Dahlias once again take centre-stage in the flower border

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Dahlia 'Top Mix' Picture courtesy HadecoDahlia 'Top Mix' Picture courtesy HadecoFlower fashions come and go, and although dahlias fell out of favour for a while, thankfully their bright and bold blooms are back in vogue. There are so many varieties, each one more beautiful than the last, that choosing dahlias for your garden is much more difficult than growing them. Read more below about the fascinating history of dahlias, where to grow them in the garden, and the care they need to keep them blooming all season long.

The bulbs shown here can be ordered online from Hadeco. Click here or on the pictures to see their lovely range of summer bulbs.

The Genus Dahlia belongs to the Asteraceae family of plants, and its close garden relatives include the sunflower, daisy, chrysanthemum, and zinnia.  The genus is native to Mesoamerica, but principally to the high plains of Mexico where, because they grow like weeds in open valleys, dahlias were declared the national flower of Mexico, in 1963.

Dahlia Cactus 'Pink' Picture courtesy HadecoDahlia Cactus 'Pink' Picture courtesy HadecoThere are about 12 original species from Mexico, out of a total of 35 recognised species in existence today, with species also occurring in other regions of Central and South America, including: Guatemala, Honduras, Nicaragua, El Salvador and Costa Rica.

These native wildflowers were cultivated by the Aztecs who held them in high regard and associated them with the sun gods. The skins of the tubers have natural antibiotic properties, and were used medicinally to treat many illnesses, and their starchy, inulin rich tubers, were eaten like potatoes.  In addition, the inulin in the tubers was converted into a natural sweetener. Even the huge tree dahlias which produce large, hollow stems, were used to collect and carry water. Sadly, much of the Aztec’s historical uses for the dahlia were lost during the Spanish Inquisition, and attempts to introduce the tubers as a food crop in Europe were unsuccessful.

After the Spanish Inquisition, Spanish botanists were sent to Mexico to study plants which might be useful to Spain, and one such botanist, Francisco Hernandez, drew illustrations of these wild dahlias, but his work gained very little interest until many years after his death when another botanist published them. This resulted in yet another botanist, Antonio José Cavanilles acquiring seeds from three varieties of wild dahlias to grow at the Royal Botanical Garden in Madrid. Towards the end of the eighteenth century Cavanilles Dahlia Border 'Carola' Picture courtesy HadecoDahlia Border 'Carola' Picture courtesy Hadeconamed these plants “Dahlia” after the highly respected Swedish botanist Andreas Dahl, and from these three wild forms he bred other unique varieties. The initial named species imported into Europe were Dahlia pinnata, Dahlia rosea, and Dahlia coccinea.

Dahlias were such a big hit that eventually the Royal Botanical Garden of Madrid shared seeds with botanists in England, France and Germany, and the ‘Dahlia-rush’ was on, with botanists all over Europe breeding and cultivating new varieties with great enthusiasm. German botanists originally rejected the name Dahlia, and instead, classified the plant as Georgina, and in some parts of Europe, dahlias may still be labelled as such.

The first dahlias grown outside of Madrid produced single, open-centred, and multi-rayed flowers, but it was not long before the horticultural growers of the day discovered the Dahlia was a natural hybrid, and when grown from seed, it readily changed both its form and colour. This was terribly exciting and the race to produce the most beautiful dahlia began, resulting in the vastness of varieties available to gardeners today.

This great variety results from dahlias being octoploids, meaning that they have eight sets of homologous chromosomes, whereas most plants have only two. In addition, dahlias also contain many transposons—genetic pieces that move from place to Dahlia Cactus 'White' Picture courtesy HadecoDahlia Cactus 'White' Picture courtesy Hadecoplace upon an allele—which contributes to their manifesting such great diversity.

Dahlia cultivars eventually made their way back over the pond to the United States, where even more cultivars were created, and the first Dahlia Society was started in San Francisco in 1917. Dahlias are now the official flowers of the cities of San Francisco and Seattle.

These beautiful cut-flowers are perennial plants with tuberous roots and garden hybrids come in many shapes and sizes, from small, 5cm in diameter, ball-like dahlias called “poms”, to the huge dinner-plate varieties which can reach 30cm in diameter. The height of the plants range from as low as 30cm in the dwarfs, to more than 1.8 to 2.4m in the giant tree dahlias.

