"what size pot i should use to put tomato plants in"
They need to finish up in a 10", or perhaps a little larger. But it helps if you "pot them on" a couple of inches at a time. So if yours are in, say, 5" now as soon as the roots have filled the pot - i.e. you can see them coming out of the bottom - pot on into 7"-8", and when that is full of root then pot-in into your 10" pots
You could probably use a 5" pot with NO bottom placed onto a grow bag - then you water the 5" pot but the roots can grown down into the growbag. That may have been what the "Use 5" pots" person meant!
When you pot things on you normally keep the soil at the same level on the plant. Try not to disturb the plants root. I normally put a little compost in the new pot and then stand the plant, WITH its pot, on top and put compost around the old pot. Firm it down reasonable well, then twist the old plant pot so it comes out. i.e. you are left with a nice hole in the compost in the new pot that is exactly the right size.
Then I gentle knock off the old pot and put the plant's root-ball into the hole in the new pot. Press it down a bit to make sure everything is firm with no gaps, and then water it well.
You may need to replace the supporting cane too. I carefully remove the old cane, supporting the plant, with my third hand!, and put the new, longer, cane in the same hole to avoid damaging the roots. If the old cane is long enough I push it down to the bottom of the new pot.
Sorry, rambling on, but I wanted to make another point about keeping "soil level" the same when you pot-on. In the final pot (i.e. the 10" one) I keep the soil the same level, but plant low - leaving an extra inch or so at the top. The basal stem of the tomato will produce some roots in a month or so - and when that happens I top up the compost with another inch or so. (When that is done I STILL have an inch (i.e. the height of the "Lip" of the pot), to the rim of the pot for watering - otherwise the water will just run off the top! |