There are a couple of ways for approaching tomato growing in a greenhouse, you can either dig and prepare the soil of the greenhouse ready to accept the tomato plants or you can grow the plants in a container such as a plant pot (12" diameter) or growbag.
Realistically most growbags that are available which can be 35 or 40 litres in size are only enough to support two plants. There are ways in which you can push this out to 3 plants but for the cost of a growbag its probably as well just to stick to 2 plants, particularly if you take good care of them and produce lots of fruit.
If you go for pots then you should steralise them before use, to do this you can use a dilute solution of a suitable disinfectant, something like Jeyes fluid is perfect for this task.
While we are on this subject, you really should wipe down the inside of your greenhouse with disinfectant as well to ensure any residual contaminants that could also cause disease are removed from the glass.
One of the reasons I am suggesting using large pots or growbags for growing tomato plants is because the soil in the greenhouse is going to need to be dug out and replaced every 2 or 3 years anyway to ensure that you get rid of any build up of disease and also because the soil will be pretty much spent in terms of nutrients.
Space is limited in a greenhouse so the best option for plant varieties is to use the indeterminate variety of tomato plant i.e. a vine plant. You can prune the plant to the required size, leaving about 4 to 6 trusses on the main stem, which is about 6' to 7' in height, greenhouse size allowing. This will give you lots of fruit over an extended period. You will have to pinch out the main stem when you have enough trusses to stop further growth.
As an aside, using the bush variety is less convenient for greenhouses because they, as the name suggests, form a determinant size of bush without pruning which is probably a little too large in diameter for the average greenhouse. The fruit also has a tendency to all come at once.
Irrespective of the growing method you choose, you should start to feed the tomatoes at the first sign of fruit and for the duration of the growing period, twice a week should be sufficient. Also as you are in a greenhouse regular and consistent watering is also very important and I recommend 2 to 3 times a day with water that has been allowed to warm in the greenhouse.
As the end of the season nears a way of helping the last remaining fruit ripen is to hang a couple of bananas in the apex of the greenhouse. If that fails to ripen the last bit of fruit you can always use the green tomatoes for making a delicious chutney.
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