The autumn show is only four months away (trust us, it’ll go quick) and as ever there will be a number of dahlia classes to enter. If you’re not used to growing or showing dahlias, there wide range of forms can be confusing. So if you don’t know your cactus from your collarette or pompon from ball, here’s a brief guide (courtesy of the National Dahlia Society) and examples of each one (click on each picture to enlarge them).

Decorative, cactus, pompon and single-flowered are usually the types that feature in our autumn show and we have provided a little more information on ball and pompon dahlias as they can be more difficult to tell apart.

Formal decorative

Decorative dahlias have fully double blooms showing no disc.  The ray florets are generally broad and flat and may be involute (curled or curved inward) for no more than 75% of their length (longitudinal axis) or slightly twisted, and usually bluntly pointed.

Cactus

Cactus dahlias have fully double blooms, the ray florets are usually pointed, the majority narrow and revolute for 65% or more of their length (longitudinal axis) and either straight or incurving.

Pompon

Pompon dahlias have fully double spherical blooms of miniature size (no bigger than 55mm in diameter except on the case of large poms which should not exceed 75mm) with florets largely involute along their length (longitudinal axis).

According to the RHS Horticultural Show Handbook, blooms are perfectly globular. Florets involute for the whole of their length, evenly and symmetrically arranged throughout the bloom and dessing back fully to the stem. Bloom facing upwards on a straight, firm stem.

Single-flowered

Single dahlias have blooms with a single outer ring of florets, which may overlap, the centre forming a disc.

Anemone-flowered

Anemone-flowered dahlias have blooms with one or more outer rings of generally flattened ray florets surrounding a dense group of tubular florets, and showing no disc.

Collarette

Collerette dahlias have blooms with a single outer ring of generally flat ray florets, which must overlap, with a ring of small florets (The Collar) the centre forming a disc.

Waterlily

Waterlily dahlias have fully double blooms characterised by broad ray florets that are slightly involute along their length (longitudinal axis) giving a saucer shaped appearance to the bloom. The depth should be not more than one third of the diameter of the bloom.

Ball

Ball dahlias have fully double blooms, ball shaped or slightly flattened.  The ray florets are rounded at the tips, with margins spirally arranged and involute for at least 75% of the length of the florets. For exhibition purposes miniature flowered varieties must not exceed 115mm and small flowered varieties must not exceed 170mm.

According to the RHS Handbook, blooms should be ball-shaped but the tendency towards flatness on the face of the largest cultivars is acceptable. Ray-florets compact and dense at the centre, symmetrically arranged, dressing back to the stem to complete the ball shape of the bloom. Florets compact and dense at the centre. Bloom poised at an angle of not less than 45 degrees to the stem, which should be straight and of a length and thickness proportionate to the size of the bloom.

Semi-cactus

Semi-Cactus dahlias have fully double blooms; the ray florets are usually pointed and revolute for more than 25% and less than 65% of their length and broad at the base and either straight or incurving.

Miscellaneous

Any dahlias which do not fall into type 1 – 9 inclusive and type 11, 12,13 & 14 e.g. Thistle Dahlias, etc. This group includes species dahlias.

Fimbriated

Fimbriated dahlias have blooms where the tips of the ray florets should be evenly split or notched into two or more divisions, uniformly throughout the bloom to create a fringed overall effect.  The petals may be flat, involute, revolute, straight, incurving or twisted.

Star

Star dahlias have blooms with a single outer ring of florets surrounding the disc.  Ray florets are uniformly either involute or revolute.

Double orchid

Double Orchid dahlias have fully double blooms showing no disc and have triangular centres.  Ray florets are narrowly lance shaped and either involute or revolute.

Paeony

Paeony dahlias have multiple outer rings of ray florets surrounding a disc, ray florets are flat or slightly involute at base and are flat or are to some extent revolute.