are determinate (grows to about 3 feet and stops) and indeterminate (grows like crazy all season). Check on the seed packet what sort you are buying. It’s the indeterminate type that needs to be pruned throughout the season, right from when the first sucker emerges. (A sucker, in tomato parlance, is a new shoot.)
Anderson is pretty certain that without consistent pruning, your tomato crop will be limited and the plants will be susceptible to disease. Pruning a tomato plant is, therefore, essential to tomato growing.
Anderson says you have to keep the tomato plant down to two leaders. (In tomato parlance those are the two main growth stems.) As the new suckers appear, you pinch them out and train the two leaders up the trellis. And you get rid of those suckers right down to the base, to invigorate the leaders and encourage fruit formation. (A tomato is a type of fruit.) You have to do this every week to ten days. You can do so at any time of day as long as the plants are not wet.
In the video Anderson shows a good way to make simple trellises. You plant ordinary stakes about 10 feet apart right down the row. Then you walk along with a ball of string speared at the end of a pole so it can wind off. Tie the end of the string on the first stake and walk down the line, winding the string twice around each stake as you go. Then train the plant up the trellis. As the plants grow, so you add string lines higher up the trellis.
About 30 days before the end of the first frost, pinch off the ends of the leaders i.e. the growing points. This forces energy into the fruit and inhibits new growth.