The biggest problem with recognising plants that have been over-watered is the confusion brought on by the symptoms they display. The trouble is that when plants are stressed in this way they normally show identical symptoms to those that have been stressed through under-watering, i.e leaf curling, stem drooping and leaf drop etc. Although at first this may seem to make no sense at all, the reasons behind this is actually quite straightforward - and it's all to do with air!

If you look at the roots of a plant you can easily spot the main body of the root which is used to transport water and nutrients to the rest of the plant. Then if you look a little closer, you should also be able to see extremely fine, hair like roots, and these are the parts that are most important here. Each of these tiny root hairs is in fact a single modified plant cell, and as with all plant (and animal) cells they need oxygen to metabolise. This is also the same for the cells found in the human body which is why we have a need to regularly breath air (oxygen) into our lungs. That way it can be absorbed into our blood where a dynamic vascular system transports the highly oxygenated blood around our bodies. It also moves poorly oxygenated blood back to our lungs where the gaseous by-product carbon dioxide is expelled harmlessly from our bodies.

The root hairs receive their available oxygen from tiny air pockets that exist in the surrounding soil, and although in their normal environment they may become filled with water from periodic rainfall, this excess of water will normally drain away allowing new air pockets to form. It's only when the water doesn't drain away through floodings or constant heavy rainfall, that problems then occur within the root environment.

By denying an adequate supply of oxygen to the root hairs, these specialised cells are unable to metabolise and although they will be able to tolerate these conditions for a short while, continued exposure to over-watering will cause them to eventually die. The trouble is that all plants need a reasonable supply of water for transpiration (breathing), to maintain temperature so that they don't overheat, and to remain turgid and upright.
As mentioned previously, plants need these specialised root hairs for their uptake of nutrients and water. If enough of these specialised root hairs die then the plant will be unable to take up enough water to ensure its survival, and of course, will begin to dry out internally showing the characteristic dessication symptoms often mistaken for drought. The point is this, even though there is more than enough water in the root environment, if the root hairs have died through 'suffocation' then the plant is no longer able to access the water to replace that which is used through its normal regulatory functions. The irony here is that the plant now enters a phase of stress due to internal drought and by trying to reduce water loss through leaf curling or drooping it exhibits the same symptoms as though it was suffering from a lack of water in the root environment. Unfortunately when people are unaware of this they will see the plant wilting and give it yet more water compounding the problem and causing further root death.

The only way to avoid this is to check the soil first before watering. You can buy various gadgets for this, use you finger or by judging the weight of the pot and comparing the weight of the pot and compost to what you know is wet or dry. You will find that the greatest cause of death among pot grown plants is in fact over-watering. It is also the easiest one to avoid.
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