The growing substrate in which plants are potted should be adequately moist, but never dump, roots need water, but they need oxygen as well to function properly. Give your plants a chance to dry out slightly before you water them.

The number one plant killer is over-watering especially if we cultivate cactus and succulents. It is easier to bring back a dying plant from under-watering than overwatering. Over watering can be more hazardous to our plants health than making them go without water.  Excessive moisture creates an environment favourable to such diseases as Crown Rot, Phytium, Phytophthora, Root Rot, damping off. Overwatering can also cause a condition known as denitrification.

A good growing medum is made to quickly absorb water and then dry out. It is loose and allows oxygen to flow around the plant's roots which the roots need to function correctly. If the potting media is kept too wet, the roots soon rot and the plant starts to wilt because it cannot absorb the water needed. When this happens the beginner usually adds still more water thinking the plant needs it.

What should we look for?

• A plant suffering from overwatering may appear to actually need water. The leaves or stems will wilt and turn yellow, sometimes dropping off. Someone seeing this might believe that the plant needs water and add to the problem. But once again, feeling the soil is the best way to avoid this.
  When in doubt: Soil + dry = water. Soil + wet = don't water.
• The media smells sour.
• One things that can happen to some plant (e.g. Christmas cactus) is after the flower buds have developed they drop off the plant. Bud drop can be caused both by lack of humidity or over-watering

What should we do?

• Succulent plants like dryness. Some dry periods between watering will maintain a healthy  root system.
• Fertilize and water in the spring and summer and early autumn, let rest in winter.
• During the winter for most species withholding water completely is the best method. Many plants will show signs of shrinkage during this dry period but this is normal and good as long as the roots do not disintegrate.
• Spring vegetative resumption: it is very important to withhold watering until the plants actually begin to show signs of growth. Some species accept watering before they are ready to grow, but it is very risky to attempt to push any plant into a growth cycle with water too soon. There will be some plants with indications of growth that are difficult to see and obviously watering can't be postponed indefinitely. If the plant has been kept completely dry through the winter and has rested for over 6 months it may be safe to resume watering without seeing signs of growth as long as the plant has daytime warmth and enough sun.
• When plants are resting in their summer cycle an occasional small watering will be fine for most species as long as they can dry out between watering. Some succulents and cactus plants will actually delay their spring growth cycle until summer in more northern climates in which case they will need plenty of water.
• Repot when there is a circular growth of root mass at the bottom.
• By the time you notice that there is something wrong, the media has already started to decay and the plant has begun to loose its roots. You need to repot immediately, cut off all dead and decaying roots and let plant roots drying and callusing for some day before to plant in a fresh substrate . Use a coarser grade of media since this will allow more rapid drying out.
• If the plant has no (or very few) living roots left, you can try re-rooting. If your plant has signs of root rot, treat it with a systemic fungicide before repotting. Be careful when you start to water again.
• Plastic pots require less watering whereas clay pots can help to mitigate the effects of over-watering..

See: Watering, Water stress, Wet sensitivity.