My apricot trees are dying. I have planted one Royal Rosa and two Blenheims apricots in Menlo Park, CA, Sunset Climate Zone 16. After growing well through spring and summer, each died over the winter and fails to come out of dormancy. I started with two, one died the frist year. I replaced it. Then one died in it's second year. And the replacement died in the third year. They are drip irrigated for one hour each evening with each tree receiving 7 gallons per week. Any thoughts on what might be killing them? They were planted in large, deep holes with non-native soil mixed in with native soil.