Dahlia come in many different varieties and can be attractive in many gardens. They are also popular for use as cut flowers. While Dahlia are fairly easy to grow, they do need some protection during the colder months in the UK. This is where a polytunnel can really come in handy – in giving dahlia protection from the frosts of winter.
Dahlia will need a fertile and free-draining soil, though they are tolerant of a wide range of soil types. They should be planted out after all risk of frost has gone where you live. An unheated polytunnel will give you a bit of a head start by mitigating the risks posed by a late frost. Tubers are best planted in their final growing position and do not usually respond well to transplanting.
Dahlia tubers should be planted into soil rich on organic matter at a depth of around 10-15cm. It is best to pinch out the growing tips once the seedlings have reached a height of around 40cm. This will encourage bushiness in your plants. Canes should be inserted when you plant to tubers and then the dahlia should be tied into these canes as they go to provide some support.
Keep your growing dahlia well watered and over the summer, get best results by fertilising your flowers with a high potash organic liquid feed to encourage flowering. A high potash organic mulch will also help to ensure the best growth.
Should large flowers be a priority, you can restrict the number of flowering stems in each plant to between three and five. For smaller, more prolific flowers, leave between seven and ten flowering stems. To encourage a long-flowering display and strong stems, remove the two pairs of flower buds developing in the leaf axils below the terminal bud. Deadhead dahlia as the flowers begin to fade.
You can cut dahlia flowers as and when required for use in floral arrangements and bouquets. Dahlia will flower from mid-summer until autumn. In milder regions, especially inside your polytunnel, you may be able to store dahlia tubers in the ground over winter. Cut dahlia's to the ground after they have begun to die back and cover the ground, providing a little extra protection to the tubers with bark or compost. In colder areas or areas where the ground may be prone to waterlogging, lift tubers and store in shallow trays of peat free compost or dry sand, leaving the crowns exposed. These tubers may then be replanted the following year.