Potting on Tomatoes

[Garvey]

I understand you should not pot up tomatoes into big pots too early,but pot them on into gradually larger pots.
Is this true and why?

[rustylady]

It is true and the same goes for any plant.

If you put a small seedling or plant into a large pot full of cold wet compost the roots will rot, and the compost can go sour.

[Two_Sheds]

Also, some plants positively thrive on having restricted roots (agapanthus, for one)

[Stonewaller]

It is true and the same goes for any plant.

If you put a small seedling or plant into a large pot full of cold wet compost the roots will rot, and the compost can go sour.

Are you sure? Why doesn't this happen when seeds germinate in the ground then? Effectively it's the same thing - a large area of cold wet soil.

I'm short of windowsill space this year so I potted my toms straight from the 2" pots they germinated in to the 9" pots they are in now and they're doing fine. Admittedly indoors on the windowsill, but there's no etiolation and they have flowers.

[Creemteez]

Generally speaking a seed will germinate when the conditions are right for it, and in open ground once the process has started the seedling is acclimatized to it's surroundings. Moving a seedling from a small quantity of warmed compost in a seed tray to a large quantity of cold, wet compost could prove a shock, but if the compost is warmed - i.e left in a green house for a few days, the shock will be minimized.
I'm sure I have read a thread on here where someone planted their tomato seedlings straight into their final pots, but only using a small quantity of compost so the plants were at the bottom of the pot. As the plants grew, they added more compost - effectively "earthing up" around the stems and encouraging extra roots to develop from the stem - until the pots were full. I believe it was quite a successful experiment.

[Alison]

Are you sure? Why doesn't this happen when seeds germinate in the ground then? Effectively it's the same thing - a large area of cold wet soil.

I'm short of windowsill space this year so I potted my toms straight from the 2" pots they germinated in to the 9" pots they are in now and they're doing fine. Admittedly indoors on the windowsill, but there's no etiolation and they have flowers.

The reasoning you've been given is sound although that doesn't mean that the you can't be successful potting on in a big jump, but it is more risky. What I don't understand though is that if you're short of space why you've gone up to a 9" pot as they'll be taking up much more room than the natural pot on size from a 2"?