Dahlia - Popularity Plus!

One of the world's most popular and widely grown garden flowers is the Dahlia. The original single, star-shaped wild species is believed to have once been native from upland Mexico and Central America, especially Guatemala.

But from very ancient times it has been widely grown throughout warmer South America and throughout the southern parts of North America as well. Dahlias were often historically thought to be natives of ancient Aztec gardens where they were seen growing prolifically.  But Native People throughout the warmer Americas soon grew them.

Today Dahlia is the national flower of Mexico, dedicated to represent the Mexican Republic in 1963. It is ranked amongst the ‘most favourite’ ‘popular’ flowers around the world. Millions are grown and sold each year and many more bought as cut flowers. They are found in annual beds and borders; containers, cottage gardens; parks and perennial borders; window boxes and the background of shrubbery displays. In many parts of the (sub) tropical world they have naturalised and can be found in a nearly ‘wild’ state. Their flowers are frequently used in ceremonies and festivals, to beautify and decorate and even to be eaten and used as medicine.

In Celestial Gardening the Dahlia is considered to be a masculine flower with a feminine name ruling the celestial days of 21-30 April (Taurus). In the Language of Flowers the Dahlia represents: Dignity, Elegance, ‘Forever Thine’, Good Taste, Novelty and Refinement.

The great Spanish Author, Botanist and Court Physician, Francisco Hernandez de Toledo first mentioned and recognised these plants for their medicinal properties when he travelled to Mexico in 1615.  Aztecs and Mexicans had long-used the flower petals and thinly sliced tubers to treat to treat rashes and skin infections; also as a soothing foot soak to relieve tired feet. Dahlia imperialis, the Giant Tree Dahlia, was recognised as the medicinal species of choice.

Historical records are confusing but suggest that this useful plant and the smaller Dahlia species were first introduced to the European garden world during the 17th Century (1787-1789) when Spaniards through Vincent Cervantes, who would later become the Director of Botanical Gardens in Mexico City, sent the seeds back to Madrid where the first plants were successfully grown. These were botanically labelled as Dahlia coccinea. It was at that time considered to be a fairly useless hothouse curiosity. But soon became more widely dispersed throughout European Botanical Collectors. A single plant and sometimes even just a few seeds would fetch 10 pounds.

At about the same time it is known that a box of dormant tubers was sent to the Netherlands from Mexico. While almost all of them perished a strong red cultivar with pointed, star-like petals survived. European and Scandinavian botanists and plant breeders quickly began cultivating and cross-breeding this plant to create some of the first fine cultivars. This first species and its related hybrid cultivars soon became named as Dahlia juarezii (and cultivars) or more commonly called ‘Dahlias’ in honour of Anders Dahl, a Swedish Botanist who did so much work to advance this species.

Around 1804 it is recorded that more Dahlia seeds were successful started by the Gardener at Holland Hall, Kensington, U.K and at least three distinct varieties were developed and then were cross-bred amongst them. These were botanically labelled Dahlia variabilis. Soon afterward four more varieties were developed in France which produced the first well-formed double blooms. Botanists and Growers continued cross-breeding and hybridising these first selections and by about 1850 there were thousands of named cultivars.

In Germany, during the same time the seedling Dahlia’s imported there became known as ‘Georgia’ or ‘Georgine’ named by Naturalist Carl Ludwig Willdenow in honour of the Swedish Botanist, Chemist, Geographer and Naturalist Johann Gottlieb Georgi a distinguished Professor of Chemistry in Saint Petersburg, Russia. All considered the still rather humble and spindly Dahlia to be a ‘hot house tropical of little value’ until Frenchman De Candole, discovered that some tubers mistakenly dumped outdoors where flourishing better than any grown inside. His experimentation with outdoor cultivation lead of the growing practices followed today when growing the Dahlia outside its (sub) tropical native habitat.

Today there are at least 36 species and an estimated 14,000 named garden cultivars varieties! These vary from the giant tree Dahlia, D. imperialis, towering at usually 8f/2.4m but sometimes upwards of 20ft/6m., down to the delicate bedding miniatures only 8-12in/ 20-30cm. high. There are thousands of intermediate sized plants in many flower shapes, sizes and colour combinations, except for true black, blue, brown and grey shades.

Dahlia flowers, and even the plants themselves, are amongst the most variable of any species. This is because Dahlias have 8 sets of homologous chromosomes rather than most other plants which have only two. Thus cross-breeding or even natural pollination often results in a remarkable range of off-spring, which frequently produce hybrids of commercial and unique potential.  Flowers can be as small as 2in/5.1cm all the way up to remarkable 1ft/30cm ‘dinner plate’ giants.

Dahlias are today classified into 15 and sometime more separate groups:

Single: Flat and open, often star-shaped petals in a single row of ray florets around a yellow centre of pollen flowers. Plants vary from medium to tall and shrubby.

