Get a head start on your summer crops with a simple cold frame

The problem

I now have a lovely greenhouse which is fantastic for protecting all my seedlings from a variety of things, be it animal or weather related. It gets really warm in there during the day but the night-time temperatures are still pretty low. The thin plastic cover only adds a couple of degrees to night temperatures but certainly keeps out the frost and wind chill factor. It is great for germinating most seeds but I want to get a head start on summer crops. I need to start tomato and eggplant seeds in July/August (but they need a soil temperature of around 22 degrees Celsius).

I have been planning to build some kind of propagation chamber for a while and I ran many ideas over and over in my head. I was going to build a great big heat mat but I decided I really didn’t want to use electricity unless absolutely necessary. I also need something simple that does not require much of my time.  I have read articles about running plastic irrigation pipes under beds and filling them with warm water each day or heating things with compost. All these are great ideas and I would not try to put anybody off trying these techniques but I don’t have time for mucking around with stuff like that at the moment.

The solution

Enter the cold frame! Here is a link to a page with some general information about cold frames. Cold frames are generally used outside a greenhouse to “harden off” plants grown inside, ready for planting out in the garden. You lift the glass lid during the day but close it at night to keep the warmth in. Note that it still needs to be open a fraction, since plants still need oxygen at nighttime.

I’ll be using them differently, keeping the cold frame inside the green house. The daytime temperatures inside the greenhouse are currently around 25ºC, when outside it has been around 15ºC. That is really about as low as it gets here on a sunny day during winter (maybe 13-14 on a cloudy day) and that is more than warm enough to germinate any summer crop. The problem is that at night it can get as low as -1 ºC, too cold for summer crops. I would like to keep the temperature above 10ºC, and ideally around 15ºC . I have read that when using a cold frame the night-time temperature should be around 10ºC above the ambient temperature outside. I am confident that this should be ok. A misconception about germinating seeds is that seeds require constant temperature to germinate. In fact, they prefer to cool down at night (just like in nature).


Cold frame materials and cost

I have made a very makeshift coldframe using materials I had lying around. I didn’t want to spend any money until I am sure this idea actually works.

The frame is large enough to hold 4 of my custom-made trays. Here is what is cost to make.

Screws: 20 ish- about $1??

Gaffer tape: 3m-  50c??

Glue: a bit-  $1??

I broke a drill bit which I will need to replace for $3

If this works well I will build a large one and use a large piece of glass from a sliding door which should look much nicer.

I will start my trial with tomatoes first using some old seed. Keep posted for updates in the next few weeks!


About Healthy Harvest Kitchen Gardens

Healthy Harvest provides a permaculture-based kitchen garden service for existing food growers and people who are inspired to grow food. We sell fresh chemical-free seedlings, seeds and seasonal produce. We also provide educational workshops, courses, gardening consultations and services. We can help with new vegetable garden designs, maintenance of established gardens, or converting and improving existing gardens ready for growing food. Please see our website for more information: Check out our blog at for news and interesting bits and pieces.

Posted on June 6, 2011, in Gardening techniques, Greenhouse, Seed raising, Seedlings. Bookmark the permalink. 2 Comments.

  1. Hi Diana,

    The frame sits directly on top of the soil which holds the heat (I believe) better than pavers etc. I does take a few days of having the cold frame empty with the lid down to get the temperature into the soil before you can use it. Thanks for the comment.

  2. GReat idea! I’ve been putting my seedlings on various metal frames to deter seedling gobblers. How about putting it on pavers / bricks / dense matter that will hold thermal heat until the evening and then release it? ie the same principle as a concrete floor in a passive solar house -giving it thermal mass. I put my pot plants in a sunny spot on pavers in winter. Seems to help.

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