The first double flowered cultivars were called “Show” and “Fancy” types. The Show types were single-coloured, ball like flowers, while the Fancy ones were multi-coloured. Around the middle 1800’s these dahlias became so popular that they attained cult status with gardeners and several thousand different cultivars were recorded.

In 1829, in Germany, the first “Anemone” flowered dahlia appeared, followed in 1850 by the first “Pompon” variety.

Dahlia Decorative 'Edinburg' Picture courtesy HadecoDahlia Decorative 'Edinburg' Picture courtesy HadecoThe “Cactus” and “Decorative” type dahlias are garden hybrids resulting from a cultivar developed in Mexico, and named “Juarrezii” after one of their presidents.  A tuber of the Juarrezii cultivar was imported from Mexico to Holland in 1872, and grown by M. Van den Berg of Uttrecht with great success. Later it was introduced to the United Kingdom, and the rest is history, with these magnificent flowers remaining a favourite of gardeners the world over.

The so called “Collerettes” form of dahlia have their origin in France, and are due to ‘sporting’ of dahlias at Jardin Botanique de Lyon at the end of the 19th century. A dahlia sport is done by taking a cutting from an arm of the plant to root and grow. These are overwintered and planted out to form tubers. The tubers are then overwintered and replanted for about 4 to 5 years to ensure that they don't revert back to the original or change in any way, resulting in a stable sport which can be introduced and sold as such.

Today, the 50,000 or more varieties of dahlias are all derived from the three original varieties that Antonio Jose Cavanilles first grew in Madrid, and they are classified by their shape, size, and the colour of their flowers. They can be single, or double flowering cactus and semi-cactus types with their pointed, rolled petals; or formal or informal decorative dahlias with their broad, flat petals. The ball forms, with their dense, globular flowers, have as their smallest relative the beguiling and ever popular Dahlia Pompon 'Bell Boy' Picture courtesy HadecoDahlia Pompon 'Bell Boy' Picture courtesy Hadecopompon dahlias. And not forgetting those in the beautiful forms of the waterlily, the peony, the orchid, the chrysanthemum, and the anemone.

Modern hybrids are available in almost every conceivable shade, except for blue, and in beautiful combinations of colours. And although there is not yet a blue dahlia, there is a wide range of violet and mauve cultivars available, and breeders are still creating new cultivars and hoping to be the first to create a ‘true blue’ dahlia. They are also hard at work developing scented and frost hardy varieties.

In the Garden:

With so many different varieties of dahlia available to gardeners in South Africa, there is a perfect dahlia for every garden style and size. Each one is more beautiful than the last, making choosing dahlias much more difficult than growing them, so visit your garden centre to select your favourite favourites.

If you plant your dahlias in late spring, the flowers should start to appear from mid-summer, and they are at their best in late summer, through autumn and into early winter, when many other summer flowering annuals, bulbs and shrubs are past their best. For this reason, they are invaluable to extend the flowering season in the garden.

Despite the fact that they have a relatively short vase life, dahlias make gorgeous bouquets, and the flowers symbolize grace, Annual Dahlia Harlequin 'Formula Mix' Picture courtesy Ball StraathofAnnual Dahlia Harlequin 'Formula Mix' Picture courtesy Ball Straathofchange, uniqueness, kindness, elegance, creativity, dignity, commitment and balance.

To keep things simple dahlias can be put into three broad groups: large growing dahlias, medium and small growers:

Large dahlias, like the well-known dinner-plate varieties range in height from about 90cm to 1.2m and produce large double blooms 25 to 31cm in diameter. These are most effective when grown in large groups at the back of perennial borders, together with other large perennials. They will require some kind of support to withstand summer winds and rain. Try thick bamboo stakes or whimsical spiral supports.

Medium-sized dahlias are often referred to as “border dahlias” because they are compact, and grow between 30 and 60cm tall, but still produce good sized blooms. These dahlias look wonderful towards the middle and front of a perennial flower bed, mixed with other late-blooming perennials. Medium-sized dahlias will also thrive in larger pots, and for something different, consider cactus-flowered dahlias, with their distinctive rolled petals.

