Natives of Mexico and first introduced into Europe in the 1600’s, Dahlias have been so extensively hybridised that there is now a form and plant height suitable for every area of the garden.
Genetically, dahlias are octoploids, meaning they possess eight sets of chromosomes - four times the number found in humans. This gives dahlias a huge range of forms, heights and vibrant colors which breeders make the most of to produce the staggering range of colours and forms available to us.
Primarily classified by the flower shape, the most common types are the cactus dahlia, with a spiky appearance, and decorative dahlias which include a range of forms from tight ball-shaped flowers through to more open forms called water lily dahlias. As well as these two, other common types include dwarf forms ideal for borders or potting, and single flowered forms which are widely used for landscaping.
Dahlias are frost tender and should not be planted out until all danger of frost has passed. To flower and grow well, they prefer a location in full sun, but will tolerate some morning shade. Drainage is very important as the tubers do not like being saturated for any length of time. In high rainfall or wet areas they can easily be grown by using slightly raised beds to ensure rapid drainage.
Plant the tubers with the crown just below the surface of the soil and water in. Little attention is required during growing other than to spray occasionally for thrips in hot conditions and with a fungicide in humid conditions as the foliage and flowers may be damaged otherwise.
Removing old flower heads will encourage further flowering, and will also prevent the plant from wasting energy with seed production.
Dahlia plants go through different stages of growth depending on the day length. During long day length in mid summer the plants produce a lot of top growth and flowers.
Early and late in the growing season when the days are shorter, the plants put most energy into tuber production below the ground, ready for dormancy during winter.
Because our winters are relatively wet, tubers can be lost to rot if left in the ground over winter. It is best to dig them up each year in late autumn after the first frosts have knocked the leaves back. Lift them carefully and trim the stems back to a length of 6-7cm.
Split any large clumps up into smaller ones. Be sure to keep some of the crown as a part of each section when splitting the clumps – it is from the crown that the new shoots develop in spring, and without a part of that a section may be blind, with no growing point. Rinse the tubers off to clean them and after drying, store them in a cool, airy place until replanting in spring.
The flowers can be picked for indoor use but will not continue to open when cut from the plant, so should be picked only when they are open already.