Tomato plants are either determinate or indeterminate. Determinate means they produce tomatoes over a short time frame and then decline. Patio tomatoes are an example of determinate tomatoes. Most modern hybrid types of tomatoes are determinate. Indeterminate tomato plants, such as heirloom tomato plants, are large vining plants that produce tomatoes throughout the season and need occasional pruning to direct their energy into growing a few bigger and better tomatoes than many small ones and lots of foliage. An example of an indeterminate heirloom tomato would be the Cherokee Purple or Brandywine variety.
How to Prune Heirloom Tomatoes
Prune off all bottom leaves and small branches of the tomato plant up to the first flowering branch or fruit cluster. This removes disease-prone lower limbs that are shaded and directs the plant's energy to the top of the plant. Also, sunlight can't reach the lower leaves and branches.
Look for suckers or new growth that appears in the crotches between the leaves and the main stem. Small ones can be thumped or pinched to remove but larger ones need to be cut with the cutting tool. Train plant to have a main stem and only two or three branches. The more branches you have, the more numerous and smaller the tomatoes and the weaker the plant. Dip cutting tool into bleach mixture and rinse with water between pruning of each plant to prevent the spread of disease from plant to plant.
Top plant one month before the first frost of the season. This is done by removing all top growth down to the limbs where large green unripe tomatoes are forming. This directs the plant's energy to the developing tomatoes rather than the new growth, which will die once temperatures get below freezing.