The most recognized forms that one will most easily recognize is that of the Cactus and Decorative dahlia. There are however a number of other distinctive flower formations in dahlias as well as a number of forms that are not so distinctive at first glance.
Group 8 Cactus dahlias.
To start with I will discuss the cactus dahlia and its various forms, of which there the group is split into three separate types that may be for show purposes often put into one class of cactus or semi cactus.
Group 9 Semi-Cactus Dahlias
The Semi Cactus form on the other hand has a far broader base to the petal whilst still being refined at the tip if the petal. For an example of this we would need to look at Piper’s Pink. The Ray Florets (petals) of the bloom of a cactus are pointed, narrow and revolute for 50% or more of their length whereas for a semi cactus they are revolute for more than 25% of their length and no more than 50% of their length
*Definition* Involute The ray floret curls inward down its longitudinal axis towards the center of the flower.
This diagram may help.
The way a petal or more correctly Ray Floret curls whether toward or away from the center of the bloom is fundamental to the understanding of dahlia classification. The easier blooms to spot the formation on are pompon’s which ate Involute and cactus which are Revolute. Look closely at the blooms and you will notice this petal formation.
Decorative and Pompon’s and Ball dahlias
Taking this knowledge of petal formation one stage further we will now look at Declarative’s, Poms and Ball dahlias. There are a number of differences between these types which is very confusing to the untrained eye, I know as I still have difficulty knowing which is which sometimes.
Group 5 Decorative Dahlias
Decorative dahlias have petals that are generally broad and flattened and may be involute for no more than 75% of their length or slightly twisted, usually they have a blunt point to their petals. Some Decoratives have petals that are slightly pointed. The bloom must be fully double and show no disc(as in a daisy flower).
Berwick wood is an example of this.
Group 6 Ball Dahlias
A ball dahlia on the other hand has a ball shaped fully double flower that may be slightly flattened. The ray florets of a ball dahlia are blunt or rounded at the tips and spirally arranged. The petals must be involute for at least 75% of their length.
Group 7 Pompon Dahlias
That is the more difficult of the form classifications discussed, now on to the easier and more unusual of the forms.
Group 4 Waterlilly Dahlias
Waterlilly dahlias, again these are fully double blooms characterized by broad and sparsely spaced ray florets. The florets are either straight or slightly involute giving the bloom a shallow appearance the depth of the bloom should be less than half the width of the bloom.
Group 3 Collerette Dahlias
The final large group of dahlias before we look at the novelty groups is the Collerete dahlias. These dahlias have blooms with a single outer ring of flattened ray florets which may overlap with a small inner ring called the collar of generally smaller florets surrounding the center disc.
Group 2 Anemone Flowered Dahlias
Anemone flowered dahlias have one or more outer rings of florets surrounding a dense group of tubular florets and showing no disc.
Group 1 Single Flowered Dahlias
Single flowered dahlias are pretty much self explanatory, in that they have daisy like flowers good examples are the top mix varieties such as Omo, Little dorrit and such like.
Group 10 Miscellaneous Dahlias
The Miscellaneous group covers any dahlia that doesn’t fall into any of the above categories these can be orchid flowered dahlias such as Jescot Julie or such wonders as the Bishop of Llandaff.