How close?

Are you pruning? Staking? Caging? Sprawling?

I cage and never prune. Huge monster plants.4 feet apart minimum. 5 feet even better.

I plant all my tomatoes 18 inches apart. These are planted in my high tunnels. I trim leaves and suckers.

If you are outside, I would go further.

Jay

General rule of thumb, if you're staking / caging, is to have 3 ft spacing between plants and 5-6 ft between rows. You can go a bit less, but the more space you can give them the better.

Tomatoes don't like to be crowded for a few reasons, but here's two:
1.) They don't like competition for sun, nutrients, or water.
2.) More space gives more air circulation and less likelihood of fungal diseases.

It's all about how much space you have, how fertile your soil is and your growing style. For me, I do 2ft. spacing between plants and 4 ft. between rows. I use 18" cages that 6 ft. tall and the plants only begin to crowd eachother by the end of the season, by which time I could care less. I've thought about increasing the spacing, but that would mean less varieties in the same space. I may get to that point next year, but for now it suits my needs just fine.

I tried tightening up my tomato spacing and got assorted fungal diseases.

Its always tempting to squeeze in a few more plants, but it never works no matter what sort of plant it is. :-(

I let most of my tomato plants sprawl and last year I planted them four feet apart in rows six feet apart and they grew into a jungle I couldn't get into to pick the tomatoes or water or fertilize them. This year I will plant them four feet apart in rows seven feet apart. Or maybe on second thought I will alternate row spacing at six feet and eight feet, which will give me a wide row access to all plants. I also plan to put down weed stop fabric on the pathways this year. The weeds and Bermuda grass were taking up too much time fighting them.