(last updated March 29, 2019)
A funny thing happens when you prune an entire greenhouse full of tomato plants: you become a green thumb—literally. You will also have a couple green fingers, and they will remain green for a few days despite lots of soap and water. Home gardeners, however, do not have the “green thumb” problem because it happens only after pruning more than a few tomato plants. Digit discolouration aside, I like everything about pruning tomato plants, from the distinctive smell of the plants to the hands-on work, and the thought that this was a small effort—a moment or two per plant—will result in an earlier harvest or larger tomatoes.
Which Tomato Plants to Prune
Prune only indeterminate varieties—the ones that grow tall and usually need staking. If you do not prune an indeterminate variety, you will have large, sprawling plants with smaller tomatoes that mature slightly later in the season.
Never prune a semi-determinate or determinate tomato plant. Pruning them will result in distorted plants with very few tomatoes.
How to Prune Tomato Plants
Pruning tomato plants simply means pinching off the shoots or “suckers” that grow out from the stems, in the joint right above a leaf branch. If you catch shoots when they have just formed, you can simply rub them out with your thumb.
Small suckers can usually be pinched off with your fingers; with ones that are a little larger you should use scissors or a sharp knife.
If you leave a sucker to grow, it becomes another big stem and takes energy away from fruit production. Tomatoes usually ripen slightly earlier on pruned plants.
Tomato plants grow very quickly when the weather is warm. New suckers are produced constantly. Prune at least twice a week during the peak of the growing season.
Never prune above the top blossom cluster to avoid accidentally pruning out the “leader” (main growing stem). Removing the leader prevents the plant from adding height and limits the potential yield. This is called “topping” and is useful later in the season, but it is not something you want to do early on.
If you accidentally break off the leader or main stem while your tomato plant is still small, get a new plant, because the broken one will never amount to much. When a leader is broken off an indeterminate plant that is already a good size, you can choose one of the suckers nearest the top and let that side-shoot become the new leader. Determinate plants with broken leaders will simply bush sideways.
Do not prune foliage, except to remove yellowing or brown leaves at the plant’s base. Tomatoes do not need to be exposed to the sun in order to ripen; rather, it is the sun on the plant’s leaves that leads to ripened fruit.