Posted: 19/08/2015 at 09:27
Rosemummy - It's just a fact that SP stems will naturally shorten has the plant ages and gets weaker. Flowering from plants of autumn sown seed will be drawing to a close soon, whilst those from spring sown seed will (hopefully) go on into the autumn.
Having said that, there are some other factors to consider. Generally modern introductions have longer stems (a show-bench requirement) than older, say Spencer varieties. Also (as Lily says) patio & other varieties will have shorter stems.
To repeat what I've said many times, buy the best quality seed you can afford and make two sowings, autumn & spring.