Drainage layer
/ˈdreɪnɪdʒ ˈleɪər/
Definition
Drainage layer refers to a bottom layer of coarse material in a plant container or terrarium, such as gravel, pebbles, or expanded clay, placed beneath the growing medium. In horticulture, it is meant to create a space where excess water can collect below the root zone rather than moving freely out through drainage holes. It differs from soil drainage, which describes how easily water passes through the potting mix itself.
Use in Gardening
- Water table effect: A drainage layer can create a perched water table, which means water may stay in the potting mix just above the coarse material instead of moving out faster.
- Container use: In standard pots with drainage holes, a drainage layer is usually less useful than a well-aerated potting mix, because root health depends more on air spaces and even moisture than on a separate bottom layer.
- Closed vessel situations: It has more practical value in containers without drainage holes, such as some terrariums or cachepots, where it helps keep roots from sitting directly in collected water.
- Material choice: Common materials include gravel, pebbles, or expanded clay, but their main role is physical separation. They do not correct overwatering or poor soil structure on their own.
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