What it is
Too much water in the soil for too long. The problem isn't the water itself, but that it fills all the soil pores and leaves no room for air: the root breathes oxygen and drowns.
How to tell
The leaves droop but stiff, swollen and dark, curled down like claws, even though the soil is wet. The pot weighs a lot. If it continues, yellow leaves, fungus gnats (they love moisture) and a bad smell in the soil appear: the root is rotting (root rot).
What to do
Don't water until the soil dries out well and the pot weighs little. Throw out the water in the saucer. Give it air (a fan helps it dry out). If the root is already rotten (brown and soft instead of white), it is very hard to recover.
Why it happens
Almost always from watering too often, without letting it dry out between waterings. Also from pots without holes, heavy soil that doesn't drain, a pot too big for a small plant (the soil without roots stays wet) or cold (the plant drinks less).
How to avoid it
Water deeply and then wait. Soil with perlite or coco fiber so it drains. Fabric pots (they let the root breathe through the sides). And extra care with seedlings, which drink very little.
🎮 Practice it in UnderGrow
An extra watering does no harm: what goes over 100 % drains off by itself. What drowns is watering too often: 3 waterings with the water bar at 70 % or more, without letting it drop below 40 %, and the root gets drowned for a whole day; if it stays drowned for more than 24 h, it adds stress and lowers health. A seedling indoors gets drowned by a pot soaked for more than 12 h. After a waterlogging the root is weak and calcium and magnesium deficiencies follow.
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