Cut plastic bottle into three sections, as indicated by the dotted lines.
Fill the bottom section with gravel for drainage.
Make a sharp inward crease down the length of the middle section to make it a little thinner.
Leave the top section alone.
Now reassemble the bottle, squeezing the middle section to fit inside the rim of the bottom section.
Fill with potting or seed compost and plant your seeds.
Then replace the top section over your middle section to create a sun roof, protecting and warming the seedlings and preventing the compost from drying out.
When your seedlings are ready, pot them up by simply removing the middle section of the propagator and pushing the entire plug of compost and plants down into a flower pot.Reuse content