Weak stems on maturing plants.

by Rachel
(Totnes, Devon, UK)

Hallo there,
This question isn't, like it sounds, about 'leggy' seedlings, or damping off, or root-rot - the problem is affecting plants that are well past the seedling stage. I grew a lot of oregano and antirhinnim from seed, and despite pricking them out quite late, potted them on deeply into bigger containers, where they grew healthily and happily for a good few weeks.
All plants now have good height, lots of leaves (with new ones appearing all the time) ***BUT!!*** the stems of all these plants don't seem to have grown at all - leaving a healthy looking plant falling over on a tiny, thin and weak looking stem/root base. The stems don't seem to be rotting, or affected by any kind of fungus... they are just ridiculously spindly - there is no way that if they stay that size they will be able to support a fully grown plant!

All of the oregano and antirhinnim plants I have just been talking about were all grown from seed and were potted on into larger containers. The real bugger is that the very same problem has happened to my broccoli plants as well, which were grown from directly sown seed straight into the ground. It is even worse with the broccoli, as obviously what with outdoor conditions they are battered about a bit more, which has put more strain on the roots. Again though - healthy seedlings, turned into healthy bigger plants with plenty of true leaves - the leaves still look perfectly healthy.....but the entire plant is now collapsing as it literally looks as though it's trying to stand on a thread of root.

What on earth has caused it, and what on earth can I do to prevent it in future, or fix it now?!? Hundreds of plants....all that work and care....all looking absolutely fine....until this. So disheartening :( ANY comments, advice, thoughts etc very welcome!!

Many many thanks, Rachel, Devon, UK