Dahlia Dinner Plate 'Lavender Perfection' Picture courtesy HadecoDahlia Dinner Plate 'Lavender Perfection' Picture courtesy HadecoThe easiest of all the dahlias to grow are the small, annual, bedding varieties which are sold in seedling trays. These grow between 30 and 50cm tall, these compact little dahlias are charming and bloom for a very long time, making them perfect for pots and window boxes, and invaluable to use as an edging for a border ,or as a filler for any small and bare, sunny spots in the garden.

The bulbs shown here can be ordered online from Hadeco. Click here or on the pictures to see their lovely range of summer bulbs.

Cultivation/Propagation:

In South Africa the best planting time for dahlias is in late spring, when all danger of frost is over and the weather is nice and warm. They will flower from mid-summer, through autumn and into early winter, or until the first frosts.

To fully understand what dahlias require in the garden to really flourish, you need to understand a bit about the climate where they originate. Dahlias are native to the mountains of southern Mexico and Central America, where summer days are warm (27 to 30°C) and nights are cool (18 to 21°C). This montane forest ecoregion is made up of forest patches occurring in island-like mosaics on the isolated tops and slopes of the highest mountains of Central America, from southern Mexico into northern Nicaragua. At such altitudes, the tropical climate gives way to a more temperate-like climate with fairly high precipitation.

Therefore, although dahlias like warm to hot gardens and require plenty of sunshine and water to flower well, they do not enjoy blazing hot and dry gardens, and when the sun is too hot and temperatures are too high the plants may stop flowering, so in these regions they are best planted where they will receive full morning sun and filtered afternoon shade, or shade at midday. Deep shade promotes lanky growth and few blooms.

Because the high altitudes at which they grow are less humid and more temperate, they may struggle in very hot and humid Dahlia Terrace 'Salsa' Picture courtesy HadecoDahlia Terrace 'Salsa' Picture courtesy Hadecoregions, where they are more susceptible to fungal diseases like powdery mildew.

Dahlias are tender to frost, and in cold regions they are often planted out as annuals every spring, or the tubers are lifted and stored in autumn for planting next season. In milder winter regions the tubers can be left in the soil to overwinter, but in the winter rainfall regions of the country, its best to lift and store them.

Both tubers and trays of seedling are available at garden centres. Always ensure that dahlia tubers are firm and plump, without any sign of rot. Trays of dahlia seedlings are available during the summer months at garden centres, ready to be planted straight into your garden, and these bedding varieties are treated as annuals in the cold and frosty regions of South Africa.

You can plant Dahlia tubers in any type of soil, just as long as the drainage is good. Sandy soil types work best, especially if they are rich in nutrients. In the garden it is good to dig the entire area over well where you are going to plant groups of dahlias, adding lots of organic matter like well-matured manure or compost before planting.

To plant individual tubers, dig a planting hole about 15 to 20cm deep and replace about 5cm of the soil before adding another 5cm layer of washed river sand (available from garden centres). The river sand helps to protect the tuber from rot. Place the Dahlia Decorative 'Edna Comstock' Picture courtesy HadecoDahlia Decorative 'Edna Comstock' Picture courtesy Hadecotuber on top of the layer of sand with its individual ‘fingers’ spread out, and the short, dry stalk pointing upwards.  Fill the hole with the remaining soil, firming it down gently.  Remember to plant stakes when you plant the tubers of tall growing varieties. For potted specimens a good, well drained yet moisture retentive potting soil is advised.

Water the newly planted tubers well and then refrain from overwatering until the plants have emerged above the soil, as overwatering may cause the tubers to rot. Once the plants are actively growing you can water them regularly, and deeply, about 2 to 3 times a week, depending on rainfall and the quality of the soil. Potted plants will always require more frequent watering than those planted in garden beds, so check your pots regularly.

Although dahlias need consistent moisture, they will not tolerate soggy soil. This can be a challenge in tropical climates with wet summers, so consider planting dahlias in containers, and position them where they have some shelter from heavy rain.

