As the flowers of high summer start to fade, the autumnal months bring forth a riot of colour when dahlias start to come into their own.
Dahlia seeds first arrived in Europe from Mexico in 1789 and were brought to England in 1798. With origins in the high plains of Mexico, with some species found in Guatemala, Honduras, Nicaragua, El Salvador and Costa Rica, it is little wonder that these stunning plants can be challenging to grow in the British climate. Dahlia pinnata, Dahlia rosea and Dahlia coccinea were the initial named species grown in Europe. However, it was not long before the dahlia growers of the day discovered that this plant, with a rhizome and tuberous roots, when grown from seed was a natural hybrid. As a consequence, over the years many hybrid dahlias have been cultivated, with the original species remaining the basis for all the different varieties of dahlias we see today. There are 42 recognised species with over 20,000 hybrid varieties.
The National Dahlia Society shows 15 different types of dahlia flower head forms on its website. Each form is based on the petal shape, arrangement and whether the flower is single or double. They vary from single rows of petals around a central disc to the fully double dahlia having broad, flat petals, often with a slightly flattened shape. The more you research the varieties available to plant in your garden, the more you will be astounded by the choice of shapes, colours and heights from which you could choose. Dahlias typically start to flower from mid-summer blooming through until the first autumn frosts occur. As flowerheads can vary in size from 2.5cm pompoms to 30cm wide giant decorative dahlias, they are a great way of adding colour to your garden late into the season. Include them in a mixed herbaceous border or plant in a dedicated display. Some cultivars grow into bushes at least 90cm wide by some 1.5m high. However you choose to include them, bear in mind they will also need to be staked and supported as their heavy flower heads can get damaged.
Dahlias thrive best in rich soil with good drainage, so it is worth raising the level of your soil by incorporating well-rotted humus-rich compost. Although dahlias require large amounts of water as they grow, they also need good drainage otherwise their tubers will rot. Their roots grow to considerable depths searching for water, so ensure the ground around is well dug with bone meal, or fish blood and bone, at the rate of three or four ounces to the square metre, added at least two weeks before the tubers are planted. They also need a warm, sheltered, sunny spot to achieve their full potential. If you have lighter soil, your dahlias may survive winter outdoors, but heavy clay soils which are colder and wetter in winter require the tubers to be lifted at the end of autumn and stored in a frost-free place over winter.
As mentioned above, there is a wide range of types of dahlias to choose from; bedding types grown from seed to more expensive tubers sold wrapped in plastic bags from garden centres, and those sold by specialist dahlia nurseries or plant sales.
When planting dormant tubers in mid-to-late spring, start them off in pots which you can keep protected until after the last frost. Make sure they are planted uppermost, positioned just below the surface of the compost. Shortly after planting, ‘stop’ the plant by removing the stem growing point: snapping the young leaves just above the fifth pair of leaves. Shoots will grow from each point on the stem to which a leaf is attached with a bushy plant soon developing. Water lightly and sparingly as leafy shoots appear and harden the plants off before planting in their final location. If you decide to plant the tubers directly into the ground (having pre-prepared the soil) mark out their positions using stout stakes or bamboo canes at four corners of the plant, allowing sufficient space between each tuber. As the dahlia grows, tie in new growth every couple of weeks and ensure you protect new shoots from slugs and snails. Water heavily weekly, and apply a mulch around plants to prevent moisture evaporation. Use a fortnightly liquid feed to keep the flowers coming.
Larger-flowering dahlias and varieties grown for cutting benefit from disbudding: flower buds develop in threes at the stem ends, so keep the central largest bud and snap off the two buds just below it. For a longer clear stem, remove the two sideshoots arising from the first leaf joint below the flower stalk.
Further, it is particularly important to deadhead regularly by plucking off the flowers or removing the flower stalks using sharp secateurs.
In autumn, when the frost has blackened the foliage, cut through the stems 15cm above soil level. If you have suitable free-draining soil, apply a thick mulch to provide winter protection, leaving the plants in the ground. Otherwise, lift the tuberous roots carefully, retaining as much soil as possible. Place the clusters upside down in a frostproof greenhouse or shed, allowing the hollow stem bases to drain and any soil to dry out. Shake off excess soil before dipping the tuber clusters in a fungicide solution to protect against grey mould. Dry the clusters once more before arranging in a single layer (the right way up) in boxes or trays surrounded with almost dry compost or sand, leaving the crowns exposed. If temperatures plummet, place inside larger boxes, packing the space between with straw or newspaper loosely covering the whole. Inspect tubers from time to time and if necessary, cut away any portions affected by fungus, treating any affected surfaces once more with fungicide.
There are a number of pest and disease problems associated with growing dahlias: grey mould (Botrytis) affects stored tubers; aphids and red spider mites can infest young plans in a greenhouse whilst summer aphids; capsid bugs and earwigs may give trouble later in the year in fully grown flowering plants. During wet summers, flowers can become distorted and should be removed immediately. Should your plants become infected with dahlia mosaic, which causes stunted growth, or cucumber mosaic, causing yellow, mottled foliage, they must be destroyed. Both viruses are carried by aphids.
There are a number of places in Oxfordshire for you to visit this September to see dahlias blooming at their best. Millets Farm near Abingdon have Dahlia Beach PYO Farm open from August to October. Working with Chelsea Award Winning Andie McDowell, Dahlia Beach has over 8,000 dahlias planted around a bright pink double decker bus and is also offering flower arranging workshops. On 7 September, Matthew Rice will be opening his garden at Ham Court in Bampton for Dahlia Day. This event is held in partnership with Bampton Gardening Club. You can enter the Dahlia competition by bringing your best dahlia to Ham Court between 9 and 11am at £2 per dahlia (maximum 4 per person) to enter the Dahlia Judging Competition which takes place between 12.30pm and 2pm. Further information can be found at ngs.org.uk.
