Thanks for writing. Good question! And one with an answer that's not as straightforward as it probably sounds...
The problem is that when a plant suffers from being too dry or too wet, it's essentially causing the same damage to a plant: roots are dying. When it's too dry, the roots die because they get dehydrated; when the plant is too wet, the roots die because they're drowning.
Another complication to this answer is that different plants need different amounts of water. And various weather conditions and soil types affect moisture, too. Plants will often wilt first when they too dry. But many varieties will wilt when they're too wet for extended periods. Whole leaves or shoots may die when the plant is too wet, as well.
There are moisture meters you can use in the landscape to help manage watering. But one good rule to follow is that most garden plants do best with an inch of water or so a week applied to the soil. This can be from a hose or from rainfall. Also: It's better to give your plants a good, deep soaking once a week than to give them several light waterings throughout a week. Deep waterings encourage the plants to develop stronger, healthier root systems that hold up better to drought.
---Justin, Senior Garden Editor, BHG.com