From that blessed source
of noding material
, New Scientist
(http://www.newscientist.com/editorial/editorial_224133.html), we learn that people are putting the "wrong" plants in their garden.
According to Tom Ogren, a California horticulturalist, we have been filling our gardens with the wrong plants and lining our streets with the wrong trees (see p 46).
(That's page 46 in New Scientist magazine
, 03 June 2000.)
People prefer male, wind-pollenated plants over female, fruit/seed-producing or insect-pollenated plants.
This, of course, increases the likelihood of hay fever.
It also reduces habitat for insects, such as bees and butterflies.