I've got two pots with sweet peas of the same variety, grown the same way at the same time, but one lot have got much longer stems than the other, and the ones with the shorter stems are now getting attacked by greenfly. The only difference is that the shorter-stemmed ones are in a less sheltered spot. Could that make a difference, and if not, any ideas? Trouble is, the short stems are really too short to make good cut flowers, but it's a shame to cut them & throw them away just to keep it flowering.
Is the light levels they get different?
Some get Virus from the Aphids and thatmakes them mottled blooms and probably shorter stems.
???
Could be down to lack of water, too - you say that the second pot is in a more exposed position, so would dry out faster.
We have suffered something similar but just with our sweet peas in general. Start of the season and they were beautifully long-stemmed, now they are so short they are virtually useless (apart from looking lovely and smelling great) - lack of water?