I have heard that you cannot grow tomatos and cucumbers together in the greenhouse - is this true?
Not to my knowledge. Both mine did well in the same greenhouse last year. They probably would have liked to have their own growing conditions slightly tweeked in their favour but they seem to do well enough.
The reason they say not to grow tomatoes and cucumbers in the same house is because tomatoes like low humidity and cucumbers like high humdity. Having said that, I too have had success growing both in the same house!
As long as you don't put them in the same growbag/pot, or grow them close (and I mean like next door to each other). Last year I grew mine on opposite sides of the green house and it was fine. My green house isn't huge either!
i put toms nearest the door & cucumbers at the back
Ditto, had very sucessful crops from both together in the green house
I put my cucumbers in pots each side of my 8x6 greenhouse & tomatoes in growbags along one side - it works well"
I have bumped this thread as Jean6 answer is I think a better answer to two different climates required for Tomatoes and cucumbers in the same green house
I grow tomatoes and cucumbers in the same greenhouse, always have done. I don't grown them in growbags I grow them in Auto Pots which are self watering and I had a fantastic crop last year from both tomatoes and cucumbers.
In an ideal world you would grow them separately as cucumbers prefer a high humidity and tomatoes prefer low humidity. The reason for this is that tomatoes are prone to fungal diseases which are promoted by high humidity and can spread rapidly under such conditions, which is why we try and grow them quite dry and not over-water them. Cucumbers on the other hand are prone to spider-mite which love hot, dry conditions so we try and grow these plants in very humid conditions which spider-mite don't like.
In reality, most of us only have one greenhouse so grow them together and, if unlucky enough to get a disease or pest problem, are unable to control it well to the detriment of one crop or the other. I much prefer tomatoes to cucumbers so when spider-mite descends on my cuc's, they are sacrificed. Unfortunately that happened every year for several years so I have stopped growing them for the last 2 years and am hoping that has cleared-up any over-wintering mites (there are no effective sprays, organic or otherwise to fully control a bad infestation.)
It's not until you get a disease/pest problem that you appreciate the facts about their different growing conditions!