If you are interested in growing coffee in the UK, the first thing you should know is that it is not possible to grow a truly exemplary coffee in our climate. The finest coffee grows in conditions very different to our own. While a polytunnel can allow you to grow produce that would not otherwise do well in the climate of the UK, coffee does not thrive when grown in these conditions. That said, as a novelty, a polytunnel can allow you to grow a few coffee plants of your own and perhaps, with a lot of work, to brew up a cup or two of your own coffee.
The best coffees are grown at high altitude and in temperatures much higher than those found in the UK. That said, growing a few coffee plants in containers in your polytunnel could provide a few handfuls of coffee beans. The lush green plants can also be attractive house plants or ornamentals, which will bloom with small white flowers before producing the cherry-like fruits from which your cup of coffee is made.
It is far easier to grow coffee plants from your plants rather than from seed. You must be certain that your polytunnel will be frost free as coffee plants cannot survive even the lightest of frosts. Pot your coffee plants into loam based compost in the early spring and aim to maintain a temperature of 24 degrees from March to September and 22 degrees Celsius for the rest of the year. This can be a costly process and so is not viable for many home growers.
Coffee plants should be fed with a balanced, organic fertiliser every month or two over the spring and summer. Water regularly with rainwater. Your plants may also require pruning, which should be done in the early spring. The coffee plants will look attractive as they grow and bloom, though you should not expect this to be the most productive of plants and from one example, you are unlikely to get enough beans for even one cup of coffee.
Coffee beans will need to be processed before they can be turned into coffee. Harvest the red cherry-like beans and remove the skins (ideally by fermenting) before drying them and then roasting them. If you have taken great care and done everything right, you will likely get an average cup of coffee for all your efforts. Many would argue that rather than expending the energy on heating a polytunnel, it is often better to simply concentrate on growing plants better able to cope with our climate.