How to heat a cold frame

Starting your gardening early in the year is all about getting your seeds to germinate properly, and then looking after them through the variable weather as we head into Spring.

First, you’ll need to get the germination temperature right. Seeds don’t normally germinate at this time of year as it’s too cold; many seeds need at least 15 C to kick into action, and some (like peppers) even more.

Many people use heated propagators on a windowsill, but even if your other half is OK with damp soil and plants spread around the house, there will be limited options for more than a few seeds.

A cold frame outdoors is another option – it gives you more space and doesn’t intrude on your living … but without a heated cold frame, the temperature inside the box is rarely going to be much above ambient temperature, perhaps apart from a few hours in the day when the low sun warms it up a little.

Heat your cold frame

Providing some extra heating for the cold frame will make a huge difference – especially if it’s an insulated polycarbonate cold frame. The compact size of a typical cold frame (as opposed to a regular greenhouse) lends itself to being warmed up nicely by not a lot of power. If it’s a portable cold frame, you can also tuck it away out of the wind, which will also help retain heat.

Heat the soil for best germination

It’s actually the soil that you want to heat, rather than the air above it. The seeds start in the soil, and their early roots are in the soil. Warm soil in a still environment will have a warm layer of air above it, which will look after the young seedlings and help prevent frost.

Don’t overheat

Here in the UK we get such variable weather, especially in the shoulder seasons as winter turns to spring, or summer turns to autumn. A heated cold frame could quickly get over 30 degrees with some added sunshine, drying out your soil and cooking your young seedlings. The small enclosed space of the cold frame makes this problem worse – the small space heats up very quickly in the sun.

To stop a heated cold frame from getting too hot, you would need automatic opening, as well as a thermostatically controlled heating system to turn off the heaters when the right temperature has been reached.

What power source for heating a cold frame?

Any reliable heating for a cold frame will need to be electrical. You can use candles, or oil/paraffin heaters, but these aren’t really going to give the same results – and of course they need attention. There are a number of options; soil warming cable, a traditional 100W light bulb, heat mats, or greenhouse heaters.

Soil warming cable

Soil heating cable is great for providing a little extra warmth to large areas of soil, but needs to be dug in … which then leaves you with the challenge of what to do when roots have grown around it. You can’t go in with your spade without risking damage to the cable. And if you’re only heating a cold frame, then you may well not have soil underneath it if it’s parked on a yard or patio. Some soil heater cables can be thermostatically controlled.

100W light bulb

A tried and tested way of heating a cold frame is with a 100W light bulb. It works (and has the added benefit of providing some extra light for the first leaves when they pop up) … but needs to be rigged very carefully so as not to get wet and short circuit. Most cold frames are outdoors, in the wind and the rain. A disadvantage of heating a cold frame this way with a light bulb, is that the air space is heated, not the soil. There’s also the problem that it will continue to heat the cold frame on a warm, sunny day – unless you fit a thermostat switch as well.

Heat pads / heater mats

There are a range of horticultural heater pads available for warming seed trays to speed up germination, many of them also thermostatically controlled. These are great for warming a small amount of soil in a protected, calm environment. Just make sure you get waterproof heat mats, as it’s not guaranteed to be dry in your cold frame.

Greenhouse bar heaters

These are low power air heaters for keeping the frost at bay in a traditional greenhouse. They can also serve to keep a cold frame warm and prevent frost – but from our experience they are not powerful enough to warm the soil enough for good germination.

A waterproof power supply

Almost all the electrically powered cold frame heating systems will run off 240V UK mains electricity, which is not good to mix with water. Make sure you have a waterproof outdoor mains socket within reach of your cold frame, and that your heater equipment has a long enough flex.

The Harvst heating systems are all safe, low voltage equipment, with a single fully waterproof power supply – perfect for adding to a cold frame.

Insulation

There’s no point heating a cold frame if you are just going to lose all the warmth to the outside air, or the ground. Try and prevent draughts. While a sealed box will stay warm, it does have the disadvantage of excess humidty – bringing with it the risk of ‘damping off’. An automatic opening cold frame is ideal here – keeping firmly shut when it’s cold, but ventilating when it gets warm. All the Sprout mini greenhouses are fitted with automatic lid openers for this reason.

The ground is also a heat sink, so if you can put your containers, heaters and everything else in the cold frame on an insulating layer (such as timber or correx sheet) it will help keep heat in the right place.