How to Use a Cold Frame

I made my very first cold frame as my second winter of growing my own vegetables approached. With no money for a greenhouse, but some scrap lengths of treated wood and a third-hand window, I measured, sawed, and screwed my very own cold frame together. Okay, so it didn’t hinge open and the only way to ventilate it was to take the (somewhat heavy) lid completely off. But it was a rite of passage: I had made my own cold frame! Surely the mark of a serious gardener.

It was worth the effort. My trusty cold frame has provided ten years of hard use. The glass has been smashed and its polycarbonate replacement blew off in a gale, but the wooden frame is still providing service as a mini raised bed for vegetables within one of my flowerbeds. The time has come to make or buy a new cold frame because, as I’ve recently rediscovered, gardening is harder without one. I made my cold frame when I didn’t have any other options, but even with a greenhouse the cold frame is indispensable.

Extend the Growing Season

A cold frame can be used to push back the end of the growing season – or at least the harvesting season – for lots of late-cropping vegetables. Root crops such as carrots and beets can happily remain in-ground in many areas with no more protection than a blanket of leaves, but by placing a cold frame over them the soil remains much drier and softer. This makes them easy to dig up when you need them and reduces the risk of rotting. Pack the frame with leaves to keep them super-snug.

Start Seedlings Earlier

At least two weeks before you sow early vegetables like spinach, radishes or peas in spring, put your cold frame in place. This will help to gently warm the soil within the frame to temperatures that are more suited to growing cool-season vegetables. If you have a soil thermometer, wait until temperatures are consistently around 7°C (45°F) for best success. This will ensure the soil is warm and dry enough to give your seeds the best start.

Plants like tomatoes, eggplant and peppers that need warmer temperatures are best started indoors in cool regions, but if you’re planning on growing them outside in summer they can be moved into the cold frame temporarily before planting out in their final positions.

Don’t risk placing the plants in the cold frame earlier than a couple of weeks before your last expected frost date, and stay alert to forecasts of overnight frosts. Be on hand with old blankets to provide extra insulation when needed. The cold frame will protect them from cold winds and other bad weather that might stunt their tender young growth, while freeing up space indoors for more sowings.

Harden Off Seedlings

All indoor-sown plants benefit from a period of ‘hardening off’ before being moved outside permanently. This can be done by simply placing them in a sheltered spot outdoors for gradually longer periods each day over a week or two, but when you’ve got lots of pots and trays to move it can become a chore. Instead move your seedlings into your cold frame, where hardening off is easy – just open the lid for longer and longer periods each day.

If you can’t be around to open and close the lid during the day, you can work around this by gradually increasing the distance your cold frame lid is open by. The first day open it just a crack. If it’s a drop-down lid rather than a sliding one, insert a thin piece of wood or similar to open it the smallest amount. The next day open it a bit more and the next a bit more still. Don’t forget to close it up before temperatures plummet in the evening though. Make it your first task when you come home from work – dinner can wait!

Using a Cold Frame to Overwinter Plants

Some overwintering plants can spend their entire lives in a cold frame. Hardy salad leaves from winter lettuce to mizuna to corn salad (lamb’s lettuce or mache) can be sown in the frame in late summer and will keep in great condition over winter. Even the hardiest leaves which are impervious to all but the harshest of weathers (claytonia/winter purslane immediately springs to mind) benefit from being covered up, if only to keep the snow off so you can find them to harvest!

Right now I’m using a temporary frame of bricks with two small panes of glass on top to protect some late-sown corn salad seedlings. This simple structure will shield them from the worst of the winter weather, coaxing them along to harvestable size a little sooner in spring.

Or pot up plants that are only borderline hardy and place them into your frame. Some plants such as rosemary can cope with cool temperatures but not with being perpetually cold and sodden. They will appreciate a drier, warmer winter abode.

The more plants you can fit in the better, as it reduces air gaps which can become cold pockets. A trick I learned when using a coolbox for food on trips is to fill any gaps with scrunched-up newspaper to slow the rate at which the coolbox warms. The same principle applies – albeit in reverse – for a cold frame. Use fallen leaves, mulch – even that scrunched-up newspaper – to plug gaps and help insulate the plants inside the frame.

So what am I to do? Make a new cold frame or take the easy-but-expensive way out and buy one? That decision is still to be made. But one thing’s clear: its year-round versatility makes a cold frame an essential item on any vegetable gardener’s kit list.