I've grown some tomato plants in growbags in a greenhouse. Got lots of
tomatoes (although they're still green).

But the stem at the base of each plant is very thin and weak (the stems are
thick at the top). Fortunately, I supported the plants on strings so
they're okay, but without the supports there's no way the stems would be
able to hold the plant.

What did I do wrong and how do I avoid weak, thin stems in future?

Thanks