Ginger is a really great plant for a polytunnel. While the temperatures can be too cold to grow ginger outside in most of the UK, a polytunnel can help to give ginger that extra warmth and humidity that it needs to grow. Ginger is most often grown for its fiery rhisomes, though ginger stems are another exotic ingredient that can be harvested if you grow your own at home.
Ginger rhisomes can be obtained for supermarkets or from online suppliers in the early spring. They will grow best in a free-draining, light, sandy growing medium. Plant rhisomes to a depth of around 10cm with the growing buds facing upwards and just above the soil surface. Keep in a warm place indoors and then transfer to the polytunnel when the weather warms up sufficiently.
Ginger will need plenty of water and so will have to be well watered and regularly watered over the summer season. Ginger also likes high humidity so is best grown with other plants that also enjoy humid surroundings. You can raise the humidity in your polytunnel on a hot day by watering the paths between your plants as well as the plants themselves.
If you wish you can simply allow the stems and foliage to die back naturally at the end of the summer, at which point you should reduce and then stop watering as the colder weather arrives. This encourages the formation of rhisomes (and you could always re-plant some of those rhisomes the following spring).
Alternatively, you can continue to use your ginger plants to harvest stem ginger, a subtle, warming vegetable with none of the fiery, dry heat of the rhisome. This can be difficult to get hold of in the UK, though it is prized elsewhere. To harvest stem ginger throughout the year, you need only take your ginger plant inside to a warm windowsill/ conservatory.
Ginger rhisomes are ready for harvest once all the leaves on the plant have died back. You can then use that ginger fresh, it will keep in your fridge for a month. You can also finely grate the ginger and dry it in your oven before grinding for use in baking recipes. Stem ginger can be harvested as and when you wish to use it in recipes. It can be used for a wide range of sweet and savoury dishes.