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ONE day last fall I was photographing some ground cover in my neighbourhood,
carefully staying on the public sidewalk. The groundsel tree Baccharis
was growing all over the place. A gentleman came out of his front door. 鈥淲hat in
the world could be worth photographing in my front yard?鈥� he asked.

He had a point. It was a drab landscape, drought tolerant and
low-maintenance, with little colour.

鈥淚鈥檓 an allergy researcher,鈥� I said. 鈥淚 need a photo of these in bloom for my
产辞辞办.鈥�

鈥淚s it bad stuff?鈥� he asked.

鈥淭hey鈥檙e all male plants,鈥� I said, 鈥渓ots of pollen.鈥�

鈥淭hat figures,鈥� he told me. 鈥淢y wife has terrible hay fever.鈥�

This was not in the least surprising to me, considering that his entire
landscape, front yard and back, was made up entirely of highly allergenic
plants.

Last week I was taking photos of another yard, shooting from across the
street. A fellow in his early forties came out of his house behind me and asked
me why I was bothering to photograph the house across from him.

鈥淚 research allergies,鈥� I said. 鈥淭hat place is a perfect example of
high-allergy landscaping. Those three big cypress trees produce lots of pollen,
the alders next to the driveway are high-allergy trees, the big fountain grasses
produce grass pollen all year round, the two pepper trees are both
wind-pollinated males, and that huge mass of juniper ground cover鈥攅very
one of them is a male clone. The whole landscape is designed to cause
补濒濒别谤驳测.鈥�

鈥淓veryone in my house has allergies,鈥� he said. 鈥淗ow does my own yard
濒辞辞办?鈥�

鈥淲ell,鈥� I said, looking it over, 鈥渢hese two ash trees here on the boulevard
are both male clones, that big yew pine (Podocarpus) is a male, but the
rest of your yard looks good. I see you鈥檙e keeping the privet hedge
肠濒颈辫辫别诲.鈥�

鈥淵ou bet,鈥� he said, 鈥渢he smell of their flowers drives me nuts!鈥�

Landscapers in every city over most of the world seem to use the same two
dozen plants over and over again. If trees or shrubs are separate-sexed, they
usually plant only males. Perfect-flowered, insect-pollinated trees and shrubs
are used less and less. Wind-pollinated cultivars are used more and more.

Female trees shed 鈥渓itter鈥�: old seeds, seed pods, messy fruit, whereas male
trees are 鈥渓itter-free鈥�. But males produce huge amounts of pollen.

The closer you are to the source of pollen, the more you鈥檙e exposed. Pollen
does blow all over the place, but most lands close to where it starts. An
allergenic, pollen-pumping tree in your own yard exposes you to 10 times more
pollen than you鈥檇 get from a similar tree down the block. Avoidance is the
key.

The dominant street trees used to be either the English elm or the American
elm. These fine, stately trees lined streets in thousands of cities worldwide.
The elms had both male and female parts in the same flowers. Elms, pollinated
mostly by honeybees and butterflies, did cause some allergy, mostly because they
were common, but elm pollen was always limited.

When Dutch elm disease swept through Europe and America, it killed most of
the old elms. The replacements were largely unisexual-flowered, wind-pollinated
trees, often lacking nectar sources. Gone was the early spring urban food source
of the bees and butterflies. Soon, honeybee and butterfly populations started to
fall in many areas.

Depending on the species planted, it usually took between ten and forty years
for these replacement trees to reach sexual maturity. Once they started to bloom
every year, allergy levels started to rise.

Japan lost most of its street trees not from Dutch elm disease but from the
Second World War. After the war the Japanese government started planting
millions of Cryptomeria japonica, also called Japanese cedar.
Cryptomeria is not actually a type of cedar at all, but is closely related
to cypress. And what鈥檚 more, like cypress, it produces allergenic
pollen鈥攍ots and lots of it.

It took the Cryptomeria trees more than forty years to reach sexual
maturity. But they鈥檙e mature now. Each spring their pollen rains on the
populations of Japanese cities; it is now the number one cause of allergy in
Japan. In Britain and the US, a variety of trees were used, but most were
wind-pollinated, often male. We are now reaping the pollen that we so
thoughtlessly sowed years ago.

Unfortunately, even today, little or no attention is given to how much pollen
will be produced by the trees and shrubs we plant. In cities all over the world,
allergies are now two, three, even four, times more common than they were just
two decades ago. The numbers continue to climb. How many of us will have to
suffer year after year before we start to do something about this situation?

Years ago, I decided to develop a plant-allergy scale. I had been researching
allergy and landscape plants for a decade and had long since tired of
classifying them as: 鈥渉igh-allergy plants, low-allergy ones, medium-low,
medium-medium-high, really-really bad, extra-extra-good鈥�. I needed a numerical
scale, something easy to use and understand.

I used a large set of factors, roughly half positive and half negative. If a
plant was insect-pollinated, that was a plus. A wind-pollinated plant would be a
negative. Flowers large and bright, again plus. Tiny and off-white or greenish
plants would be negative. But if a plant shed pollen for a very brief period,
positive. If it rained pollen for months on end, negative. And so on. The scale
runs from 1 (best) to 10 (worst). It鈥檚 called the OPALS+TM scale and is used by
the US Department of Agriculture.

How can we cut back on allergies? The answer鈥檚 simple. Only plant those
plants with low numbers on the scale, preferably 5 or less. Now is the time to
start turning back the pollen clock.