seed planting and propagation
seedling propagation
heated seedling propagation boxes
There are many heating units available to artificially heat seedling propagation trays.  This page looks at  the pros and cons of three  units I have personally used to artificially heat seedlings.

Sage Single heat tray with thermostat Control

Photo of a Sage single heat tray with themorstat control.
Sage single heat tray
with thermostat control.
Has a variable thermostat that gives you greater temperature control and accuracy.

At $155 AUD(Dec. 2009) this unit is quite expensive. 

In order for the thermostat to work properly the tray's mat has to be keep permanently moist.  If it dries out the heating unit will simply switch off.  If this happens on a very cold night you can lose seedlings.

Sage flexible single heat mat

Photo of a Sage Flexible heat mat.
Sage Flexible single heat mat.
At around $85 AUD(Dec. 2009) it is much cheaper than the thermostat control version.  It is also more reliable as it heats continuously with an eleven watt current.  I find this unit perfectly adequate for growing seedlings with artificial heat.

Note that there are a wide variety of heat mats available for the reptile pet industry, many of which are cheaper than this sage unit.  Check out your local pet shop that specialises in reptiles for what's available.

As it has no thermostat the temperature varies more than the Sage's single heat tray with thermostat.

Sage flexible heat mats heat trays with thermostat control can be purchased directly from the Sage website.  For details view the Sage Horticultural Products page.

aquarium heater propagation hot box
Photo of a home made propagation box with an aquarium heater as it's heat source.
Home made propagation box that uses
an aquarium heater as it's heat source.

Is a very cheap way to make a heated seedling propagation box with full thermostat control.  The whole unit should cost you no more than $35 AUD (Dec. 2009), which is around a quarter of the price of the Sage heat tray with thermostat control.

For details on how to make an aquarium heater propagation box see the How to make an Aquarium Heater Propagation Hot Box  page.

Aquarium heaters are not designed for this sort of use and are much more fragile than heat mats or trays.  The body of water that you are heating is usually not that large and heated water evaporates very quickly.  Which means you will have to regularly  top it up with water.