Equipment: Which cold frame would be best?

[newstart]

Is it best to have a clear glass or plastic cold frame for winter when light levels get low. I thought about polycarbonate etc but am wondering if plants do okay over winter in them. If polycarbonate is adequate and does not loose plant vigor etc over winter I would prefer to buy this as its cheaper.

What are folks opinions ? I come from the UK.

Would really appreciate peoples input. Many thanks!

[David Shaw]

I prefer glass, however glass breaks! I regularly have glass lights breaking either due to the wind or something falling on them. The main problem with this is that the glass shards and pieces fall down onto the plants beneath.

[vivienr]

We have had plastic coldframes but found it impossible to keep the lids on in windy weather and sometimes the whole frame would take off. We made a large wooden sided coldframe with acrylic sheets for a lid but gave up with that too. Last weekend we lugged it down the garden to turn it into a new vegetable bed.
So I could only recommend plastic if you have somewhere very sheltered to put it.

[newstart]

Thanks for both you opinions much appreciated! Does any one else want to add to this?

[ellen&dan]

Hi there
 We have polycarbonate and clear plastic on our cold frames and they both perform well for us. When it is windy we just close the lids and put a piece of wood on top.

[Giles]

I use Halls Aluminium Jumbo Frames.
They are aluminium, and can be fitted with toughened glass (doesn't break!).
The lids are hinged, and can be slid to one side. They will not blow off in the wind.
The sides of the frame are glass too.
I like them due to:
Maximum light for plants; Strong; Easy to clean/very hygienic.
http://www.hallsgreenhouses.co.uk/index.php

[Rodger Whitlock]

While different cold frame designs offer various advantages and disadvantages, in my opinion most of these differences are of only minor significance. There are some operational issues that demand attention, however.

1. Resistance to wind

If your area ever gets high winds, it's critically important that your coldframe not take flight in a powerful gust. Weight alone is not necessarily sufficient to stabilize a cold frame. Be prepared to fasten your coldframe(s) down to some kind of foundation. The cover must be especially resistant to the wind, lest it be lifted off and blown into the next county. Even if hinged on one side, you don't want the cover to start flapping.

2. Transparent vs. opaque sides, and depth.

If feasible, affordable, and practicable, it is better for a cold frame to have transparent sides (glass or plastic) down to the level of the tops of the pots. Below that level, opacity is desirable to prevent the sun shining directly on the walls of pots and causing the soil in them to overheat.

My own cold frames are 48" wide × 32" front to back, wooden boxes cobbled together out of heavy yellow cedar fence boards. The front is 12" high, the back 18". This is too deep, as the pots are generally about 6" high, so the plants get somewhat drawn reaching up toward the light.

3. Whether to plunge pots or not

A cold frame resting above a sand bed allows pots to be plunged into the sand. Even with plastic pots, this helps greatly to stabilize moisture levels in the pots as long as the soil in the pots is in capillary contact with the sand via the drain holes. My own frames rest on a concrete slab, and I cannot recommend that. Even a gravel bed would be better! When water is draining out of the pots, it washes fine silt out to form a layer of mud in which the pots rest.

4. Ventilation

It is critical that you be able to keep your frames well ventilated even in bad weather. Generally speaking they should be shut down tightly only during serious cold spells. You want to evaluate your options in part by asking the question, can I crack the cover open a little without it being blown away and without rain getting inside?

5. Front to back depth

Besides depth, the distance back to front is critical. I determined that my own frames should be 32" by kneeling and seeing how far I could reach if I were picking up a pot. Someone with long arms can make good use of cold frames deeper back to front than someone with short arms, who will need a shallower design. Of course, a two-sided frame will be twice the optimal arm-reach distance.

Perhaps it goes without saying that the cold frame design you select has to fit your budget. If you can't afford a purpose-made frame of glass and aluminum, a simple wooden box with an old window as a cover can make a perfectly satisfactory — and much cheaper! — substitute. The plants are sublimely indifferent to such issues.

[Diane Clement]

We all have different needs and you need to prioritise what is important for you. 
I bought a new frame last year.  My priorities were that I wanted a large frame around 10' x 4'.  It was to be a shade frame to grow woodland type plants in pots: arisaemas, cypripediums, dwarf lilies, erythroniums, anemonellas, etc.  I didn't want a traditional frame placed on the ground, I wanted it to be raised off the floor with sides so that I could have a sand plunge.  I did not have the time nor expertise to build this.  I have in the past used polycarbonate, but in the long term, I prefer glass.  After much research, I chose an Access frame, which were on special offer last summer.

http://www.garden-products.co.uk/customer/product.php?productid=22247

The base is made of tanalised wood and is available in add on 6" depths.  This was an expensive extra but in the end I think was worth it.  The frame has sliding doors along the sides.  The roof is sliding but I may just remove the roof glass in the summer.  I am in the process of sorting out some shading and that should be in place shortly.    
Here it is in situ a week ago.  I had to clear a large bed to site it, and I now need to make paths round it, but I was waiting for the spring to see what plants came up round the edges!  So it still needs finishing off, but I have been very pleased with the results.