If tey are yellowing and dieing of quickly, it sounds to me like they are not getting pollenated properly. I always pollenate mine by hand to reduce this, other swear by not needing to. Why not give it a try on a couple and see if things improve...? You can sually tell in a couple of days after pollenating them if they're going to survive as they sudenly put on a 'swelling spurt'!
Good luck and let us know how you get on
__________________ Shortie "There are only two lasting bequests we can hope to give our children; one of these is roots, the other wings" - Hodding Carter |