Drip irrigation is ideal for dahlias, and if you are hand watering, water at the soil level to avoid wetting the foliage, as this can encourage powdery mildew. However, if you are growing dahlias in hot and dry summer regions, they will thrive if they can be ‘misted’ down occasionally with water to keep them cool.

To create a bushier plant, pinch off the top growth of the dahlia shoots above the third set of leaves while the plants are still young. Shorter varieties are naturally bushier and won't need pinching.

Feed all your dahlias, and especially those in pots every 3 to 4 weeks from mid-summer to early autumn using a low-nitrogen, water-soluble fertiliser, like Pokon Flowering Plant Food. Avoid using a high-nitrogen fertiliser, or you will grow lots of leaves at Dahlia Cactus 'Alvas Doris' Picture courtesy HadecoDahlia Cactus 'Alvas Doris' Picture courtesy Hadecothe expense of the flowers. Never over feed dahlias as this can cause low-yield blooms or tuber rot.

Mulch the soil around your plants with straw, compost, or shredded bark to keep weeds to a minimum and to retain moisture and keep the roots cool.

To keep the plants blooming for longer, cut the flowers regularly for indoor flower arrangements, and remove all old blossoms.

The tubers will go dormant in winter, and in most regions of South Africa dahlia tubers can survive the winter in the soil if they are protected with a thick layer of mulch, which protects them from freezing, and if the tubers are kept relatively dry. In the winter rainfall regions it is best to move pots out of the rain, and to lift tubers growing in garden beds to store over winter.

In very cold regions, or if you simply wish to make space for winter flowers, the tubers can easily be lifted and stored for next season. In autumn, after the first frost has blackened the foliage, cut off all but 5 to 10cm of top growth, before gently lifting them out of the soil without damaging them. Allow the tubers to dry for a few days in a frost-free location, out of direct sunlight. Once dried, brush off any excess soil, and store each clump of tubers in a ventilated box or basket. Fill the box with slightly moistened sand, peat moss, or vermiculite, and place it in a cool, dry location where the Dahlia Pompon 'Amusing' Picture courtesy HadecoDahlia Pompon 'Amusing' Picture courtesy Hadecotemperatures remain between 7 and 13°C.

Check the tubers periodically through winter for rotting or drying out. If the tubers appear shrivelled, mist them lightly with water; and if any start to rot, cut the rotted portion out of the clump so it won't spread. The tubers are fragile, so be careful when handling them. When warm weather arrives, you can plant the overwintered tubers and begin the cycle again.

Problems, Pests & Diseases:

Well-grown and healthy dahlias are relatively pest and disease free, and pests are usually confined to sucking insects and some treatable fungal problems like powdery mildew and grey mould, however, there are a few diseases which can actually kill the plants.

Often the first sign of trouble in dahlia plants occurs just as they sprout and develop leaves, and it is common to find new leaves completely chewed. The culprits are usually caterpillars or some form of larvae which feed on leaf tissue and make Swiss cheese of the foliage. If you catch them early enough you can pick the caterpillars off by hand, or to control the larvae and break the cycle you will need to spray.

Kirchhoff’s Margaret Roberts (Dipel) Biological Caterpillar Insecticide contains Bacillus thuringiensis var. kurstaki bacterium, which targets destructive leaf-eating larvae of Lepidopterous spp, and can be used on vegetables, fruit, flowers, bulbs, clivias, lawns, and herbs, and harvesting of edible crops can be done directly after application. The caterpillars eat the sprayed foliage and stop feeding within a day or so, and they may hang from the leaves before rotting and dropping to the ground, usually within Dahlia Top Mix 'Yellow' Picture courtesy HadecoDahlia Top Mix 'Yellow' Picture courtesy Hadeco3 to 4 days.

Biogrow’s Neudosan contains potassium salts of fatty acids, and is an environmentally friendly insecticidal soap with improved efficacy against soft-bodied insects and mites. It may be used in organically certified crops, and the crops can be harvested immediately after spraying. This product is known as a fast acting, contact insecticide – miticide, and is very effective against soft-bodied insects and mites, particularly those of the order Homoptera (e.g. aphids, leafhoppers, mealy bugs, psyllids, scales and whitefly). However efficacy is also obtained in the following orders: Thysanoptea (thrips), and Lepidoptera (caterpillars).

