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I've been growing my tomatoes up a 6ft trellis of nylon netting for the last couple of years, with good results. But they tend to grow out of control, eventually toppling over the trellis and breaking their branches, and parts of the branches will die out. I've never really bothered with pruning them unless a vine looks diseased or like it's dying.

I've read that I really ought to be pruning the tomatoes to the "main vine", which will result in higher fruit yields. But I'm not sure how to tell which vine is the "main" vine (or if it really matters), and which vines are just branches. After a week, both vines look rather strong. And when both vines have flowers, I'm anxious about cutting away potential produce! Will pruning it really result in higher yields?

What rules of thumb do you use for pruning your trellised tomato vines?

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I always pinch out side shoots on mine and in years where all my neighbors have complained about how poorly their tomatoes are doing, I've had enough to give away. "No tomatoes this year? Take some of mine, I've more than enough. giggle" :) –  Niall C. Jun 11 '11 at 13:56
    
@nicholas what differentiates 'vine' from other tomatoes? –  David Jul 10 '11 at 17:46
    
@David - Nicholas is probably thinking about the difference between "bush" (determinate) tomatoes and indeterminate. You don't need to prune determinates since they'll stop growing at a certain size anyway. –  bstpierre Aug 25 '11 at 12:06
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3 Answers

up vote 20 down vote accepted

Check out this video. I use these techniques with my tomato plants with good results.

Summary:

  • Leave two leaders (main stems).
  • Prune all other suckers.
  • Start when suckers 3" length.
  • Prune every week to 10 days.
  • Don't prune determinate types ("bush" tomatoes).
  • Don't prune when wet to avoid spreading disease.
  • 30 days before frost, prune tips of leaders to force energy into fruiting.

The video shows techniques used at Johnny's Selected Seeds farm in Maine.

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Wow, that's a truly great video. I'll watch it again a couple of times before I go out and do it. Thanks! –  nicholas a. evans Jun 10 '11 at 16:30
    
Ok, I finally took the time to watch it... great video, thanks. Even if you already know how to prune tomatoes, you may learn something. –  bstpierre Jun 22 '11 at 20:42
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You will get more yield (total season fruit weight) if you don't prune. You might get bigger fruit (but less of them) if you do prune. By pruning, you're effectively thinning the fruit.

Tomato Cultivar Trial and Pruning Observation

The most striking difference at harvest was larger fruit size (.69 lb vs. .55 lb) from pruned plots. However, pruning also reduced the number of fruit harvested by 32% and total marketable weight by 15%.

http://www.public.iastate.edu/~taber/Extension/Progress%20Rpt%2000/tomato.pdf

Its possible to get bigger fruit by pruning, but its not guaranteed. I've read studies saying there was no significant difference between fruit size of pruned and unpruned plants. Though there is always a significant difference in total yield.

Removing side shoots may reduce overall tomato harvest. In fact, in a study by Purdue University, and published in Organic Gardening Magazine, scientists found that removing side shoots was shown to increase the average fruit weight some of the time but did not increase the total harvest for each individual plant.

http://www.grow-tomato-sauce.com/pruning.html

If the fruit size you have now is not big enough, you might try pruning. If the size is acceptable, I wouldn't suggest pruning... I would suggest a more sturdy trellis.

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Pinch for sure. Another trick is to let the plants grow in pots and when you transplant, bury them four inches deeper. This promotes more root growth. Stay on it. Pinch frequently until they are producing.

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