Mignon: Single species but considerably more dwarf to 18in/45cm.

Anemone: Flat or pleated, open-centered single star flowers with a distinctive inner row or rows of frilled, fluted or often tubular smaller inner petals creating a pincushion effect. Can be produced in many sized plants.

Collarette: A very similar single species but with an inner row or rows of smaller petaloids arranged around the pollen disc. These create an inner circle or ‘collar’ often in a distinctly different colour shade producing a two-tone flower with two rings of petal colour. Plants can be short to shrubby and tall.

Duplex: Two rows of daisy-type, fluted, or ray-like petals around an open pollen centre. Can be medium short to robust sized plants

Peony: Up to four rows of fluted or ray petals and many curled or twisting small petals or petaloids clustered around the centre. Often robust and shrubby plants.

Incurved Cactus: Medium to large fully double to semi double blooms with many ray petals whose margins fold back to create long thin tubes coming to a pointed tip. Pointed petal tips tend to recurve inward toward the centre creating a star-waterlily effect, pinwheel or incurved near-ball. Plants can be short to tall.

Cactus Recurved or Straight: Fully double star-like flowers with tubular petals that are either recurving away from the centre or are straight creating a star-burst effect. Plants can be short but are often robust and tall

Semi-Cactus: Fully or semi-double star-like blooms but with petals folding back only at the tips to less than half of each petal, creating a solid disc with pointed petals. Often robust plants.

Formal Decorative: Mostly very double flowers with broad, flat or fluted petals arranged quite symmetrically around a central disc. Outer petals often recurve back and inner petals cup inward creating a flattened-ball effect. These are very often bold and impressive blooms; sometimes amongst the largest. Often strong and robust plants but dwarf forms also exist.

Informal Decorative: Semi-double to double with broad, often longish petals, sometimes fluted or twisting, occasionally pointed and somewhat irregularly arranged to form a double Daisy or sometimes near semi-double Sunflower appearance. Frequently large flowers on robust plants but dwarf forms also exist.

Ball: Very double blooms, often quite large and impressive, on strong and sturdy stems but sometimes quite pendulous flowering stems. Petals are broad, often blunt, recurved or rounded at the tip, occasionally quilled and frequently turned-in at the margins creating a flattened to near round ball. Petal arrangement can be quite formal and symmetrical or informal and Peony-like. Often robust and tall plants on stout canes.

Miniature: Flowers always less than 4in/10 and usually much less. These can be produced in any of the preceding group flower forms. Plants are often dwarf or of no more than medium height.

Pompon: Fully double, ball shaped blooms often with fluted, formal or incurving, very symmetrical petal arrangement. Flowers are seldom more than 2in/5cm across and carried on medium to short and shrubby plants.

Tree: Dahlia imperialis and related species and cultivars. Flowers are single, semi-double, occasionally Anemone, Collarette or fully double in mauve, pink and several shades of bone, cream and white but new hybrid cultivars are being developed all the time. Plants produce enormous canes 8-20ft/2.4- 6m or more that resemble Bamboo. Leaves are very large and multiply compound. Flowers are produced as large branching clusters at the tops of the canes. Further side shoots continue flowering down the cane. Late flowering Autumn into Winter.

Dahlias are herbaceous, perennial tuberous rooted often bushy, cane-like or shrubby plants. The tuberous roots are quite substantial; spreading off the central stem or crown of stems. They are cream, yellow, bone white and fleshy within and covered with a brown and papery but thickish skin somewhat resembling Kumara or Sweet Potato roots.

In ancient times Dahlia tubers were grown and harvested almost exclusively as a food crop by Inca and Aztec Native American Indian People. They grew them just like Potato and Sweet Potato. The entire plant could be eaten, especially the tuberous roots and used medicinally plus the flowers were frequently used in (religious) ceremonies and for decoration.

The Native South American Indian word for the Dahlia, ‘Akokotlya’, means ‘water pipe’. The Dahlias’ hollow stems were used especially for drinking water, much as we use soda straws today. The cleanest water always lies just below the surface. So by using the Dahlia stem dipped into any pool or stream, it was possible to drink the cleanest and freshest water. At other times the hollow straws were filled with cannabis, various herbs and tobacco which was then smoked as an important part in social and particularly special religious rituals and ceremonies

The arrival of warm Spring weather is the ideal time to start planting both the tuberous roots, seeds, and seedlings for a Summer and Autumn display. In mild and frost-free climates, Dahlias that are left in the ground all Winter often begin flowering in very Late Spring or Early Summer. While in colder climates with shorter seasons, Dahlia tuberous roots can be started either in the glasshouse for earliest blooms, or semi-dormant roots are planted directly into their growing position once all danger of frost has past.

