Trees can be one of three sexes – monecious, dioecious male or dioecious female. Naturally, there is a relatively even split between all three, so the amount of pollen wafted into the air is regulated. But when dioecious males are planted independently of dioecious females, as often the case in urban areas, their pollen is unchecked by any capture by female flowers.

Eight years ago Tom Ogren, a horticulturist, was in Sacramento, California, when he noticed that the ground around the State Capitol building was covered in thick yellow pollen.

Scanning the trees along the street with his binoculars, he saw the trees were all deodar cedars (Cedrus deodara) and all cultivated males.

Naturally, the deodar is monoecious, having both male and female cones growing on the same tree. But cultivation has produced wholly male trees – plants favoured by planners since they have no seeds or pods to drop but only pollen. This was the case at this Sacramento site, Ogren said.

Growers’ breeding of purely male diodar trees had created, said Ogren, “something that doesn’t even exist in nature”.

Ogren said he had seen similar pollen-coated cityscapes in Christchurch, New Zealand, London, UK, and all over Canada. “Everywhere I’ve gone, same situation,” he said.

Male trees are often favoured by urban landscape planners as female trees can shed fruit as well as seeds and pods.