On the North Coast, as elsewhere, over a period of time, the types of plants selected by homeowners, landscapers and government agencies have changed significantly -- and
the changes have made the plant allergy problem substantially worse.
For reasons of convenience, more and more shrubs, trees and other plants are
selected for their "litter-free" characteristics -- that is, they are
male plant types and generate little or no seeds or fruits.
In 1949 the USDA first recommended that American cities begin planting male tree and
shrub cultivars on our streets and in our public parks. Over the past
fifty years that advise has become firmly rooted in the psyche of
American ornamental horticulture. M
any American cities have plant ordinances that ban the planting of seed, fruit or nut producing
cultivars on our streets or in our public spaces. As a result, modern
urban landscapes have evolved to consist predominantly of trees, shrubs,
grasses and ground covers that are:
* Dioecious male cultivars selected because they produce no fruit or
* Monoecious cultivars selected, at least in substantial part, because
they minimize female sexual characteristics (fruit and seed).
These plant choices, of course, incidentally maximize the most
significant male sexual characteristic: pollen production. Thus, in
order to avoid the problem of cleaning up fruit and seeds from female
trees we have -- unthinkingly -- altered the natural mix of male and
female plants in such a way that pollen loads have increased, and
correspondingly, so have allergies and asthma.
Pollen is the invisible "litter" in urban America. We have increasingly clean sidewalks and,
from a pure health perspective, dirty air.