why are my plums splitting?

Did you have a period of very heavy rain followed by a dry spell? This could cause the plums to split.

...or the other way round maybe? :-)

A grower from a fruit tree nursery also said a possible (other cause) can be over feeding. This is only for young trees of course while their roots are able to access surface feeds. Larger trees are unaffected as their roots go down too far.

I would still go with MG's suggestion though.

I havnt fed the trees. yes we did have a dry week. it was great to get out in the garden and even mow the lawn in mid September but what a sad way to loose the plums. Thanks guys

Sorry couldn't help more

we just had two hot fine weeks then a large amount of rain and very humid .our christmas plums split over night .they still taste lovely great for jams u just need to pick them each morning . the remaining crop should be allright . hope we were of help