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?