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To prune or not to prune your tomato crop

By: Trish Grenfell, Placer County Master Gardener
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Why prune? From the University of Illinois Extension: “Pruning is the removal of small shoots that join the stem. This reduces competition between the suckers and the fruit. Pruned plants produce larger and earlier fruit as most of the plant energy is channeled into the fruit.”  In general, more stems means more but smaller fruits, which are produced increasingly later in the season.
A sucker is a small stem that is growing between the main trunk and stem of a tomato. It is usually growing at a 45 degree angle. Remove a sucker (see image) by taking it between your thumb and second finger and bending it to the side until it breaks. UC Davis advises to wait until the sucker has two leaves and pinch off just beyond those first two leaflets. The advantage of this method is that there is more foliage left for photosynthesis (food production) and better leaf cover to help protect the developing fruits from sun-scald. It will be necessary to check your plants weekly for sucker development.
The number of large stems you retain on your tomato plants depend on (1) whether your plants are determinate or indeterminate and (2) how you keep your plants off the ground.  
Pruning is more critical in indeterminate tomatoes than determinate ones. Determinate varieties have short- to medium-length branches and are not heavily pruned since they are self-topping, growing to a genetically pre-determined size and then stopping. All of the blossoms and fruit on a determinate tomato develop at the end of growing tips at about the same time. Some horticulturists instruct us to pinch all suckers from the ground level to the first flower cluster, as these stems are not productive. Others say don’t prune anything. Your choice.
Indeterminate varieties are heavily pruned when trellised, moderately pruned when staked, and lightly pruned when caged. Caged plants generally are pruned to four or five main fruiting branches. You choose the best stems to keep. As plants grow, keep turning ends of the remaining branches back into the cages.  
Limit staked indeterminate plants to two or three fruit-producing branches. A popular method is to select the main stem, the stem that develops immediately below the first bloom cluster (a very strong stem), and one other stem below that. Remove all other suckers.
Trellising is only for indeterminate varieties. Prune to just the main stem, or occasionally to the main stem plus one strong stem originating just below the first bloom cluster.
At the end of the growing season (about 30 days before a potential frost), top your tomato plant if you see immature fruit set on your plant. That baby fruit needs every opportunity to ripen. If you remove the growing tips, all sugar produced by the plant will go to the tomatoes and not to further leaf growth. Admit it. The season is coming to an end and you want those homegrown fresh tomatoes for your Thanksgiving table. Fight your gardening instincts and top that plant.
 
Trish Grenfell is a Placer County master gardener. Contact Placer County gardeners at 530-889-7388 or pcmg.ucanr.org/Got_Questions.