Tomato pruning typically involves the removal of side growths called suckers that sprout from the joint between branch and stem. Too many of these cause the plant to sprawl. By careful pruning you can correct this tendency, creating a custom-plant architecture conforming perfectly to your greenhouse, closet or other specialized hydroponic environment. Thoughtful pruning creates space between the branches, allowing better airflow and easier manual pollination. Most importantly, it helps concentrate the nutrients you've painstakingly added to the system's reservoir into fewer but plumper tomatoes.
Verify your tomato variety is an indeterminate. Indeterminates include such varieties as Supersonic, Big Boy, Big Beef and other beefsteaks. They grow as large as you'll let them, producing many branches and suckers. They will benefit from a thorough pruning.
Choose one to three suckers sprouting from the base of the plant to preserve. These, along with the original main stem, will produce all of your tomatoes.
Remove all other suckers. Allow each to grow to about 3 or 4 inches long before gently breaking them off at the base by hand.
Verify your tomato variety is a semi-determinate. Semi-determinates include Celebrity and Mountain Pride. Not as compact as determinates nor yet as sprawling as indeterminates, they will produce many suckers but only grow to between 3 and 5 feet. They prefer a more restrained pruning.
Wait for the plant to reach a height of 8 to 10 inches and begin producing flower clusters, also known as fruit trusses.
Find the first (lowest) fruit truss and identify one or two suckers immediately below it. Remove all suckers below that.
Repeat the previous steps about seven to 10 days later, if necessary.
Allow all other suckers to flourish.
Support and Maintenance
Support remaining stems from above with plastic twine or a plant yo-yo suspended from an overhead wire, clothesline or closet hanger bar. This will encourage vertical growth.
Remove leaves as they begin to yellow and wither near the bottom of the plant. This further encourages air flow and helps prevents infection.
To prevent sunburned fruit in a greenhouse situation, allow five to seve leaves to grow above the last (highest) flower cluster you want to produce fruit. After this, remove all new fruit trusses from above these shade-providing leaves.