Dahlia pinnata plant notes
Dahlia pinnata is in the Asteraceae (or Daisy) family.
Tuberous, herbaceous perennial, with pinnate leaves, striking flowers.
Plant in full sun.
Varies according to cultivar. Typically between 70 to 120 cm (2 to 4 feet)
Tender, but may be hardy down to -5 degrees C (41 degrees F) if not waterlogged and well mulched. USDA Hardy zones 7 to 10.
Origins of Dahlia pinnata
Dahlia pinnata (the pinnate dahlia, or d. pinnata) along with Dahlia imperialis , is one of the two original varieties of dahlia that were described by Francisco Hernandez, physician to King Philip II of Spain, when he visited Mexico in the 16th Century.
Dahlias (as we know them) had been cultivated in Central America, principally Mexico, for centuries. Whilst we grow dahlias for the beautiful flowers, in Mexico Dahlia tubers had been used as a food source and the long canes of the Dahlia Imperialis (the tree dahlia), had been used to carry water. In fact one of the names used by indigenous peoples for the plant, Cocoxochitl, means “water pipe plant.” Unsurprisingly, the Dahlia is Mexico’s national flower.
It was not until 1789, (whilst revolution raged in France), that Abbe Antonio Jose Cavanilles, Director of Madrid’s Royal Gardens , raised dahlia plants in Europe for the first time. Based on material sent to him from Mexico, Cavanilles cultivated Dahlia pinnata, Dahlia coccinea and Dahlia rosea . He named them dahlia, after the Swedish Botanist and student of Linnaeus, Anders Dahl.
Incidentally, you will often see Dahlia pinnata referred to as Dahlia pinnata Cav., as Cav. signifies that it was a plant named by Cavanilles.
Seeds and tubers of these attractive new plants quickly found their way around the horticulturalists of Europe and it soon became apparent that Dahlias were quick to hybridise, leading to much confusion and debate amongst botanists even to this day as to the true classification of the genus dahlia and its multitude of species and hybrids.
But, the fact that Dahlia naturally hybridised when grown from seed meant that it readily changed its form and colour. As far as growers were concerned, this was a massive opportunity to create endless new varieties to attract the plant buying public.
Dahlia pinnata and dahlias species
Technically Dahlia pinnata plants today should properly be described dahlia x pinnata. According to scientists Hansen and Hierting, Dahlia pinnata is genetically a variant of a true species plant that they named Dahlia sorensenii (d sorensenii) and and had probably hybridised before its introduction to Europe in the 18th century (see Hansen, H. V. and J. P. Hierting. 1996. Observations on chromosome numbers and biosystematics in Dahlia (Asteraceae, Heliantheae) with an account on the identity of D. pinnata, D.rosea and D. coccinea. Nordic Journal of Botany 16: 445-455. A pdf of the paper is available here on Researchgate.
Today, there are approximately 30 garden Dahlia species and perhaps 20,000 cultivars. However, it is reasonably widely accepted that it is from Dahlia pinnata (or the pink d sorensenii) and Dahlia coccinea that today’s garden varieties of Dahlia are derived.
The genetic diversity of dahlias arises because of something called polyploidy. Polyploidy is the condition by which the cells of an organism have more than two pairs of chromosomes.
Most creatures have a single pair of chromosomes – one inherited from each parent. Creatures with a single pair of chromosomes are called diploids.
Most plants and some creatures, such as fish and amphibians are polyploid, meaning that have three or more sets of chromosomes.
Dahlias are polyploid but with a much higher number of matching chromosomes than most plants. They are octoploids, which means they have 8 matching pairs of homologous chromosomes.
This level of polyploidy is rare in the plant kingdom and accounts for the wide range of genetic expression in dahlias that we see in their many forms and colours.
Today’s Dahlias come in virtually every colour except blue and black and in a wide range of sizes from the tiny Lilliput Dahlias to small pompon dahlias, to giants, with blooms over a foot (30cm) wide.
