Dahlias are all grown basically the same way, but the flowers vary greatly in size and shape.
Some are better than others for the border, for cutting, or for showing off your giant dinnerplate blooms that are so heavy they need staking to hold them up!
Involute vs Revolute Petals
Involute petals curl inwards, closing into the centre of the flower:
Revolute petals curl outwards to the rear of the flower, swept back away from the centre:
The craziest looking cactus dahlias have petals doing both:
Dahlia Groups
Single Flowered: One outer ring of florets, which may or may not overlap, and make up a disc in the middle.
Anemone Flowered: One or more outer rings of ray florets (usually flattened) around a mass of tubular florets, with no visible disc in the middle.
Collarette: One outer ring of ray florets (usually flattened) that overlap, and an inner ring, or collar, of small florets, with a visible disc in the middle.
Waterlily: Wide but shallow, fully double, open blooms with wide ray florets. The depth of the bloom is not more than one third of its diameter.
Decorative: Fully double, with no visible disc in the middle. The ray florets are normally wide, and either flat or twisted, and typically have a blunt end.
Ball: Fully double, and form either a near-perfect or flattened ball. The ray florets are rounded at the tips.
Pompon/Pompom: Fully double & spherical, with florets largely involute along their length (longitudinal axis).
Cactus: Fully double, typically with pointy ray florets, most of which are narrow and revolute for over 65% of their length (longitudinal axis) and either straight or involute for the rest.
Semi-Cactus: Fully double, typically with pointy ray florets that are revolute for between 25% and 65% of their length, wide at the base, and either straight or involute for the rest.
Star (formerly Single Orchid): One outer ring of florets around the disc. All the ray florets are either involute or revolute.
Double Orchid: Fully double, no visible disc, with triangular centres. The ray florets are narrow, and all of them are either involute or revolute.
Paeony: Several outer rings of ray florets around a central disc. The florets are flat or slightly involute at the base, and the rest is flat slightly revolute.
A Bit of Dahlia History
Dahlias are named after the 18th century Swedish botanist Ander Dahl, who also has a snake named after him – Dahl’s whip snake, Platyceps najadum dahlii.
Dahlia Flower Fact
Dahlia petals are edible and make a pretty garnish or salad addition.
However, the same is not true of the tuber: the wild varieties were a staple food source for Aztecs, but it seems that cultivation has made them unsuitable for eating.