Building a Cold Frame

by Darren (Plot 219)

We can get a head start on propagating our seedlings by building a cold frame.

Essentially, a cold frame is a miniature greenhouse – a (usually bottomless) box with a clear lid that captures sunlight, insulates plants and warms the soil.  It can be as simple or fancy as you like.

The key component is the clear top, which should be removable (or hinged if attached) so that once seeds germinate, or if the day gets too hot, it can be propped open to allow ventilation.  The size of your cold frame will most likely be determined by what you use for the clear top.  Probably the most common option is an old window frame, which is ideal, but you can also use a sheet of clear polycarbonate, clear plastic or even plastic/kitchen wrap stretched on a frame.

Once you have the top selected, construct the sides to support it above your seed trays – ensure you leave room for them to grow to transplanting size.  Sides can be made from stacked house bricks, timber (which has the advantage of being able to be cut to give a sloped roof), more clear sections or even a large polystyrene box.

Place it somewhere it will get sun, ideally against a north-facing wall where it can absorb some extra heat.
If it gets cold at night, cover the glass with a something (e.g. a towel) to retain heat and remove it when the day warms up.

A few examples are shown below for inspiration (a Google search will show lots more).  So don’t wait for summer, and save money by planning seeds rather than buying seedlings!

Cold Frame