Urban Food Garden


Aquarium heater seedling propagation boxes are a cheaper way to artificially heat seedlings than using commercially made thermostatically adjusted heated trays.  This page explains how to make a two-seedling tray aquarium heater propagation box.

About aquarium heater seedling propagation boxes

Thermostatically adjusted heated seedling trays are the best way to raise seedlings that need warm soil temperatures to germinate.  The  heating units are more powerful than heat mats without a thermostat, which means they bring the seedling tray up to the desired temperature more quickly.  And once at the desired temperature the thermostat ensures that the tray does not overheat.  But commercially made thermostatically adjusted heated trays are expensive.  It is possible to make a thermostatically controlled heated seedling box for a fraction of the cost with the use of an aquarium heater. 

An aquarium heater seedling propagation box is a watertight container with an aquarium heater in it and a seedling tray sitting on top.  The heater brings the water to the desired temperature, the water then gives off heat to warm the seedling tray suspended above.

There are any number of ways to build an aquarium heater seedling propagation box, an example of a single tray one can be found by following this link: HOW TO MAKE AN AQUARIUM HEATER SEEDLING PROPAGATION BOX.  Below are instructions on how to make a double tray aquarium heater propagation box.

    These crates are commonly available.  They can vary in volume but the top dimensions are uniform.
  • 42 x 19 MM WOOD
    Two pieces 640 mm & two pieces 330 mm long.
    One piece 370 mm long (width and depth can vary)
    Twelve  65 mm  screws(for the top frame and centre support wood).  Six 15 mm screws (to secure the top frame to the tub).  Nails can also be used to put together the top frame.
    Wattage size will not make a great deal of difference in terms of electricity use, larger wattage heaters will simply be on for less time.
Constructions steps
1. Insert the centre support

Secure a piece of wood in place across the centre of the crate so that its top is level with the indents at either end. While the length of the wood must be the same as the inside of the crate the width can vary, as long as it is wide enough to support the sides of the seedling trays when placed on top 

I used button head screws to secure the piece of wood in place.

2. Build the frame that sits on top of the crate

Screw or nail the four 42 x 19 mm pieces of wood.  The shorter 330 mm long pieces should be on the inside.  Using corner clamps makes the job easier.

The frame when completed.

3. Screw the frame to the top of the crate

Screw the frame to the top of the crate.  The internal dimensions should allow two standard seedling trays two fit snugly on top of the crate.   Adding a strip of rubber window seal on the inner edge will make the fit even tighter.  However, when I added a rubber seal the damp conditions caused it  to perish relatively quickly and the frame itself produces a good enough fit without the addition of the seal.

Profile illustration of a two-seedling tray aquarium heater propagation box. 

An alternative would be to make the wooden frame thicker.  The inner fame would then support the seedling trays without the need of a centre support.  Note that I have never made one using this alternative design, though if I was to make another two-seedling aquarium heater propagation box I would seriously consider this option.

Internal view of a two-seedling tray aquarium heater propagation box.  It is important that there always be enough water in the bottom of the crate to keep the aquarium heater fully submerged.


Whether you follow these instructions verbatim or just use it as inspiration to design your own version, a two-seedling tray aquarium heater propagation box is a cost effective and reliable way to grow seedlings using artificial heat.

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.