Heating a cold frame in winter

[Nickjr44]

I have made a wooden cold frame with polycarbonate sheet windows on the top. I'm wanting to start my seeds in this then plant them out at the right time. To keep a temperature at about 18c has anyone used a tubular heater electric before? I. Just intending to use this in the winter. My box is 1.2m x 600 mm wide and raised off the ground on a stand.

[Bigmallly]

Hello Nick & welcome to the Jungle, heating a coldframe goes against what it is intended for which is to acclimatize your plants prior to planting out............having said that here are a few ideas.

10 Ways To Heat A Cold Frame Over Winter - ThePrepperProject.com

[Nickjr44]

Thank you for your ideas, its just to get my seeds started just like a propagator so was just wondering if an electric tubular heater would be any good!

[Bigmallly]

I don't see why not Nick. I maybe under use mine as you can grow all year round in one:


https://www.hws.edu/fli/pdf/cold_frame.pdf

[fishpond]

The problem with using a tubular heater in this situation when most (if not all) of the plants will be at ground level, is that there will be a big difference between the air temp (warm) and ground temp (cold).
Assuming a power supply is available to use and you are able to completely water proof the box on the themostat then I would think a Parasene soil warming cable( either 3 or 6 metre--(you will need to work it out given the area you need to heat) with a suitable thermostat would be of more use, as this will keep the roots warm, whilst also supplying gentle heat to the leaves to stop them getting frosted.
A max/min thermometer within the cold frame would also be useful.

[Nicos]

I can't offer any help...but my first thought is won't there be condensation...humidity..possibly causing problems?

[fishpond]

Hi Nicos
I assumed the lid could be opened very sightly to allow air to circulate (it may be possible to fit an automatic greenhouse window type opener or 2 to the lid, but it may not be worth the trouble when a twig or a small stone would do the job). I have never seen condensation problems on the big Victorian cold frames you find in Victorian walled fruit /veg gardens. But then again those cold frames never appeared to have closely fitting lids.

[ESBkevin]

If you dig a big hole under it and fill with fresh manure the composting process creates steady heat for weeks and you can place seed trays on top inside the cold frame.

[JusPotterinTim]

Hi Nickjr,

A well sealed cold frame with polycarbonate glazing should be a good environment for using tubular heaters, I guess it could then be called a warm frame? Obviously you want to ensure the electric supply cables etc. don't get drenched. Most of the tube heaters are splash proof which should mean they'll be fine unless their constantly exposed to water.

I've got a wooden mini greenhouse with double-walled polycarbonate glazing, about 1.2m high, and 60cm wide and deep, which I've got a 90W tubular heater in.

My advice would be to seal all gaps between panels and around the glazing, to stop rain water and cold air getting in. I used clear silicone which worked very well. I also stapled bubble wrap along the frame in areas which don't get sunlight coming through, for a bit of extra insulation, and laid a double layer of bubble wrap on the ground.

As Nicos says moisture buildup can be an issue in a small grow house, but with my wooden greenhouse, a few hours with the lid open on a sunny day and the moisture evaporates. I also fitted a Bayliss XL greenhouse vent opener to the lid of mine, which does a great job in opening and ventilating the greenhouse when it gets hot. It could probably be adapted to fit on a cold frame too if you have a hinged lid. If it's cool and wet for a couple of days with no sun to dry it out I wipe the condensation off the panels.

For me, the 90W heater allows a 10 Celsius increase in temperature at night. I also use a cheap wireless thermometer with an alarm on it, so I can see if it's geting too hot or cold. The thermostats on the tube heaters aren't brilliant, so the temperature knob needs turning up on a cold night and reducing once it's a bit warmer outside to maintain a fairly constant temperature. I aim for 15C minimum for my chillies.