Answer: There is no specific formula for watering plants. Water needs differ among plants, and is dictated by their growth spurts, the type of soil they are growing in, and of course the weather. However, a good rule of thumb is that most garden trees and shrubs need one-inch of water per week, applied slowly so it can trickle down and wet the entire rootmass. If the weather is very hot, you may want to water twice a week, especially if you have fast-draining sandy soil. Clayey soils will hold moisture longer than sandy soils. Probably the best way to measure how often you need to water is to water as you normally would then wait 3-4 days and dig down into the soil. If it is still moist 2" below the surface, you won't need to water for a few days. If it is dry, it's time to water.
Overwatered plants just need time for the soil to drain. There's not much else you can do except wait for things to dry out a bit.
Best wishes with your landscape!
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