Greenhouses are a familiar feature in many gardens and they offer a number of advantages. Some plants will only grow with the heat of a greenhouse and others do a lot better in this environment or are simply more likely to grow when started off this way.
However, many people are concerned about the amount of space greenhouses take up and instead consider a cold frame. Cold frames are essentially small glass boxes that offer some of the same advantages as greenhouses. However they are not as many people think, just a miniature greenhouse. There are many differences between them apart from size and each has its own advantages.
A greenhouse is largely made of glass, supported by a frame that is usually aluminium but may also be wood. They tend to take up a reasonable chunk of garden space though greenhouses as small as 4'x4' are available for the space-conscious.
The “greenhouse effect” caused by global warming is so named for a reason. Greenhouses allow heat to enter but do not allow it to leave quite as easily. This raises the temperature providing an ideal environment for plants that like a warmer environment than they would get outside. This can be enhanced, particularly in cold weather, through the use of a heater. They also shelter plants from wind and rain.
Many plants will grow entirely in a greenhouse and may not grow any other way in the UK. Others will thrive in the condition and will be easier to germinate within a greenhouse before planting outside.
As greenhouses are usually tall, walk-in structures it is not uncommon to have racks to allow plants to be placed on multiple levels though this is usually only done in the early growing stages as it reduces the amount of light they get. It can also simply allow for tools to be stored and then plants placed on the top.
Cold frames offer some of the advantages of greenhouses. However, there are a number of very practical differences.
Cold frames will provide the same warming effects as a greenhouse though they are too small to use a heater to combat very cold weather. Few if any plants can be grown entirely in a cold frame. Instead, they are generally used for “bringing on” plants that will germinate and sprout better with the extra heat. They are also used to essentially extend the growing season. Plants can start growing in a cold frame sooner than they would outside and may survive a bit longer if placed back into it at the end of the season.
Space in cold frames is very limited. Not only are they small but their low height eliminates the use of racks in most cases meaning that you only have floor space. However, taller cold frames designed specifically to incorporate racks are available.
Essentially, cold frames serve a separate purpose from greenhouses. At first glance they may seem like an inferior, if more compact, version of the same thing. However, their size advantage is significant and ideal for smaller gardens. Furthermore, while greenhouses are all-purpose growing environments, cold frames are specifically designed for purposes such as germinating. If this is all you want to do then it will be far better and cheaper to buy a compact cold frame than a full greenhouse.