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Overview
Dahlias are tuberous perennials that produce a spectacular display of flowers through summer and into autumn.
Orchid group Dahlias have foliage that is mid to dark green and elliptic to lanceolate in shape, the edges can be serrated.
Flowers are open centred featuring one ray of florets surrounding a central golden disc. Petals have an attractive inwardly curving or involute form and come in many colours.
This is still a new category, having been bundled under 'Miscellaneous' for many years, and the number of cultivars available is still fairly limited.
They are however excellent border flowers providing amazing colour and one of the very best cut flowers, a florists delight.
Soil: Any average, well drained, friable soil will do. Some added organic matter like compost or animal manures will help, but not too much. Ridging up the soil will assist drainage and help warm it too.
Choose a sunny location, around 6 hours per day is ideal, with shade from the scorching afternoon sun.
Plant after the colder nights have passed (end October in SE Australia - just before the Melbourne Cup). September is OK in Queensland.
Moisten the soil before planting but not directly afterwards - moist is good, overly wet is not.
Lay the tuber on its side and cover with soil around 10-15cms deep. They will need around 80-100 days to bloom.
Maintenance: Water sparingly after planting then increase to 2-3 times per week once the first sprouts come through. Do not let them dry out.
Liquid fertilise with a low nitrogen formulation one month after planting and again a month later.
Pinching out smaller buds helps produce bigger blooms.
Keep well watered as the buds set and from then on until they die back in autumn when you can trim back the dead foliage until next year.
Note: Fertilisers produce foliage and water produces blooms.
Hand weed, do not use herbicides.
When cutting flowers include plenty of stem, in fact the length of your forearm is best even if that includes side buds, this helps stimulate more flower production.
In warmer climates they can be left in the ground over winter but Dahlias generally go better when lifted and kept in a cool, dry location over the cold months. This also gives you the opportunity to remove any weakling tubers.
Keep your tubers from drying out - storing in peat moss helps and try to maintain temperature around 7-8c.
Diseases: Two spotted mite, aphids, slugs and snails.
Powdery mildew can be a bother so try not to water the foliage, better to deep soak the roots only.
Other Species: There are around 35 original species in the Dahlia genus but thousands of named cultivars have been segregated into fourteen main groups to help identification.
These include:
#1. Single flowered #2. Anemone flowered #3. Collerette #4. Waterlily #5. Decorative #6. Ball #7.Pompon #8. Cactus #9 Semi-Cactus #10. Miscellaneous #11. Fimbriated #12. Star #13. Orchid #14. Paeony
There is also now a Double Orchid Group - similar to Orchid Group but doubled.
Practically all modern cultivars have Dahlia pinnata and Dahlia coccinea as original parents though Dahlia hortensis is also present in the overall mix.
Comments: Named after Dr Andrew Dahl a Swedish botanist it is also the national flower of Mexico.
The Dahlia was first brought to Europe from Mexico where it was grown initially at the Madrid Botanical Gardens. From there it spread across Europe and the British and Dutch have been largely responsible for the breeding of thousands of cultivars.
Author: Bob Saunders.