Dahlia pinnata Seeds | Dahlia
LATIN NAME: Dahlia pinnata
COMMON NAME: Dahlia
Plant Profile
Dahlias come from the cool highlands of Mexico and Guatemala, where they grow as tuberous perennials on volcanic soil at 1500-3000 m. Dahlia pinnata is one of the parents of the modern garden hybrids, and it shows the family traits: divided, deep-green leaves, stiff hollow stems, and a flowering period that runs from midsummer until the first hard frost.
The wild species is single-flowered, with one row of ray florets around an open yellow disc, and it is a far better plant for insects than the dinner-plate cultivars. Seed-grown dahlias are variable, which is either the point or the problem: every seedling is a new combination, and a few will be worth keeping.
Science & Biotechnology
Before insulin, dahlia tubers were a serious object of medical research. In the 1920s and 1930s, investigators on both sides of the Atlantic extracted a fructose polymer from dahlia roots and studied it as a carbohydrate source for diabetics, since inulin-type fructans are not digested to glucose the way starch is. The work was overtaken by insulin therapy, but the chemistry it established is still cited in inulin research today, where the same fructans are studied as prebiotics.
Modern dahlia research is dominated by horticulture rather than medicine: flower colour and form genetics, incompatibility systems, and virus indexing, since dahlias accumulate several viruses that reduce vigour without obvious symptoms. The plant is also a model in studies of the Asteraceae inflorescence, because the difference between a single and a double flower is a well-defined developmental switch.
Uses & Benefits
As a cut flower dahlia is close to unrivalled for late summer, producing more stems per plant over a longer period than almost any other flower. As a garden plant the doubles are structural and the singles are ecological, and the singles are the ones that feed bees and butterflies through August and September.
The tuber is edible in its homeland and was grown as a vegetable before it was a flower, though the garden cultivars are starchy and bland. The one caution worth repeating: dahlias are not frost-hardy, and in cold climates the tubers must be lifted and stored dry over winter, which is the main labour the plant demands.
Growing from Seed
Sow dahlia seed in late winter or early spring at 18-21°C, 5 mm deep in a free-draining mix. Germination takes seven to fourteen days and is usually strong. Seedlings grow fast and will flower in their first year if started early enough, which makes them a cheap way to fill a cutting bed.
Pot on twice before planting out, and pinch out the growing tip once plants have three pairs of leaves to encourage branching. Plant out after the last frost, 40-50 cm apart, in full sun. Water deeply rather than often, and feed with a high-potash fertilizer from midsummer; too much nitrogen gives leaves at the expense of flowers.
Our Take
Seed-grown dahlias are a gamble worth taking once: most will be ordinary, a few will be genuinely good, and you cannot buy those few anywhere. If you want specific colours and forms, buy tubers; if you want the pleasure of finding something, sow seed.
Related Reading
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