Numerous insect pests find dahlias delicious, and in addition to the caterpillars and larvae, sucking insects are probably going to be the biggest issue with the most widespread offenders being: Aphids, Thrips, Mites, and Leafhoppers. Although these are relatively tiny pests, their habit of sucking sap from the plant can diminish its health, cause stunting and distorted plant parts and even transmit disease. Horticultural soaps like Biogrow’s Neudosan are great to treat these, and even blasts of water can minimize their presence.

Dahlia Top Mix 'White' Picture courtesy HadecoDahlia Top Mix 'White' Picture courtesy HadecoSeveral types of borer are also dahlia flower pests, and using systemic insecticides may offer some protection if applied early in the season.

Slugs, snails and cutworms produce topical damage that is visually obvious, and also reduces the plants health and attractiveness. Slugs and snails may be controlled with diatomaceous earth or organic baits. Cutworm baits can be applied to the soil before planting.

Fungal issues such as powdery mildew are a universal complaint. To prevent this, avoid overhead watering and use a horticultural fungicide.

Margaret Roberts Organic Fungicide is a broad spectrum organic fungicide/bactericide solution for the control of diseases on edible crops, ornamentals, herbs and roses. Because of the waxy nature of dahlia leaves, a spreader added to the spray will give better coverage. It is registered in RSA for powdery mildew on roses, ornamentals and peas, and its uses include: bacterial leaf spots, anthracnose, damping-off, downy mildew, botrytis rot and many more diseases.

Sprays that contain copper, like Copper Count or Copper Soap are used as a preventative spray and can also be used on vegetable crops. Always add a sticker to the spray mixture to make it more effective.

Biogrow Copper Soap is made by combining a soluble copper fertilizer with a naturally occurring fatty acid. The copper and the fatty acid combine to form a copper salt of the fatty acid, known technically as soap. The copper soap Natural fungicide controls Dahlia Decorative 'Nepo' Picture courtesy HadecoDahlia Decorative 'Nepo' Picture courtesy Hadecomany common diseases using low concentrations of copper, down as low as 90 ppm. The net result is an excellent vegetable, fruit and ornamental fungicide. This natural organic fungicide is used as a preventative spray to control diseases on a wide range of plants. As with most fungicides it acts to protect plants from infection.  Therefore, it is important to have the copper soap on the leaf, flower or fruit before the pathogen is able to cause an infection.  This product will control fungal diseases such as: black spot, powdery and downy mildew on vegetables and ornamentals, rust on ornamentals, early and late blight on potatoes and tomatoes, and Peronospora and downy mildew on grapes.

More sinister complications with dahlias can arise which can kill the plant. These may include:

Stem rot usually occurs when dahlias are growing in heavy, poorly drained, wet soil. Look for a white ring in the soil around the stem. The rot will creep in and kill the stem and advance down into the soil to kill the tubers.

Mosaic virus dwarfs plants and distorts leaves. You must destroy the plant, as there is no cure.

Botrytis blight causes buds to rot and covers the plant with powdery grey mould. Remove any affected part and destroy it.

Aster yellows disease is spread by leafhoppers, which are frequently found on plants, and are the vector for Aster yellows, a Dahlia Border 'Autumn Fairy' Picture courtesy HadecoDahlia Border 'Autumn Fairy' Picture courtesy Hadecodisease where leaf-like tissue forms where you should be getting flower buds. Plants are sadly a loss.

Problems with dahlias also include verticillium wilt and necrotic spot virus. Infected soil causes the former and leaves become black, brown or greenish-brown. Necrotic spot virus is caused by the feeding of thrips, so control these by spraying regularly with Biogrow’s Neudosan.

Warning:

Dahlia tubers and leaves contain phototoxic polyacetylene compounds that can cause skin irritation in humans who are handling the plants leaves and tubers in sunlight.

The plant can be toxic if eaten in large amounts, according to the North Carolina State University Extension.

Dahlias are toxic to dogs, cats and horses. Clinical signs are mild gastrointestinal signs and mild dermatitis. Its toxic principles are unknown.