An aster flower has more than 21 petals so it does not fit in with the pattern of "the golden section". (Aster is the name of the street that Zoe lives on)
Aster flower does not have 21 petals; as any biologist knows, every single aster flower has exactly five petals. What is commonly perceived as an aster flower, is in fact a large inflorescentia composed of dozens of small flowers, tubular (with all five petals fused together to form a pipe) at the centre of the inflorescentia, ligular (where the fused petals form a small tongue, resembling a single flower petal) at the perimeter. The plant family to which an aster belongs, known nowadays as "Asteraceae" (aster-likes), was once labeled "Compositae" (composed) to underline this.