sometimes there are variable stem lengths but some are bred specifically for this trait. not ideal for cut flowers unless you use a posy vase.

It seems so unfair, given how well they started out :o(

Water and feed feed feed. A friend of mine shows them nationally. We occasionally get his superb looking rejects! The amount of care he lavishes on his is amazing. The first lot of flowers are the plant's main throw of the dice, aimed at producing seed for the following year. We breed them to keep on flowering (along with many other plants), but they don't know that, so their second lot is back-up. They don't care how long their stems are. Any reduction in resources is a signal to a plant that it's time to set seed and finish.

Thank you, Worthy, that's just what I needed to learn. I prepared the bed very carefully, with lots of well-rotted muck back in early spring, but sweet peas seem to be really gross feeders. I am off this very minute to give them extra rations. Would a slow release general fertiliser or something like tomato food in solution work best, do you think?

They need plenty of water to keep stem length going. They don't need much extra fertiliser if the beds were well prepared.
To keep stem length going, apart from water, a top dressing of dried blood about a month after first flowering helps.
Unfortunately, once the length has started to reduce, it won't get longer again.

Oh Dear! I'll know for next year, though......Thank you all.