When growing tomatoes as the plant matures, it will need to be " potted on" which means repotting the plant into a slightly larger pot. During the growing season, you may need to do this about 3 times from the small plug plant stage, until the tomato plant reaches a size large enough to go into full size pot or grow bag.
It may be tempting to save time and put the tomato into a larger pot at the beginning, but this not ideal. If you put a small plant into a large pot or grow bag, it will not thrive. Pot the tomato plant into a slightly larger pot each time increasing the size of the pot by a few centimetres, doing this 2/3 times until planted into its final large growing container.
It is a good idea to plant tomatoes deep, especially if your seedling is a bit on the weedy side. The only exception is in respect of grafted plants. For these plants it is very important that the graft is above the soil level.
The last pot into which you plant your tomato will be the growing space for the rest of the summer. This means you should select a good sized pot with a diameter of between 20- 30 cm and the larger the better. Grow bags are promoted as a good growing medium for tomatoes. They are convenient, but the problem is the depth of compost is shallow, which is not ideal. In addition, this can cause problems when watering, as the water has a tendency to run off. With time the bag also has a tendency to flatten, increasing the difficulty in watering.
For tomatoes, regular watering is essential and irregular watering impairs the flavour and can cause the skins to split later in the season. It is important to get the water to the plant's roots. If you are using grow bags, to increase the soil depth, which will help with the watering problem, use tape to strap up the bag to make the bag more rounded.
You can grow tomatoes in a greenhouse or outside, but tomatoes need a warm, sunny sheltered spot and not all gardens or parts of the country enjoy these conditions. An alternative is to grow in a greenhouse or poly tunnel which creates an ideal growing environment for tomatoes, and offers protection from blight.
Blight can be a real problem but is a rare visitor to greenhouse-grown tomatoes. If planting outside, pick the most sheltered spot in your garden such as a sheltered sunny patio.
When potting on a tomato plant is a good time to add a water bottle for effective watering, see below.
Also a short 2m.30sec video on illustrating how to pot on tomatoes