Drainage is Critical
Did you know that oxygen is critical for your plants’ roots?
Since plants do not have good circulatory systems, they need to be able to get oxygen near where it is needed. Every cell in a plant “breathes” and most of the plant is exposed to oxygen. Plant roots do not help deliver oxygen to the rest of the plant, they use the oxygen. And since the roots do so much of the hard work of delivering nutrients for a plant it is critical that they have oxygen available.
Well drained soil is full of air pockets that keep oxygen available to plant roots. Rain and watering will fill those pockets with water, so it is critical that the water be allowed to drain from those pockets. If your soil remains wet for long periods after rain or watering or you notice puddling and standing water in your garden you have a problem with drainage.
Soil with fine particles, like clay, do not allow much space for pockets of oxygen and the fine particles can compact very easily. The fine particles tend to stick together, especially when wet, preventing water from draining away. Even if your surface soil is loose and loamy, they could be a layer of hard packed clay just underneath. And of course with good soil if you overwater the air pockets will constantly be filled with water and push the oxygen out. So, yes, you CAN drown your plants by overwatering.
Of course we are all aware of species that can survive, if not thrive, in wet soil These plants roots have adapted to living anaerobically. Making use of such plants can solve soil and drainage problems not easily corrected. But when possible, amending the soil to aide in drainage will accommodate a greater variety of plants. Improving the soil is not especially difficult. Loosening existing soil deeply and mixing in appropriate compost and organic matter will do wonders for your garden.