How to Build a Small Propagator.

Latest Update: 14th March 2017.

These Small Propagators have a plastic box containing a shallow water tank and sieved homemade compost on top.  It's housed inside a purpose made cold frame designed to protect the seeds and cuttings growing in it from extremes of weather and from pests all year round.

The mini and jiffy pots I use to propagate seeds are buried up to their rim in the biologically active compost which is kept moist by water from the tank.  Here's how to make one for your own garden.
This is the bin containing the water tank and growing area of the propagator.  The photo shows the water tank overflow outlet complete with a pest exclusion filter.
The filler, distribution and overflow system is a push fit with no silicone sealing required, and the hole drilled in the bin's sidewall is a tight fit over the overflow T junction (set 150mm above the bottom of the bin, the overflow controls the maxium water depth in the tank).  The black corrugated distribution pipe at the bottom of the assembly is slotted to allow water to flow quickly into the tank. The corrugations stop Scoria from blocking the slots.
The water tank area is filled with 20mm grade Scoria.  The tank is filled to overflowing with water and the Scoria made level with it.  A layer of horticultural fabric is used to cover the tank and keep compost out.
A layer of sieved homemade compost fills the bin to 20mm below the rim.  44 mini pots fit into the propagator buried up to their rims as shown above.  

Water wicks up from the tank through the layer of Scoria into the compost.  The moist compost layer keeps the seedling mix supplied with water.  Beneficial microbes from the compost gain access to the plant's root zone through the drainage holes in each pot.  The improved germination and growth rates of the seedlings grown this way is very noticeable. 

Cuttings are simply pushed into the compost leaving half their length exposed.  I don't use growth stimulants on cuttings other than the beneficial microbes in the compost which quickly establish supplies of nutrients to the plant and stimulate root formation.

Consistent moisture levels are maintained even when the water tank is nearly empty, and the tank will last at least a week between refills even in very hot weather.
The cover on my cold frame is fitted with a protective layer of polycarbonate in the cooler months It maintains a mild environment for seeds and young seedling to grow when cooler weather would slow that growth and damage them when its frosty. It providing me with a continuous supply of winter vegetable seedlings, and I can start spring and summer vegetables much earlier than would otherwise be possible. 

In summer I replace the polycarbonate with a 21% shade factor netting to keep out airborne pests and protect my tender seedlings from the worst effects of intense sunlight.  When temperatures are over 35 deg C, I clip a piece of 75% shadecloth over the propagator for extra protection.

The cover is removable (see handle extensions) which is cheaper and more reliable than hinges.  The 2 short legs at the bottom of the cover protect it when its parked out of the way.  The bottom of the cover overlaps the wall so that rain will run off and not into the cold frame.  There is a stop under each side of the cover to position it and stop it sliding off the cold frame.
The condensation on the inside of the lid is caused by the temperature difference between the inside and outside of the propagator.  The outside temperature was 7degrees overnight and this photo was taken at 11 am the following morning.  It indicates that the inside environment is being kept warm and moist.
The propagator's body is made from 9mm marine plywood, and is painted to keep the timber's chemical treatment locked away from the beneficial microbes in the unit.  The sheet of black polythene at the back of the body is a solar collector helping maintain a warmer temperature inside the propagator during the cold months.  Its easy to remove when the warmer months arrive.  I have not added slug and snail protection, because there is no access for them, and they don't seem to be a problem.

There are adjustable rotary vents in the top corners of the frame to allow hot air to escape during warmer daytime weather in spring.  The polycarbonate cover is left on as long as night time temperatures are still low.  21% shade factor netting keeps flying pests out of the propagator when the vent is open.