Cranwell was born in
Auckland, New Zealand, in 1907. She grew up in
Henderson, on an orchard at the conjunction of the
Ōpanuku and
Oratia streams. Swan states her initial interest in science and botany was supported by
Henry Charles Swan. Swan had a small orchard in Henderson and experimented with propagation to combat orchard pests. She was strongly influenced by her conservation-minded and artistic mother. It has been suggested that Cranwell inherited the unpredictable aspects of her fearless and adventuresome spirit from her mother's Cornish roots. Her father was a trained nurseryman who had planted an extensive orchard in the family property. She attended Henderson public school, and attended
Epsom Girls Grammar School from 1921 to 1923. She entered the
University of Auckland in 1925 where she undertook an initial BA degree that was a mixture of English and botany, followed by a master's in botany with a thesis on the epiphytes of the
Waitākere Ranges. She graduated in 1929. During her university studies she developed a love of
tramping and gained a reputation as talented, in the University Field Club. Her love of the New Zealand wilderness stood her in good stead for the many botany field trips she began to embark on, most often with fellow botany student and friend
Lucy Moore, to various remote and inaccessible parts of the country. ==Auckland Museum==