How urban planners' preference for male trees has made your hay fever worse

Horticulturists urge better sex mix of city trees to mitigate rising asthma and CO₂ pollution levels

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.