My tomato plant is about 6 months old and about a foot and a half tall.
How often should you prune your tomato plant during this stage? Is it possible to over prune the plant? I usually use the pinch method of removing stems.
About my tomato plant. This plant is potted and it lives in the hot sunny weather in north Texas.
As a general rule, you can prune tomatoes every 7-10 days.
Yes, it is possible to over prune the plant. If you remove too many stems, you will reduce production. And you don't need to prune a determinate tomato -- they will stop growing at a certain height.
I wouldn't prune yours. At 6 months old, I'd expect it to be much larger than 18". Is it fruiting? (Mine are about 3 months old and 4-5' tall.)
What to prune is a lot more important than how often. If you know what to prune, you can't prune too frequently. Then you're better off pruning when sucker growth (and whatever else you want to prune) is small, so you reduce the amount of wasted growth. So it really depends on how fast the plants are growing.
In July, if the weather is good, and they're really taking off, I try to hit them a couple times a week. (I'm in New Hampshire.) Once a week would be fine, except I sometimes overlook suckers, and then I hate to prune them when they're a foot or more long, which they would be after two weeks. Once fruit growth has taken off in August, the plants slow down.
For bigger suckers, use pruning shears, not your fingers, to avoid pulling off a long strip of "skin" off the remaining stem. Which brings me to...
Fungus. If you get unexplained, sudden, yellowing and die-off, it's probably a fungus disease. If you can afford to pull the plant, do so immediately. If not, be careful when pruning nearby plants - try to minimize the chances that you'll introduce fungus spores into the wound.
