Home & Garden Columns

Green Neighbors: Pollen, Cloning and Why We Need Healthy Trees

By Ron Sullivan
Tuesday April 10, 2007

Why did I spend most of last week sneezing? Why do half the people on the street seem to be sneezing along with me? Is it a peculiarly Berkeley sort of performance art? 

No, right now it’s mostly the trees. I’m breathing an unrestrained sigh of relief because the fruitless mulberries on my street have mostly stopped their incontinent pollinating and their flowers are falling off. What looks like a mass die-off of homely gray caterpillars on the sidewalks—that’s them.  

Over the last couple of decades, as Thomas Ogren points out in his intriguing book Safe Sex in the Garden, there has been a shift in the sort of trees being planted, especially by public entities. Lots of “fruitless” trees and shrubs are being used because they’re “cleaner”—they don’t drop fruit or seeds on the sidewalk. As we’re seeing, that doesn’t mean they don’t make a mess. They’re being mass-produced, as big nurseries take on the methods of any other industrial enterprise. And they’re all males.  

All our trees are flowering plants, unless you’re counting tree ferns. Trees like pines and other conifers; oaks, alders, olives, and those mulberries, whose flowers are inconspicuous, are usually pollinated by wind. That’s why they don’t need showy blooms to attract pollinators.  

Male flowers produce pollen, and if there aren’t any handy females, where’s all that pollen to go? Into your lungs and mine, and that might be one reason pollen allergy rates are rising all over North America. When your exposure to some random allergen hits a certain lifetime threshold, you discover you’re allergic because you’re sneezing or wheezing or itching or worse. Here’s the rub: that threshold is unknown before you hit it, and so’s the particular allergen.  

A lot of these “non-messy” trees are cultivars that have been planted as replacements for the street trees killed by Dutch elm disease in the 50s and 60s. People had all sorts of ideas about streamlined living then, same as we do now. 

A cultivar—the name of a plant that you see in single quotes on its tag—is often a clone, reproduced by a scaled-up version of what Aunt Tillie did when she rooted a slip of tradescantia in water. All-male clones are produced either from a male of a dioecious species, which has staminate (male) and pistillate (female) flowers on separate plants, rather the way we mammals do it; or from the staminate parts of a monoecious species, which has both kinds of flowers in different places on each plant.  

Plants have a complicated assortment of sexual arrangements; a Falwell among them would either prosper on an endless indignation supply or confuse himself to death.  

And there’s a funny thing about cloning trees: If you clone a redwood from cuttings that come from the top of the tree, it will usually be taller and skinnier, more upward-tending, than a clone from the bottom branches of the same tree. (Yes, people do. They get cuttings from redwood tops by firing a shotgun up into the tree and gathering the bits of tree that fall. I kid you not.) 

Similarly, if you clone a twig that has only male flowers on it, you’ll get an all-male tree of a species that is normally hermaphroditic, with both sexes usually occurring in one plant. if you’re a big wholesale grower, you’ll do this a few thousand times and sell a few thousand genetically identical trees to your big clients and voila, drifts of pollen are wafting across the city.  

Add to the pollen count any number of other allergens like molds and mildews that got their boost with winter’s first rains, insects and their leavings, and just plain dust (including rubber from tires wearing on roads) and it’s a miracle we’re all breathing.  

Ogren and tree-lovers in general agree on some common-sense considerations for our trees. A less-than-healthy tree will typically carry a greater load of insects and molds (including the molds that flourish on the insects’ droppings), and the insects’ dander and the molds’ spores are powerful allergens. Choosing trees that will prosper where they’re planted and keeping them healthy will reduce that load.  

That will also conveniently and thriftily reduce the need for safety pruning, line-clearance pruning, and pesticide spraying. Healthy trees do all their good work more vigorously, and live longer.  

That good work includes trapping and filtering literally tons of airborne particulates and noxious gases that we’d otherwise be breathing—and, of course, producing oxygen and cooling our cities and mitigating cities’ effects on climate. 

That, plus much-needed bird and beneficial critter habitat, efficiently vertical. In a civilized society, we’d even be getting lots of free fruit from urban trees. Maybe that’s a goal to work toward, someday. 


Ron Sullivan is a former professional gardener and arborist. Her “Green Neighbors” column appears every other Tuesday in the Berkeley Daily Planet, alternating with Joe Eaton’s “Wild Neighbors” column. Her “Garden Variety” column appears every Friday in the Planet’s East Bay Home & Real Estate section.  


Photograph: Ron Sullivan 

Good riddance: The flowers are falling off the mulberries, finally.