Bonnie, why do you want to wash your trees? Do you have buggies on them? I use SaferŪ Insecticidal Soap on my shrubs when they get pest bugs on them (& even on the veggies, when I have a Whitefly problem). It's organic/non-toxic, so it's safe around pets.
It's a tad on the spendy side, but unless you're in the middle of a rainy season, it's worth the money you'll spend. You can buy it in ready to use in spray bottles, or in concentrated form, if you want to mix up larger batches of it for big jobs. I buy the concentrated, cuz it's cheaper in the long run. Directions are on the label for use in pump sprayers, spray bottles, or as Tina mentioned, hose-end sprayers.
The only tip I can think of to give you, is to be sure to spray ALL the plant/tree... which means the under sides of the leaves, as well as the tops. I also spray down the stems/branches (all the way to the ground), cuz bugs will hide anywhere the solution isn't.