As the four boards aren't fixed in position but are moveable, as always in the system I use, they can easily be moved somewhere else, or used for a completely different function. In fact, they were moved from somewhere else, and their function was completely changed, in a striking illustration of the flexibility of the system. A very short time before, the four boards had been used to make up, not two cold frames but two corner composters, one full, one empty -
Just as easily, the four-sided cold-frame could be converted in a minute or two into two corner cold-frames. The two corner composters could be converted into one four-sided composter.
Joy Larkcom writes in 'Grow your own vegetables' that a garden frame is 'more substantial than a cloche, it is less affected by wind, better insulated and offers more protection from frost.' This certainly applies to the PHD frame, which offers excellent insulation and excellent wind resistance (although the PHD cloche is rock-solid in high winds.) And, later: 'Broadly speaking, frames are used in the same way as cloches, although they cannot be moved from one crop to another so easily - if at all.' The PHD frame eliminates these difficulties. It can be moved very easily - simply lift the boards, so that the supporting stakes are pulled out of the soil and reinsert the stakes, with boards attached, in the new position. Crops can be left to grow under the protection of the frame and then, when the protection is no longer needed, the frame is simply removed: a matter of moving the separate boards, not a frame as a heavy unit.
The height of the frame shown above is 30cm. This makes it suitable for a very wide variety of crops. A crop such as lettuce, low-growing at maturity, is perhaps better sheltered by the low board system described in the page 'Bed and board.'
The roof of the frame is flat, allowing rainwater to be collected. Although there are periods in this country when drought is a distant memory, the trend is very much towards drought conditions and water ought not to be wasted. I don't provide further details here of the system used, but the roof of the composter is in three parts and allows the water to be diverted to the crops. In winter, it's better if water is diverted to 'the soil at the base of the plant, rather than the leaves, to lessen the risk of fungal disease' (quoting again from Joy Larkcom's 'Grow your own Vegetables.') I've devised a system which makes this possible.
Adequate ventilation is easy to achieve. One, two or all three of the roof sections are removed, or, alternatively, propped open. There's an additional means of ventilation which isn't possible using the traditional cold frame: since the boards are supported on stakes, the boards can be raised from the soil to a greater or lesser extent, giving air gaps at the base. The PHD cold frame, then, gives complete flexibility in hardening off seedlings and young plants.
The PHD cold frame can also be used as a 'greenhouse composter,' a composter which increases the length of the composting season and the temperature inside the composter by means of the greenhouse effect. The equipment is identical, even if the uses are very different. There's more information about this use in the page on composting and composters.