Helianthus annus 
f. lenticularis

Showy Sunflower

G.F. Ledingham Herbarium

The cultivated sunflower (Helianthus annus var. giganteus) has flower heads 15 to 30 cm across.

Flora of Alberta keys genus Helianthus as having "at least the lower leaves opposite".  Yet it keys Helianthus annus as having "leaves mostly alternate".  Budd's Flora also states that the leaves of this species are "usually alternate".

Budd's Flora keys genus Helianthus as "plants with entire, undivided leaves".  The leaves are, indeed, undivided, but many leaves of Helianthus annus are toothed

Flora of Alberta states that the achenes of genus Helianthus bear a "pappus of small scales or awns, soon falling off".

Budd's Flora keys genus Helianthus as having "achenes without barbed awns" (italics my own).

It also keys genus Helianthus as having "pappus composed of scales, bristles, or barbs"; not "pappus absent, or present merely as a short crown".

I have found that neither flora easily distinguishes between Helianthus and Rudbeckia (Rudbeckia hirta is Black-eyed Susan) in their keys.  However, they do note that the receptacles of Rudbeckia are more conic and the leaves always alternate.

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Annus Characteristics
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annual plants

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pubescence rough

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leaves mostly alternate

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leaves commonly ovate, the lowest often cordate and toothed

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involucral bracts abruptly attenuate, with conspicuous marginal hairs

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receptacle flat or nearly so

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disk purple or dark brown

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scales between disk florets not very hairy

 

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Helianthus Characteristics
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juice watery, not milky

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plants not heavy scented

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plants more or less leafy-stemmed

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plants not with a woody caudex

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leaves not compound

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leaves not linear

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at least the lower leaves opposite

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involucre not glandular nor glutinous

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involucre bracts in one or more series, but all of the same nature

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receptacle flat or convex, not conic or columnar

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receptacle with chaffy scales between the florets

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flower heads with both tubular and ray florets

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rays conspicuous

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ray florets yellow

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ligulate (ray) flowers either pistillate or neutral

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pappus composed of small scales or awns, soon falling off; not composed of hairs