The very best blooms are created in mild, temperate climates. Here the tuberous roots are planted in Early Summer. The plants quickly rocket away in the Summery heat and begin producing blooms about 90 days later in Early Autumn just as the weather begins to cool. This produces the finest quality exhibition blooms throughout Autumn until heavy frost. In winterless climates, the plants continue to bloom sporadically all Winter. Then the following Late Spring/Early Summer the large clump of tuberous roots is dug up, divided and replanted to start the flowering cycle again.

In colder climates after the first frost the clumps are cut right back and dug up. After removing soil from around their roots, these are buried and stored in boxes of old potting mix, peat, sand, sawdust, etc in a cool, dark cellar or similar frost free position. Or the clump of tuberous roots can be left underneath the bench in a glasshouse or protected cold frame. There they will remain dormant until lightly watered again in the Spring.

Once the crown of tuberous roots breaks dormancy, small often greenish-red-purple  ‘eyes’ or shoots begin to appear where the ‘neck’ or thin end of the tuber attaches to the main growing stem. Once these eyes begin to advance only slightly, the supporting tuber with the eye intact can be carefully cut away from the old parent stem and can be planted out in its growing position of started in an individual pot or seedling flat for planting out later.

Once these small eyes develop into strong growing stems, they can be cut back to just one or two pairs of leaves. These cuttings will usually strike quite easily; so will mature sections of growing Dahlia stems. Stem cuttings taken just below a leaf node will strike quickly in peat and sand or a sterilized seed raising mix if kept warm, moist and bright. Simply dip their cut end in hormone gel or powder (optional), pinch out the tender growing tip so that a small 4-6in/10-15cm stem remains with two to four leaves attached near the top. Place these cuttings in a bright (but not hotly sunny), humid, moist and warm terrarium-style environment and they will usually strike quite rapidly to produce new plants identical to their parents.

Dahlias also are extremely easy to start from seed. Grow them as one would Marigold or Zinnia in a good quality potting or seed-raising mix. Start in flats or individual containers kept very bright, moist and warm. Their seed is best started once weather is consistently warm both day and night; or in the glasshouse with bottom heat or a climate-controlled environment.

Warning! Dahlia seed and seedlings quickly collapse and rot in cold, damp ground and low light. But when provided with (sub) tropical growing conditions, the seed often germinates in as little as a week and young growth is rapid. Transplant into their permanent growing position once soil and air temperatures are consistently warm and the seedlings are at least 2in/5cm tall. Keep well watered, but never sodden until all wilting stops.

Seed sown directly into a moist, sunny and warm garden bed, especially in Early Summer, will often germinate in days. The young seedlings can be later thinned or are easily transplanted to their permanent growing position. This is worth the effort because it is from seed that we get our new varieties.

Dwarf bedding varieties like the classic Unwin series can be reliably expected to bloom in 10-12 weeks from seed. Robust and taller Giant Dahlias take 12-14 weeks from seed. As mentioned earlier, Dahlia’s large chromosomal number means that the seed produces quite variable results. Even when gathered professionally with the greatest of care, new cultivars almost always result. Many if not most of what is produced from seed will be attractive, if not delightful and very worthwhile. Then to save whatever has been produced from seed, the parent must be multiplied by stem cuttings, tissue culture or tuberous root division.

Dahlias will bloom in part shade, especially if they receive at least 5-6 hours daily of strong sunlight. But there is no question that Dahlias prefer full sun in a consistently warm position that is sheltered from strong winds. This produces robust, stocky and strong plants with the greatest number of high quality blooms.  Sometimes bedding Dahlias can be successfully grown in dappled light in drier positions provided they are allowed to sprawl and vine over the ground like a groundcover. Even though the stems flop, their growing tip continuously rights itself upright with blooms held erect above the foliage. But plants are always spindly and flowers are of poorer quality.

Rich, loamy soils that drain very freely are best, but Dahlia will grow in almost anything. Well draining garden loam on a slight mound or slope facing into the sunshine provides the ideal location. Avoid water-logged soils and lighten heavy clays with compost, gravel, Gypsum and/or sand.

The finest Dahlia soils are deeply dug and enriched several weeks prior to planting with Lime, mature compost or very well aged manure plus a generous dusting of a good quality balanced General Garden Fertiliser and another dusting with Muriate of Potash or well-aged and untreated wood ashes.  Dahlias are quite partial to extra Potassium. These must be dug in well in advance to avoid burning tender roots.

Dahlias are notoriously brittle, especially as stems mature and become laden with heavy blooms. These often split or break away during heavy rain and windy weather. To avoid this, Dahlias are usually staked at the time of planting. Sometimes they are grown within wide mesh; wire cones or tubes or against trellis. Dwarf bedding varieties do not need staking

In the wild, (sub) tropical situations, cottage gardens, or ‘naturalised’ garden environments that experience no freezing or heavy frosts, Dahlia clumps can be left to develop and spread undisturbed over many years. This will result in large and sprawling clumps of shrubby growth with many smaller blooms flowering over a long period.

