Borage is a sturdy, annual plant that will readily self-seed and return year after year if you allow it to do so. While borage will generally also grow outside in the UK climate, it may be a valuable addition to the polytunnel as it is said to be beneficial as a companion crop to a range of edible plants that you may be growing. Borage itself also has edible flowers, which resemble cucumber and could be perfect topping for a summer pudding or cocktail.
Borage can be sown indoors around a month before the last frost date in your area or direct sown as soon as risk of frost has passed where you live. When choosing where to grow your borage, remember that it can grow to a final height of up to 1m. Borage should thrive in a polytunnel as it likes a sheltered location out of strong winds that can knock it over. Generally, however, borage will do well anywhere, as it is not fussy when it comes to soil type or conditions.
Borage is a wonderful bee plant and will attract bees into your polytunnel to pollinate your crops. It is considered to be a particularly good companion plant for strawberries, tomatoes and squash, though the truth of the matter is that any crops that require bees for pollination should benefit when you plant borage nearby. Borage is also said to repel or distract a wide range of different pests who might otherwise be problematic in your polytunnel.
If you do not want borage to pop up all around your polytunnel the following year (which may, however, not be a bad thing!) then you should remove the seed heads when they form or collect the mature seeds before they are dispersed and plant them in a more controlled manner the following spring.
Borage has been used medicinally in a number of different ways throughout history as well as for its minor culinary uses. Borage flowers are attractive as well as giving a sweet, cucumber taste. They are often used for flavour and decoration in a salad, or atop a pudding or cake. Borage flowers are also sometimes frozen inside ice cubes and used to make attractive drinks for a summer occasion. Borage will bloom over a long period in the summer and flowers can simply be pinched off as and when they are required. Young leaves are also edible and can be used in a sauce for pasta or a range of other dishes.