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Pruning Tomatoes | Summer 2010 Out Here Magazine

Trim and train your plants for better health, increased yields

By Francis J. Ferrandino
Illustration by Tom Milner

The natural vigor and hardiness of tomatoes almost always guarantees a successful harvest. However, the rapid growth of a healthy tomato plant can also lead to problems.

A tomato is a solar-powered sugar factory. For the first month or so, all of the sugar it produces is directed toward new leaf growth. In the next few weeks, the entire character of the tomato plant changes.

If unsupported, the increasing weight of filling fruit and multiple side branches forces the plant to lie on the ground. By season's end, it will be an unsightly, impenetrable, disease-wracked tangle.

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, because the only competition is a single growing tip. The result is large fruit that are steadily produced until frost.

In contrast, more stems means more, but smaller, fruits, which are produced increasingly later in the season.

Pruning also affects plant health. Pruned upright plants have fewer problems with leaf spots and fruit rots because their leaves stay drier and free from pathogen-laden soil.

There's no one right way to prune and train your tomatoes, but these "rules" can help:

  • Get tomato plants off the ground.
  • Give plants plenty of room.
  • Never prune or tie plants when the leaves are wet.

The Techniques

Keep tomatoes free of side stems below the first fruit cluster. To encourage a strong stem, trim all suckers and don't tie plants to their supports until the first flowers appear.

When removing suckers, avoid the top 10 inches of the stem so that you don't remove the growing tip from the plant.

There are two ways to deal with a sucker that isn't destined to become a stem. The simplest is to pinch it off entirely; this is called "simple pruning."

This should be done when the sucker is still small and succulent. Grab the base of it between your thumb and index finger and bend it back and forth. The sucker should snap off, producing a small wound that will heal quickly. Avoid cutting the sucker with a knife or scissors, because the resulting stump can become easily infected. Once a sucker becomes too tough and leathery to snap off, however, you'll have to use a blade.

Another method is Missouri pruning, in which you pinch out just the tip of the sucker, letting one or two leaves remain, allowing more leaf area for photosynthesis and to protect developing fruit from sun-scald.

This is labor intensive, because suckers grow very quickly during hot summer months. It helps to know that side stems started late in the season will always be spindly and produce inferior fruit. You must be heartless and tip them all.

The payoff: large, robust, tasty tomatoes.

Dr. Frank Ferrandino is an associate scientist in the Department of Plant Pathology and Ecology at the Agricultural Experiment Station in New Haven, CT.