How you prune a tomato depends on if the tomato is of the determinate or indeterminate type. You need to identify which it is before you begin. This article assumes you have already started your tomatoes from seed or bought them from a greenhouse.
1) Determinate tomatoes are often called bush or trailing tomatoes. They tend to grow in a dwarf habit and seldom require staking. They tend to be more cold-tolerant and so are good for growing in containers outdoors. Also, they tend to bear their entire crop all at once. In Calgary, we usually get one crop from them in late august. Watch out around that time for an early frost which will ruin the fruit. Determinate tomatoes do not require pruning.
2) Indeterminate tomatoes will tend to grow in longer vines, and are often called staking or greenhouse tomatoes. They are ideal for growing in a greenhouse since they are both heat-tolerant and can produce fruit all season long. These tomatoes will produce “suckers” in between the main vine and the branch (at about a 45-degree angle) which you will need to prune continuously through the season. You do this so that the plant sends its energy to the flowers and fruit which are already developing, rather than trying to produce a new branch entirely.
You also prune the lower branches, starting about midsummer. When you do this, leave 12 branches, counting from the top, and never remove more than 1/3 of the plant’s branches/foliage at one time. When a bottom set of flowers has produced fruit, you can prune all of the leaves below that set. Doing this sends the plant’s energy to those fruit, not the leaves below them.
To “balance” your fruit, allow only 3 tomatoes from the first set of flowers, then 4 fruit, then 5. Eventually, the plant will be large enough to manage 6 fruit on the remaining sets until the end of the season.
Author: Jorgen Laursen