Warning: horticultural oils (dormant oil, summer oil, etc.) and soaps (insecticidal soap, dishwashing soap used as a pesticide, etc.) will melt away the bloom that covers the leaves of many conifers, hostas and other plants: that is, the waxy white powder that gives them their blue coloration.
These products have the same effect on the bloom that covers many fruits (blueberries, grapes, plums, etc.). So if you spray your blue spruce or your blue hosta, their leaves will turn green! And you will see your blueberries go from blue to deep purple! The effect is not harmful, but it can be disappointing.
Fortunately, the effect is temporary. No, the discolored leaves and fruit will never regain their original color, but the new ones that grow back the following year will be as blue as they ever were.
So, you can spray these plants with water, but avoid fats (like soap) and oils.
Translated from French by Mathieu Hodgson