There are two types of plums - Japanese and European. They cannot pollinate each other. Japanese plums
flower earlier than the English. I have one Methley (Jap, self fertile) and two European (Stanley & Green Gage). The European ones are only two years old, so they are not producing yet. The Methley is 10 years old. It's a beautifully shaped tree and has gorgeous blooms every year. The problem is that the Japanese plums
flower very early. I have only had plums from this tree one year because of late freezes.