This year has been a really good year for plums, too good in fact, if you ┬áleave plums hanging like grapes they’ll get too heavy and branches will break under the weight.Then they use their energy to form stones etc on lots of small fruits. Best practice is to thin out the fruits & it will divert the plant food (sugar) into the remainder. If you have lots, remove excess to give fruits at a couple of inches apart, they’ll ┬ábenefit from the air circulation and have space to develop into good sized fruits.