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How To Build A Cold Frame

Hannah checking progress of tomatoes in the cold frame. Photo by Natalie Mendham

A cold frame is a welcome addition to any cool temperate garden as it provides a warm microclimate for heat-loving plans to thrive. The magic behind them is that they help extend the season of your crops, meaning you’re stretching summer at either end. This is always a good thing when your winters are long and drizzly.

Unlike hothouses or glasshouses, cold frames are built close to the ground (usually 0.5 m–1 m high) and are designed to raise seedlings, harden off young plants or grow annual heat-loving vegies that don’t get very tall.

While we were initially thinking of building a large glasshouse, we eventually changed our plans in response to our context. Our steep slope and lack of available flat land has seen us prioritise outdoor growing and play spaces. This all meant a smaller cold frame would be more appropriate. It’s also a heck of a lot easier and cheaper to build. Wins all round!

Cold frames come in all different shapes and sizes depending on your needs. Ours is 6 m long, 1 m wide and 1 m high at its highest point (it slopes down to 800 mm at the front). We use it to grow short annual vegetables like eggplants, bush tomatoes and basil. Over winter we use it to propagate seedlings and grow a mixed green manure crop to rejuvenate the soil.

Cold frame with early tomato. Photo by Hannah Moloney

Where Should You Locate A Cold Frame?

Place your cold frame facing the sun. For us that’s north, for folks in the northern hemisphere it’ll be south.

What Materials Can You Build It From?

Often a cold frame is made with a stone or brick base, a durable material that provides wonderful thermal mass, holding precious heat for longer. The top is generally made from glass that’s angled towards the sun, letting in 100% light and warmth.

However our climate in Hobart isn’t that extreme so we used what we had available to us—ethical hardwood timber from a local saw miller’s property, polycarbonate roofing left over from a building, and our existing sun-facing stone wall.

When building with timber you have to be prepared to eventually replace parts or rebuild the whole thing as the timber will rot over 5–10 years when it’s in direct contact with the soil. We’ve made some efforts to slow this by making the base sleepers that sit directly on the soil easily replaceable. If you can access and afford it, Cypress macrocarpa timber is the most durable timber to use in the landscape. We didn’t have access to this, so we have mixed Eucalyptus timber.

Construction wise, the timber frame is stabilised by timber stakes in each corner and along its length. We’ll actually replace these with steel pickets shortly as these will last longer in the ground.

Building the frame. Photo by Hannah Moloney

The Lids

Across the whole length we have two large lids each 3 m long that can be lifted and propped up with timber ‘elbows’ so we can easily work inside the cold frame and/or allow in air on really hot days. The lid has two settings, fully open (best when we’re working in there) and low (which is just to let in fresh air).

As we have incredibly strong winds (mostly in spring), we added some adjustable latches onto each lid to make sure they don’t get ripped off in big gusts. This might not be necessary for your place.

Aeration

This cold frame is not airtight, which we actually like as this means there’s no risk of plants getting cooked on hot days. Air can still get in between the stone wall and the frame— overall these small gaps have created what appears to be just the right airflow for our climate.

Cold frame illustration. Illustration by Hannah Moloney

Planting directly into the soil.

Once the timber frame is built and before you put the skin on, make the most of having easy access and prepare the soil. First aerate the ground with a garden fork or a broad fork. Don’t turn the soil, just pierce it with the tines.

After this aerating process, we put down a layer of cardboard (with sticky tape removed) to slow weeds coming back (they always eventually come), then a good layer of top soil around 200 mm deep to match the height of the sleepers and a sprinkle of compost on top.

Cold frame location diagram. Illustration by Hannah Moloney

Planting

Normally people in Tasmania plant their tomatoes on or just after our annual Show Day on October 25th. Traditionally this is when you can safely say there’ll be no more frost, however there is the occasional freak frost. This year we planted a small batch of tomatoes on September 21st. One whole month early, and we were eating the first cherry tomatoes from the cold frame in mid-December—that’s around 6–8 weeks earlier than normal.

Hannah Moloney runs Goodlife Permaculture, together with her partner Anton Vikstrom in Hobart Tasmania. Hannah is also guest presenting on Gardening Australia.

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