Overwatering generally means watering too frequently. Watering for too long at any one time doesn’t harm plants, but only wastes water. The excess water runs off or flows below the root level of the plants and is inaccessible to them.
Watering too frequently, however, keeps the soil from drying out between irrigations. Plants, unlike animals, do not circulate oxygen within their bodies. Every part — including roots — must have oxygen available from the air. Saturated soil doesn’t contain enough air for most plants, and the roots die.
Plants can generally tolerate a few hours or perhaps a couple of days without soil oxygen, but longer than that begins to kill roots.
Most of the fungi that cause root-rot diseases need very moist soil. The most important root-rot fungi ( Pythium and Phytophthora species) are called water molds because of this affinity for wet soil. Drying periods kill the fungi, and are necessary for the health of many plants. Since water molds are killed at a soil dryness that is easily tolerated by plants, it is simple to dry the soil a little between waterings and avoid root rot.
The length of time you can go between waterings depends, among other things, on the porosity of the soil. Porosity refers to the number and size of the pores between the crumbs of soil. The size of these pores determine the rate at which water drains from the soil, allowing more air to enter. Soils with extremely large pores, such as some artificial potting mixes, drain so rapidly that it is hard to overwater them. Even moments after watering, the soil has adequate air.
Some clay soils, however, have minute pores. Water drains slowly from them, so they are slow to aerate again. Watering too frequently can deprive the roots of oxygen and promote root rot. This type of soil requires careful management of watering to keep air in the soil. Luckily, these types of soils also hold a great deal of water, so are slower than other soils to dry out.
The usual symptoms of overwatering are dying, blackened areas on leaves. These areas don’t always appear at leaf edges and tips, but might occur anywhere. They usually appear first on older leaves.
Plants may also wilt, making gardeners think they need more water. However, the wilting is caused by root death. Because the roots are dead, the plant cannot take up water and dies from a lack of water in waterlogged soil.
Root rot normally starts at one part of the root system and progresses around the plant. Because of this, one branch or one side of a shrub or tree may die at a time.
You can check for overwatering by digging into the root zone or knocking a potted plant out of its container. Soil that has been too long without oxygen usually smells sour or rotten, with an odor of rotting eggs.