Dahlias will bloom from mid-summer until they are knocked over by frost. With careful deadheading, you can have dahlia flowers for long periods in summer and autumn. Read about how to deadhead dahlias and, especially how to distinguish the dead heads of dahlia flowers from the flower buds here. Dahlias, like dahlia ‘Wine Eyed Jill‘, are often grown for cut flowers, so deadheading ensures that more blooms are available for longer.
The showy Dahlia flower is actually made up lots of individual flowers called florets. What we tend to think of as the petal of the dahlia is actually called the ray floret. The yellow parts of the the flower in the centre are called the disc florets. The disc florets are the parts of the bloom that are pollinated and generate seed, although some varieties of dahlia have been bred so that the disc florets are not visible.
Dahlia pinnata cultivation
Dahlias can be grown from seeds but they develop tubers and tuberous roots, and it is from the tuber that we normally grow dahlias in our garden. You normally pot up dahlia tubers in Spring, keeping them out of frost, and then plant them out in early summer when the frost risk has passed.
Dahlias thrive best in moist, fertile but well drained soils with a neutral PH. They are quite shallow rooted, so can dry out in hot weather. They put on a lot of growth in the course of the growing season, so need to be well nourished. Feed and mulch them well in Spring for the best displays.
In North America, dahlias are hardy in USDA Zones 7 through to 10. In very cold areas or where there is a risk that the ground becomes waterlogged, it is best to lift the tuber when the first frosts hit in fall or winter. There are full details on how to overwinter dahlia plants and tubers here.
Dahlias can be troubled by aphids, as well as powdery mildew and other fungal diseases. They can also suffer from crown gall, a disease that enters plants through a wound in the stem and causes tumour like growths on the roots and the tubers.
But, in my experience, dahlia’s biggest enemies are earwigs, slugs and snails. Earwigs can ruin the blooms of the plant and slugs and snails will often chomp off new shoots as they emerge where tubers have been left to overwinter underground.
Dahlia pinnata is the species that has given rise to so many beautiful dahlia varieties and yet in its simply flower form it is still very beautiful in its own right.
More on Growing Dahlias
You can also get much more guidance on growing dahlias in these posts:
- Taking care of dahlias: the trick with deadheading
- Dahlia pinnata: a dahlia original
- How to grow dahlias: the complete guide to dahlia care
- How to overwinter dahlia plants and tubers
- When to plant dahlia tubers: solving the dahlia grower’s dilemma
- All you need to know about dahlia tubers and dahlia bulbs
- Dahlia varieties: your complete guide to all types of dahlias
- Dahlias in my garden: Six on Saturday
- Dahlia Wizard of Oz – Beautiful pink pompon dahlia
- Can you grow dahlias in raised beds?
- Do dahlias grow in Florida?
- Dahlia Wine Eyed Jill
- Can you grow dahlias in a tropical climate?
- Dahlia Ivanetti: magnificent magent dahlia
- Dahlia Night Silence – dusky pink dahlia beauty
- Visit a dahlia farm near you
- Best mulch for dahlias: a comprehensive guide
- Dahlia Islander: large, pink, loud, showy. What’s not to like?
- Heat tolerant dahlias: beat the heat with these 120 choice varieties
Martin Cole has been an avid gardener for more than 20 years and loves to talk and write about gardening. In 2006 he was a finalist in the BBC Gardener of the Year competition. He is a member of the National dahlia Society.
He previously lived in London and Sydney, Australia, where he took a diploma course in Horticultural studies and is now based in North Berwick in Scotland. He founded GardeningStepbyStep.com in 2012. The website is aimed at everybody who has been bitten by the gardening bug and wants to know more.
Gardening Step by Step has been cited by Thompson and Morgan, the UK’s largest mail order plant retailer, as a website that publishes expert gardening content.
Check out my comprehensive step by step guide, with plain language explanations and ultra-useful images and illustrations. This is for you if you love dahlias and want to the best out of the dahlias you grow.