As the heat of summer ascends and and spring crops wither and fade, tomato plants take off, and often become unruly garden residents. Most heirloom tomatoes are indeterminate, meaning they keep growing and growing, sometimes reaching upwards of 6 feet high if trellised. Just as you would with fruit trees and bushes, these varieties of tomatoes can be pruned and trellised to yield a higher volume of better tasting fruits. Since indeterminate varieties set so much foliage, the idea behind pruning is to cut back and shape the foliage as much as is reasonably possible in order to direct the plant's energy and nutrients to its fruit.
Striking a balance between taking away too much and too little is key. Here is how we approach pruning, based on years of taming the tomato plant:
Identify what to remove. The first step in pruning is to identify what to take away, and what to keep in tact. The tomato plant has a main stalk, which is also known as the leader. The leader is always preserved when pruning. Leaf branches grow on the leader stalk, and are the main leafy foliage along the stalk. Between the leader stem and the branches grow suckers, shoots that can grow and form their own branches, suckers and fruit.
When pruning tomato plants, you are always removing the suckers. I leave the main stalk plus one sucker; you might choose to prune to only one stalk, which can result in more air flow and larger fruit, but less foliage. More foliage can help prevent sun scald. Pruning is about striking a balance between striking a balance between taking away too much and too little. No matter which method you choose, always leave the branches on the main stalk intact, unless they are yellow or diseased.
Prune early and often, in the beginning. A few weeks after transplanting, you will notice that your tomato plants are full, tall and filling out with many branches and suckers. This is the most important time to do your first pruning! By catching suckers early and often, the plant's energy is most efficiently directed to the main stalk. If you wait too long, the suckers grow into thick, lush stalks that have taken up a lot of energy. Remove unwanted suckers every week or two for a few weeks. If you trellis with the basket weave method, it is helpful to time your pruning when the plants are ready for another layer of weaving.
Have a blade on hand for bigger suckers. Tiny suckers are easy to pinch off, but you can easily damage and tear the plant snapping off larger ones. Have a small knife, snips, or orchard pruners on hand. Anything that is very large and fruiting might actually have a negative effect if removed, so consider leaving very thick stalks, and be more diligent about pruning tender suckers in the future.
Scale back heavy pruning at fruit set. Once you get used to pruning, it might be hard to stop! However, some moderation is needed to keep a good balance with your plants. In the beginning of the season, it is very important to prune, but as the season progresses, heavy pruning is not as necessary, especially if you do a good job shaping and staking the plant. In fact, by the end of the season, I generally just remove any suckers that don't behave in the basket weave trellis. Having additional foliage at heavy fruit set can prevent sun scald and enables the plants to have more fruiting stalks. This is especially nice if the season happens to be a bit longer. Remember, the plants needs foliage to survive, just not all of it.
Don't prune determinate varieties. This post covers indeterminate tomatoes, but there are 3 more classifications: Semi-determinate, Determinate and Dwarf. Treat semi-determinate tomatoes grow like indeterminate ones. Determinate varieties should not be pruned. Dwarf tomatoes are determinate as well, and should not be pruned either, except to remove any yellow or diseased leaves.
Don't prune plants when wet. Keep your plants healthy and only prune and and stake them when they are completely dry. Most devastating tomato diseases spread through water contact.