The finest exhibition blooms are produced from individual tubers planted separately with plenty of growing space between each plant. These are either purchased ready to plant or cut away from established clumps. First dig around the clump with a garden fork and lift it free from the soil. Wash away any clinging soil so that the tubers are exposed. Select fat and larger tubers with almost no neck (where the tuber connects to the growing stem). Carefully cut the tuber away from the stem leaving a small amount of the old growing cane intact with the tuber. If the clump of tubers has been resting dormant all Winter, eye shoots will possibly be obvious near the neck of the tuber. Make sure every tuber has at least one eye attached. Tubers that break away without an eye or neck are considered to be ‘blind’ and will usually not produce shoots and grow. Narrow and smaller tubers, especially those with long and narrow necks usually produce spindly and weak plants.

When planting individual Dahlia tubers, lay the tuberous root horizontally 10-20cm/5-8in deep in the planting hole with the eye or bud facing upward and toward the stake. In heavier soils, plant the tubers 7-10cm/3-4in deep and 60cm-1m/2-3.4ft apart each way for tall Cactus and Decorative varieties; 2m/6.7ft apart for tree forms and 12-24in/30-60cm for bedding and Pompon types. Closer plantings will still produce excellent results but may result in crowding and so less healthy and/or weaker plants. In colder growing positions Gardeners often dig a generously deeper hole but only cover each tuber with 5-7cm/2-3in of soil and fill the remainder of the hole in later as the shoot(s) develops. This shelters the young growing stems and keeps them warm while sprouting

Dahlias are notoriously brittle, especially as stems mature and become laden with heavy blooms. These often split or break away during heavy rain and windy weather. To avoid this, Dahlias are usually staked which is best set in place at the time of planting. Sometimes they are grown within wide mesh; wire cones or tubes or against trellis. Dwarf bedding varieties do not need staking

Strongest plants and best blooms arise when only 1 or 2 shoots are allowed to develop. So once new shoots appear remove all but the strongest three shoots or less Some Growers pinch out the central stem when approximately 15-20cm/6-8in tall, or when they have reached the 4-6th pair of fully formed leaves. This will create compact, stocky, wind-resistant plants with about 6-8 strong flowering stems per cane.

To create the large exhibition blooms, remove up to half the side branches and smaller side buds to eliminate inferior growth and so encourage bigger blooms.  The very largest blooms are created by growing each Dahlia cane as a ‘standard’. This requires removing all side branches and shoots except for the main cane. This main cane is allowed to develop only one or two top buds. Once the main bud is reliably developing, its side bud is also removed. This encourages all energy into produce one giant bloom. When this technique is applied to giant decorative cultivars, the result can often produce a massive and very tall plant with one immense bloom easily the size of a dinner plate on a plant quite reminiscent to a giant Sunflower.

To create these exceptionally large and/or productive or prolific Dahlias requires some extra attention. Dahlias respond well to such additional care. They need plenty of water without ever becoming over-saturated or chilled. Generous organic mulching helps immensely in providing them with a constant supply of nutrient-enriched moisture.

Exhibition Dahlias respond to feeding with a mix of Superphosphate, Bone Meal, and Potash or wood ashes mixed with well aged compost spread around each plant as a surface feed and mulch. This is most important as soon as the first buds become visible and start to expand. Avoid heavy, strong concentrations of fertiliser or manure coming in direct contact with the plant stem or the shallow surface roots as burning will result. Feed little and often with this enriched mulch or a complete soluble food, compost, aged manures, or a commercial granule mixed with blood and bone. Water the plants thoroughly first, then feed, then water over this fertiliser again. Always avoid overfeeding, especially on dry soil, as this will often burn the plants.

It is better to continuously provide a steady stream of food to keep the plants at full production. For the best exhibition blooms combine this granular/mulch feeding with near continuous watering as buds develop and possibly also apply a liquid feed weekly.

As the budding stems begin to develop, continue to thin-out these buds so that only one or two develop on each branch or cane.  After a branch finishes flowering, cut it right off or at least reduce its length by half. This will encourage the formation of additional buds.

Dahlia blooms make excellent cut flowers. They are best cut in the early morning or cool of evening. Soon after cutting immerse about 2.5cm/1 inch into very hot (but not boiling) water for about 15 seconds, just enough time to darken the stem. Then plunge the stems in cool, deep water for at least an hour before arranging them in their display vase. Change their vase water every couple of days. Flowers can sometimes last for upwards of a week.

Dahlias have been rewarding Gardeners and Growers for hundreds of years with their beautiful, colourful, often vibrant and versatile; even edible and medicinal qualities. It is for all these reasons that Dahlias are a world class gardening favourite that has easily earned the reputation as the plant with ’popularity plus’.

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