Urban Food Garden


The cheapest heated seedling propagation box with full thermostat control is one that is heated with an aquarium heater.  This page explains how to make and use an aquarium heater single tray propagation box.


  • A plastic container roughly the size of a standard seedling tray.
    Ideally the seedling tray should fit snugly on top of the 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.  The one used in the photos is a cut down 30 litre square storage crate.
  • Aquarium heater.
    A 50-watt heater is ideal, but you can use one with a higher wattage, it will not use more electricity than a 50-watt heater as it will be on for shorter bursts of time.

  • Aquarium heater protection cover (Optional). 
  • Standard size seedling tray and grow top lid.

50 watt Aquarium heaters, the bottom one has a heater protection cover on it. 

  1. Place container in a sunny position with access 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 seed raising mix in it  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 be slightly cooler than the water temperature.
  6. Plant seeds in the seed raising mix, cover with a grow lid (plastic mini greenhouse dome). Manage the seedlings as per normal seedling care, but making sure there is always enough water in container to cover the top of the aquarium heater.

Cut away illustration of an aquarium heart hot box. 

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