Seedlings in styrofoam seedling boxes are grown on a base of several litres in volume of compost or garden soil, which means that when you water your seedlings the soil base below acts as a sponge and absorbs water for your seedlings to draw upon. 

This means that you don't have to water as often and there is a much greater margin of error should you forget to water your seedlings.

The undulating top of the styrofoam box means that when a pane of glass is placed on top of it there are plenty of side vent holes.  If you unexpectedly strike a hot day and you haven't removed the glass top then the seedlings are unlikely to overheat as the excess heat easily vents out these side holes.

Because styrofoam has good insulating properties and there is plenty of mass in the form of the soil/compost base styrofoam propagation boxes retain more heat at night than commercial propagation boxes.  This means that your seedlings will grow quicker and seeds that need higher soil temperatures, such as eggplant, basil and capsicum, to germinate are more likely survive.

A styrofoam propagation box costs little to make.  The boxes themselves can usually be picked up at supermarkets or green grocers for nothing and if you have some second hand glass all you will need to buy is a glass cutter.  Otherwise a sheet of glass can be bought from a glazier for around $11 AUD.  By contrast commercial propagation boxes sell for $25 to $30 AUD.

As long as you don't break your glass a styrofoam seedling box will last for years  In contrast the plastic tops on commercial propagation boxes usually become brittle and crack after four or five years.



The size and insulating properties of styrofoam boxes means you can' place them on a heating tray.

While the seeds are germinated in a thin layer of sterilized seed raising or potting mix the base is made up of garden soil or compost which is not sterilised.  So any weed seeds in your compost or garden soil base are likely to also germinate.