1. Native H. annuus.
Although the same genetically plants can vary in appearance making descriptions confusing.
Native to North and Central America the following is from the Manual of Vascular Plants of North Eastern US
and Canada and Flora of North America @ efloras.org.
The erect annual herbs, 1 to 3 m high have round green stems that are rough with stiff hairs.
There is usually a single basal stem with many branches above.
Leaves, on petioles from 2 to 20 cm long may be opposite at the base but are alternate above.
Up to 20 cm long and 10 cm wide they have ovate to heart-shaped or broad lanceolate blades.
The tip is pointed, there are 3 veins from the base and most have a toothed edge.
The lower surface has stiff hairs or feels rough.
Plants have 1 to 9 heads or capitula on hairy peduncles 2 to 20 cm long.
Each head has 20 to 30 ovate to wide lanceolate involucral bracts at the base.
There are hairs or cilia on the edges and long hairs on the outer or lower surface.
The ovate paleae among the florets on the receptacle have a long bristle-like end and 2 side teeth.
The single row of ray florets around the edge have a yellow ligule 2.5 to 5 cm long.
The over 150 to one thousand spirally arranged disk florets are typically a purplish-brown.
They are fertile bisexual flowers with a pappus of scales and a corolla tube with 5 pointed often reddish lobes.
Anthers of the 5 stamens form a tube which the style passes through before it splits into 2 branches.
The inferior ovary has 1 locule and 1 ovule.
The fruit are dark brown cypselae, commonly called achenes around 5 mm long.
They have a pappus of 2 deciduous scales and may have fine hairs.
2. Naturalised Australian H. annuus.
Flora of Australia vol. 37 page 554 gives basically the same description except that plants naturalised here are
shorter at 0.5 to 1.5 (2) m high with leaves 3 to 20 cm long by 1 to 10 cm wide.
3. Cultivated Helianthus annuus hybrids
Virtually all plants seen here, both commercial and ornamental are hybrids.
These are the same plant but look very different from the wild and naturalised ones.
FOA divides them into subspecies:
H. annuus subsp. macrocarpus are plants grown commercially for the oily seeds and occasionally seen in gardens.
They are tall plants with a single very large flower head.
H. annuus subsp. annuus are the ones typically seen in gardens. They range from 20 cm to over 3.5 m high, some are
branched and some have heads up to 50 cm across.
The ray florets can yellow, orange, red, purple or brownish and some are striped.