Overwatering is less about the volume of a single watering and more about frequency: watering again before the medium has dried enough to let air back into its pores. Roots need oxygen for respiration. In saturated medium they suffocate, take up less water and nutrients, and become vulnerable to pathogens such as Pythium.
In practice
Typical signs are leaves that droop while firm and swollen (curled down from stem to tip, not limp and papery), a heavy pot, slow growth, yellowing, and fungus gnats breeding in wet surfaces. Seedlings in large pots are the classic case. The fix is to wait until the pot is noticeably lighter before watering again, improve drainage, add perlite to dense mixes, or move to a smaller container or more air-porous pot.
Note that coco and rockwool can be watered many times a day without overwatering, because they drain freely and hold plenty of air.
Don't confuse with
- Underwatering: limp, papery droop from too little water; the pot feels light.
- Root rot: often the consequence of chronic overwatering, but a disease in its own right.
A genus of oomycete water moulds, such as Pythium myriotylum and P. aphanidermatum, that cause brown, slimy root rot, especially in warm hydroponic systems.Small dark flies of the genus Bradysia (family Sciaridae) whose larvae live in wet growing media, feeding on fungi, organic matter and young roots.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.Coco coir is a growing medium made from processed coconut husk, prized for holding water and air at once; it binds calcium and magnesium unless buffered.Underwatering is letting the medium dry too far between waterings, so the plant loses turgor and wilts, with limp, papery leaves and crisping edges if repeated.Decay of roots by pathogens such as Pythium, Fusarium, Rhizoctonia or Phytophthora, usually triggered by low oxygen, heat and waterlogging around the roots.The wet-dry cycle is the alternation between watering a container and letting it dry back, which refreshes oxygen in the root zone between irrigations.Air-filled porosity is the share of a container's volume that holds air rather than water after it has drained, a key factor in root oxygen and overwatering risk.Field capacity is the amount of water a medium holds after it has been saturated and allowed to drain freely; in pots it is called container capacity.
