Sunday, August 4, 2013

Step One - Building The Cold Frame

The plan:
1) build and install one permanent, in-ground cold frame
2) start winter greens from seed
3) transplant the seedlings into the cold-frame
4) enjoy fresh mixed greens all winter!

The first step towards a winter garden and - hopefully! - a winter harvest is the construction of a permanent, in-ground cold frame.  But what is a cold frame?

A cold frame is an enclosure with a transparent lid which serves as a season extender for growing hardier plants.

There are many different types of season extenders, from very simple things like cold frames built out of bales of hay to elaborate constructions, such as greenhouses.  In this case, my cold frame is built from wood, insulated with rigid blue foam, and partially buried in the ground (which further helps to ameliorate heat loss).  It has no source of heat, other than capturing solar energy behind the twin wall polycarbonate lid.  This is in contrast to a close relative, the hot bed, which is a similar structure save for the fact it is heated (for example, by heating tapes buried in the ground).

Wooden cold frame

This is the basic construction of the cold frame.  Note that it slopes downward back to front to present a greater surface area to the low winter sun.  It measures approximately 4 feet wide, 8 feet long and varies from 8-12" deep.  While building it out of naturally rot resistant wood (for example black locust or cedar) ensures it will last, this is likely going to be difficult to source; this cold frame is simple pine.  We'll see how long it survives in the humid, wet environment of this tiny Atlantic island.  Resist the temptation of chemically preserved wood.

Siting the cold frame is very important.  Avoid low-lying areas where cold air may pool.  Facing the south in full sun, if available, is best - one needs to take every advantage of the thin winter sun.  If you're lucky enough to be able to place it at the foundation of your house or another heated building without sacrificing sunlight, the heat from the building will help to keep the cold frame warm; otherwise, placing it in the lee of a windbreak (fence or line of trees) will improve heat retention.  Finally, since the cold frame is partially buried, remember that whatever site you choose you will need to be able to dig it down into the ground.  While this is not particularly difficult in PEI, it may be quite a bit more difficult in other Atlantic locales where the ground is rocky and hard.  You will need to be able to dig a hole large enough to accommodate the cold frame such that the top of the front edge is flush with the ground around it.

Polycarbonate cold frame lid

The lid of the cold frame is composed of a wooden frame set with a 4x4' sheet of twinwall polycarbonate.  This material is light, strong and thermally insulating; it lets the sunlight into the cold frame to warm the air within, and keep the plants inside from freezing solid.  In my region, it is available for order from the Halifax Seed Company.

Thermal energy storage

The final step is the use of a thermal energy storage device.  Sounds impressive, but it's really just a 10 litre plastic container of water placed inside the cold frame.  To aid in capturing heat, the side of the plastic container facing south is painted black.  During the day, the sunlight warms the water in the container, and at night the water releases the stored heat back into the cold frame.  I picked up these containers at the local supermarket, where they are sold as water to store in case of emergencies, spray-painted one side black, and will line the back of the cold frame with them.

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