I have a friend who is building a sunroom garden, and we need to make sure the soil drains well drain into the overflow tank, while still allowing the soil to always be moist. How deep should the soil be so that it gets to the well draining sand, and brick scraps?
This is the same problem as a potted plant with the same best practices:
- do not add a layer of gravel, crushed brick or sand at the bottom. This does not improve drainage but just creates a perched water table.
- sand does not improve drainage as much as you think. It tends to move in the soil and pack down
- do use things which promote drainage: perlite or leca which is a baked clay product which is quite useful
- do not use topsoil, use a soil less potting mix and add more perlite or leca
- if your container is large enough you can use my favorite garden tool:
- four inch drain pipe with sleeve
- Run a section down the bottom of the pot
- angle up to above the soil level and cap it
- this allows you water from the bottom or, more importantly pump out excess water if overwatering occurs