Q.When To Fertilize My Fruit Trees
Plum trees and the nectarine trees tangerine trees, orange trees, lemon trees, apple trees what should I be adding this time of year in California?

Certified GKH Gardening Expert
These trees have different fertilization times, unfortunately. Generally, you can be sure that summer and spring are safe times, but it depends on the tree. Generally, winter is not a safe time to fertilize anything.
Here are some articles that will get you started:
https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-fertilizer.htm
https://www.gardeningknowhow.com/edible/fruits/nectarines/growing-nectarine-fruit-trees.htm
https://www.gardeningknowhow.com/edible/fruits/oranges/growing-an-orange-tree.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm
https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-feeding.htm