seed planting and propagation
seedling propagation
how to make an aquarium heater single tray propagation Box
The cheapest heated seedling propagation box with full thermostat control is one that uses an aquarium heater.  This page explains how to make a single seedling tray heated propagation box using an aquarium heater.

Photo of an aquarium heater propagation box without it's grow top lid on.
Aquarium heater propagation box without It's grow top lid on.

Photo of an aquarium heater with it's protective coverAquarium heater with it's protective cover on the right.  The protective cover slides over the heater.
materials required

  • Plastic container roughly the size of a seedling tray. 
    Ideally the seedling tray should fit snugly on top of the plastic container.  If it does not fit snugly you might need a piece of wood or plastic sheet to cover the gap.

    The container should be deep enough to hold at least ten litres of water.  I use a cut down milk crate container.
  • Aquarium heater. 
    Any size is fine as a 200 watt box will use no more than a 50 watt one, it will simply be on for less time.  But it is best if the heater be no bigger than 100 watts
  • Aquarium heater protection cover (Optional)
  • Seedling tray and grow top lid.

building steps
  1. Cutaway illustration of a hot box heated by an aquarium heater
    Cutaway illustration of a hot box
    heated by an aquarium heater.
    Place container in sunny position close to a power point.

  2. Lay heater horizontally in the container with the cord hanging out over the edge.  Use the heater's suction pads or a heater protection sleeve to keep the heater off the bottom of the container.

  3. Fill with water so that the water level is well above the top of the heater.

  4. Place a seedling tray with Grow Lid on top of the container.  If there is a gap then cover it with a piece of wood or plastic.
  5. Plug in the heater and allow about an hour for the water to heat up to the right temperature.  Heat rising from the surface of the water will heat the seedling tray directly above it.  Adjust the thermostat on the heater to get the temperature that you want, keeping in mind that the air temperature above the water will always be slightly cooler than the water temperature.

WARNING: Aquarium heaters are not designed for outdoor use. To avoid any possibility of electric shock make sure water always covers the heater in the crate, the heater and extension cord plug connection is protected by a double insulated cover suitable for outdoors and the electricity is connected through a safety trip switch.  

Page modified 2016-08-04