I believe it is a combination of things. The trees are planted from 3 to 5 feet apart for better cross pollination. Lavender is planted near by attracting bee activity. Chill hours are usually very good, 800 to 1100 hours. The property that I am on was agricultural and supported fruit orchards before World War Two. Silicon Valley has covered up most of the surrounding land. This area was previously called “Valley of Hearts Delight” or the “Prune Capital of The World”. The soil is fertile loam and drains well and yet holds moisture, established trees find their own water source. Fruit trees prefer a mild climate, it does not get too hot or cold here. When it gets warm to hot (less than 100 deg. F) humidity is low. Damage from disease or pests to trees is usually very minimal. Birds squirrel and insects usually do not bother plums/pluots/pluerries here, no bagging or protection is necessary.